first commit

2026-07-31 10:46:40 +00:00
commit ae8c4f5232
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+**/build/**
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+#Fri Jul 31 09:53:09 UTC 2026
+gradle.version=8.10.2
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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Toolchain
+
+Shell state does **not** persist between Bash calls. Source the toolchain in every command that runs
+Gradle:
+
+```fish
+source /home/user/tools/env.sh && ./gradlew <task>
+```
+
+That sets `JAVA_HOME` (Temurin 21.0.12+8), puts Gradle 8.10.2 on `PATH`, and sets `GRADLE_USER_HOME`.
+
+## Reading Minecraft's source
+
+Decompiled, mojmapped sources are already extracted for grepping — **use these, not `javap`**:
+
+- `/home/user/mc-sources/minecraft/` — 5364 Minecraft `.java` files
+- `/home/user/mc-sources/neoforge/` — 953 NeoForge `.java` files
+
+Verify vanilla behaviour there before asserting it in code or tests. Nearly every subtle bug in this
+project came from guessing at a constant instead of grepping for it.
+
+## Commands
+
+```fish
+./gradlew build                  # compile + unit tests + jar
+./gradlew test                   # unit tests only (fast, no game runtime)
+./gradlew test --tests 'dev.chunkinspector.analysis.LevelPropagationTest'
+./gradlew test --tests '*.aChunkQueuedThenSupersededKeepsTheStrongerAnswer'
+./gradlew jar                    # the mod jar, build/libs/chunkinspector-<version>.jar
+./gradlew e2eTest                # full end-to-end: installs and drives two real dedicated servers
+./gradlew runServer              # MDG dev server, for manual poking
+```
+
+`e2eTest` is deliberately excluded from `check` — it downloads NeoForge (~300 MB, cached in
+`~/.cache/chunkinspector-e2e`) and boots two servers, ~90 s once the cache is warm. It `dependsOn jar`,
+so it always tests the shipped artifact.
+
+The server half can be run without Gradle: `./e2e/run-servers.sh` (honours `E2E_OUT`, `MOD_JAR`,
+`NEO_VERSION`, `E2E_BOOT_TIMEOUT`). Servers are left in `build/e2e/<mode>/` afterwards — read
+`build/e2e/light/console.log` and `build/e2e/*/world/chunk-summaries/latest.txt` when a test fails,
+they are far more informative than the assertion message.
+
+## Architecture
+
+Minecraft 1.21.1, NeoForge 21.1.247, Mojang mappings, ModDevGradle. Java 21 + Lombok.
+
+### The layering rule that everything else follows
+
+`analysis/`, `report/` and `origin/` contain **no Minecraft types**. `game/` is the only package that
+touches them, and its job is to reduce live game state to plain records (`TicketRecord`,
+`ChunkRecord`) via `LevelCapture.of(ServerLevel)`.
+
+This is not stylistic. It is what makes 62 unit tests runnable without a game runtime, and it is why
+`build.gradle` adds `fastutil` and `gson` as *test* dependencies at the versions Minecraft itself
+ships. Keep new logic on the Minecraft-free side of that line; if you need a game value, capture it
+into a record rather than importing `net.minecraft` into `analysis/`.
+
+### Two cost tiers, decided before Mixin application
+
+The always-on tier injects **nothing**. `META-INF/accesstransformer.cfg` widens five
+`DistanceManager`/`Ticket`/`ChunkMap` members for read-only access, and a snapshot walks them on
+demand.
+
+Deep mode (`-Dchunkinspector.deep=true`) adds a `StackWalker` capture at
+`DistanceManager.addTicket(long, Ticket)` — the single choke point every ticket passes through.
+`InspectorMixinPlugin.shouldApplyMixin` *declines to apply* `DistanceManagerMixin` and `TicketMixin`
+when deep mode is off, so Minecraft's bytecode is untouched rather than carrying a disabled branch.
+
+Because that decision happens during Mixin config load, all configuration is system properties
+(`InspectorConfig`) — a config file would be read too late. Adding a new tuning knob means adding it
+there, not to a NeoForge config spec.
+
+### Snapshot lifecycle
+
+`InspectorService` splits the work: `capture()` runs on the server thread and only copies two
+collections; analysis, propagation and JSON serialisation happen on the single-threaded
+`chunkinspector-report` executor against the immutable snapshot. That single thread is also what
+makes `ReportWriter`'s check-then-create filename logic race-free.
+
+Triggers are `periodic` (every `interval` ticks), `command`, and `shutdown` (always full analysis,
+blocks up to 30 s). Output goes to `<levelDir>/chunk-summaries/`.
+
+### The analysis itself
+
+`LevelPropagation` replays vanilla's `ChunkTracker` rules — a ticket at chunk C level L gives every
+chunk at Chebyshev distance d a level of L+d, lowest wins, stop above `ChunkLevel.MAX_LEVEL` — using
+Dial's algorithm (bucket queue) so the pass is O(loaded chunks). Crucially it also records *which*
+ticket won each chunk; the game throws that away, and it is the entire point of the mod.
+
+`LevelAnalysis` turns that into totals, per-type breakdowns, suspects, hotspots and unattributed
+chunks. "Influence" is a partition, not a sum: each loaded chunk is credited to exactly one winning
+ticket, so per-ticket numbers add up rather than overlap.
+
+Attribution is opt-in per snapshot (`attribution` flag / `snapshot full`) because it is the only
+part that is more than O(tickets).
+
+### Ticket identity without instrumentation
+
+`TicketKeys.render` is what lets the free tier often be sufficient — a NeoForge forced-chunk key
+already carries the requesting mod's id. Deep mode is only needed when the key is opaque.
+
+## Gotchas
+
+- `net.minecraft.server.level.Ticket` is `final`, so `ticket instanceof TicketOrigin` will not
+  compile even though `TicketMixin` makes it true at runtime. Cast through `Object`.
+- `OriginRegistry` filters `dev.chunkinspector.*` frames out of every capture, so a test that calls
+  `record()` from within that package captures nothing. `src/test/java/example/mod/LeakyChunkLoader`
+  exists to be an outside caller.
+- `/forceload add` takes **block** coordinates, not chunk coordinates. The e2e script force-loads
+  `1600 1600 1663 1663` to cover chunks (100,100)..(103,103).
+- Snapshot filenames resolve to the second, so they carry the trigger (`summary-<stamp>-<trigger>.json`)
+  and uniquify on collision — a periodic and a command snapshot really do land in the same second.
+- The mixin config's `defaultRequire: 1` means a silently non-applying injector fails the build at
+  load time. Check `console.log` for `Mixin apply failed` when a deep-mode e2e test goes quiet.
+
+## Tests
+
+`src/test/java/dev/chunkinspector/Fixtures.java` holds the shared scenario (a player ticket, a leaky
+permanent ticket, a forceload, a portal ticket, and a chunk set) used across the analysis, report and
+digest tests. Extend it rather than hand-rolling another fixture.
+
+`src/e2e/` is a separate source set whose `compileClasspath` includes `main`'s output — the e2e tests
+parse the servers' JSON back through the production `InspectionReport` records, which is deliberate:
+it proves the file a server writes is a file this mod can read.
+
+See `README.md` for the user-facing documentation: all nine system properties, the commands, and how
+to read a report.
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+# Chunk Inspector
+
+Finds out **why chunks stay loaded** on a heavily modded Minecraft **1.21.1 / NeoForge** server.
+
+Every loaded chunk on a vanilla-or-modded server is loaded because some *chunk ticket* holds it, or
+because it is within propagation range of one. Chunk Inspector reads the live ticket table, replays
+the game's own level propagation, and writes a report naming the tickets, the chunks they keep alive,
+and — if you ask for it — the exact line of code that created each one.
+
+Reports land in `<levelDir>/chunk-summaries` (i.e. next to `level.dat` in the world folder).
+
+## Two cost tiers
+
+| | Always on | `-Dchunkinspector.deep=true` |
+|---|---|---|
+| Bytecode injected into Minecraft | **none** | `DistanceManager.addTicket`, `Ticket` |
+| Per-ticket cost | zero | one stack walk |
+| Names the ticket type, level, age, owner key | yes | yes |
+| Names the *code* that created the ticket | no | yes |
+
+The default tier is genuinely free: an access transformer widens a few ticket fields, and a snapshot
+walks them on demand. Nothing runs between snapshots. The instrumenting Mixins are not merely
+disabled without `deep` — a Mixin config plugin declines to apply them, so Minecraft's classes are
+left untouched.
+
+Reach for `deep` when the always-on tier shows you *which* tickets leak but the owner key is opaque.
+Many tickets already identify themselves (a NeoForge forced chunk carries the requesting mod's id),
+so it is often unnecessary.
+
+## Commands
+
+All require permission level 2.
+
+| Command | What it does |
+|---|---|
+| `/chunkinspector status` | Cheap in-chat summary of the current dimension |
+| `/chunkinspector why [<chunkX> <chunkZ>]` | Every ticket contributing to one chunk's level, strongest first. Defaults to the chunk you are standing in |
+| `/chunkinspector snapshot` | Writes a report now |
+| `/chunkinspector snapshot full` | ... including the expensive chunk-to-ticket attribution pass |
+
+A report is also written every `interval` ticks and once on shutdown. The shutdown one always runs
+the full analysis, because it is the one people come back to.
+
+## System properties
+
+Pass these on the server's JVM command line, before `-jar` / `@libraries/...`.
+
+| Property | Default | Meaning |
+|---|---|---|
+| `chunkinspector.enabled` | `true` | Master switch. When off, no hooks and no reports |
+| `chunkinspector.deep` | `false` | Record the stack trace behind every ticket |
+| `chunkinspector.interval` | `600` | Ticks between automatic snapshots; `0` disables them |
+| `chunkinspector.attribution` | `false` | Run the chunk-to-ticket pass on *periodic* snapshots too |
+| `chunkinspector.retention` | `48` | Snapshot files to keep; `0` keeps everything |
+| `chunkinspector.textReport` | `true` | Also write the human-readable `latest.txt` |
+| `chunkinspector.stackDepth` | `24` | Frames kept per call site (deep mode) |
+| `chunkinspector.maxOrigins` | `8192` | Distinct call sites remembered (deep mode) |
+| `chunkinspector.sampleRate` | `1` | Capture one stack per *N* ticket adds (deep mode) |
+
+Example — free monitoring, one snapshot per minute, a day's history:
+
+```
+-Dchunkinspector.interval=1200 -Dchunkinspector.retention=1440
+```
+
+Example — a full investigation:
+
+```
+-Dchunkinspector.deep=true -Dchunkinspector.attribution=true
+```
+
+## Output
+
+```
+chunk-summaries/
+  latest.json                            the newest report
+  latest.txt                             the same thing, readable
+  summary-20260731-103326-periodic.json  timestamped history, pruned to `retention`
+  summary-20260731-103326-command.json
+  summary-20260731-103340-shutdown.json
+```
+
+`latest.txt` of a server with a 4x4 `/forceload`:
+
+```
+══ minecraft:overworld ══  game time 528
+  chunks: 113 loaded, 61 block-ticking, 25 entity-ticking (1741 tracked)
+  tickets: 17 on 17 chunks, 17 never expire, 16 from /forceload
+
+  by ticket type
+    forced        16 tickets   16 chunks  level>=31  keeps 64 loaded  oldest 26s
+    start          1 tickets    1 chunks  level>=30  keeps 49 loaded  oldest 26s
+
+  most suspicious tickets
+    chunk 103, 102       forced      level 31  chunk 103, 102
+      -> permanent ticket, alive 26s, keeping 9 chunks loaded; entity-ticking, so mobs and machines run here
+
+  loaded chunk clusters
+       64 chunks  centre 101, 101  bounds 98, 98 .. 105, 105     forced x64
+       49 chunks  centre 0, 0      bounds -3, -3 .. 3, 3         start x49
+```
+
+The JSON carries the same information plus everything that did not fit — see
+`InspectionReport` for the field-by-field documentation.
+
+### How to read it
+
+- **suspects** are tickets with *no timeout* that the game does not manage itself. Only these can
+  leak forever. `keeping N chunks loaded` is a partition, not a sum: each loaded chunk is credited to
+  the single strongest ticket holding it, so the numbers across tickets add up rather than overlap.
+- **loaded chunk clusters** group contiguous loaded chunks. A leak usually shows up as one cluster
+  far from spawn with a single cause.
+- **level** is the vanilla chunk level: `<=31` entity-ticking, `<=32` block-ticking, `<=33` full.
+  An entity-ticking leak is far more expensive than a merely-loaded one.
+- **unattributed** lists loaded chunks no ticket explains. Normally empty; entries here mean
+  something is holding chunks outside the ticket system.
+
+## Building
+
+```
+./gradlew build          # jar + unit tests
+./gradlew e2eTest        # installs a real NeoForge dedicated server and asserts on its reports
+```
+
+`e2eTest` is deliberately not part of `check`: it downloads NeoForge and runs two servers end to end,
+taking a few minutes. It boots a dedicated server in each tier, force-loads a 4x4 square of chunks
+over the console, and verifies the reports name it.
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+plugins {
+    id 'java-library'
+    id 'net.neoforged.moddev' version '2.0.143'
+}
+
+group = mod_group_id
+version = mod_version
+base { archivesName = mod_id }
+
+java.toolchain.languageVersion = JavaLanguageVersion.of(21)
+
+repositories {
+    mavenCentral()
+}
+
+neoForge {
+    version = project.neo_version
+
+    // Expose the mojmapped, decompiled Minecraft sources so they can be grepped.
+    // `./gradlew createMinecraftArtifacts` produces them under build/ + the MDG cache.
+    accessTransformers.from('src/main/resources/META-INF/accesstransformer.cfg')
+
+    runs {
+        configureEach {
+            systemProperty 'forge.logging.markers', 'REGISTRIES'
+            logLevel = org.slf4j.event.Level.DEBUG
+        }
+        client {
+            client()
+        }
+        server {
+            server()
+            programArgument '--nogui'
+        }
+    }
+
+    mods {
+        "$mod_id" {
+            sourceSet sourceSets.main
+        }
+    }
+}
+
+// The end-to-end suite asserts on reports produced by a real dedicated server, so it is kept out of
+// `check`: it downloads and installs NeoForge and takes minutes. Run it with `./gradlew e2eTest`.
+sourceSets {
+    e2e {
+        compileClasspath += sourceSets.main.output
+        runtimeClasspath += sourceSets.main.output
+    }
+}
+
+configurations {
+    e2eImplementation.extendsFrom testImplementation
+    e2eRuntimeOnly.extendsFrom testRuntimeOnly
+}
+
+dependencies {
+    compileOnly 'org.projectlombok:lombok:1.18.34'
+    annotationProcessor 'org.projectlombok:lombok:1.18.34'
+
+    testCompileOnly 'org.projectlombok:lombok:1.18.34'
+    testAnnotationProcessor 'org.projectlombok:lombok:1.18.34'
+
+    testImplementation platform('org.junit:junit-bom:5.11.3')
+    testImplementation 'org.junit.jupiter:junit-jupiter'
+    testRuntimeOnly 'org.junit.platform:junit-platform-launcher'
+    testImplementation 'org.assertj:assertj-core:3.26.3'
+
+    // The analysis, report and origin packages are deliberately free of Minecraft types so they can be
+    // unit-tested without a game runtime. These two are the only libraries they touch, at the versions
+    // Minecraft 1.21.1 itself ships.
+    testImplementation 'it.unimi.dsi:fastutil:8.5.12'
+    testImplementation 'com.google.code.gson:gson:2.10.1'
+}
+
+// Interpolate mod metadata into neoforge.mods.toml
+var generateModMetadata = tasks.register('generateModMetadata', ProcessResources) {
+    var replaceProperties = [
+            minecraft_version      : minecraft_version,
+            neo_version            : neo_version,
+            mod_id                 : mod_id,
+            mod_name               : mod_name,
+            mod_license            : mod_license,
+            mod_version            : mod_version,
+            mod_authors            : mod_authors,
+            mod_description        : mod_description,
+    ]
+    inputs.properties replaceProperties
+    expand replaceProperties
+    from 'src/main/templates'
+    into "$buildDir/generated/sources/modMetadata"
+}
+sourceSets.main.resources.srcDir generateModMetadata
+neoForge.ideSyncTask generateModMetadata
+
+tasks.withType(JavaCompile).configureEach {
+    options.encoding = 'UTF-8'
+    options.release = 21
+    options.compilerArgs << '-Xlint:all,-processing,-serial'
+}
+
+tasks.named('test', Test) {
+    useJUnitPlatform()
+    testLogging {
+        events 'passed', 'skipped', 'failed'
+        exceptionFormat 'full'
+        showStandardStreams = false
+    }
+}
+
+var e2eRoot = layout.buildDirectory.dir('e2e')
+
+var e2eServers = tasks.register('e2eServers', Exec) {
+    group = 'verification'
+    description = 'Installs a NeoForge dedicated server, runs the mod on it twice and keeps its reports.'
+    dependsOn tasks.named('jar')
+    workingDir projectDir
+    commandLine 'bash', 'e2e/run-servers.sh'
+    environment 'E2E_OUT', e2eRoot.get().asFile.absolutePath
+    outputs.upToDateWhen { false }
+}
+
+tasks.register('e2eTest', Test) {
+    group = 'verification'
+    description = 'Asserts on the reports the dedicated servers produced. Not part of `check`: it is slow.'
+    dependsOn e2eServers
+    testClassesDirs = sourceSets.e2e.output.classesDirs
+    classpath = sourceSets.e2e.runtimeClasspath
+    systemProperty 'e2e.root', e2eRoot.get().asFile.absolutePath
+    useJUnitPlatform()
+    testLogging {
+        events 'passed', 'skipped', 'failed'
+        exceptionFormat 'full'
+    }
+    outputs.upToDateWhen { false }
+}
@@ -0,0 +1,156 @@
+#!/usr/bin/env bash
+#
+# Provisions a real NeoForge dedicated server, drops the built mod into it and drives it through a
+# scripted session — once with the default (zero-instrumentation) configuration and once with deep
+# mode on. The resulting worlds are left in build/e2e/<mode> for ChunkInspectorE2ETest to assert on.
+#
+# Runnable on its own:  ./e2e/run-servers.sh
+#
+set -euo pipefail
+
+project_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+neo_version="${NEO_VERSION:-$(sed -n 's/^neo_version=//p' "$project_dir/gradle.properties")}"
+mod_jar="${MOD_JAR:-$(ls "$project_dir"/build/libs/chunkinspector-*.jar 2>/dev/null | head -1)}"
+out_dir="${E2E_OUT:-$project_dir/build/e2e}"
+cache_dir="${E2E_CACHE:-$HOME/.cache/chunkinspector-e2e}"
+boot_timeout="${E2E_BOOT_TIMEOUT:-420}"
+
+# The forceloaded square the assertions look for: chunks (100, 100)..(103, 103) — 4x4, far enough
+# from spawn to form its own cluster and small enough to stay under the report's suspect cap.
+# /forceload takes *block* coordinates, so these are the corners of those chunks: 100 * 16 = 1600
+# and 103 * 16 + 15 = 1663.
+force_from="1600 1600"
+force_to="1663 1663"
+
+if [[ -z "$mod_jar" || ! -f "$mod_jar" ]]; then
+    echo "no mod jar found; run ./gradlew jar first" >&2
+    exit 1
+fi
+
+log() { printf '[e2e] %s\n' "$*"; }
+
+# --------------------------------------------------------------------- provision
+
+installer="$cache_dir/neoforge-$neo_version-installer.jar"
+template="$cache_dir/server-$neo_version"
+
+mkdir -p "$cache_dir"
+if [[ ! -f "$installer" ]]; then
+    log "downloading NeoForge $neo_version installer"
+    curl -fsSL -o "$installer.part" \
+        "https://maven.neoforged.net/releases/net/neoforged/neoforge/$neo_version/neoforge-$neo_version-installer.jar"
+    mv "$installer.part" "$installer"
+fi
+
+# The installation is cached and copied per run so the two modes can never share a world.
+if [[ ! -f "$template/.installed" ]]; then
+    log "installing dedicated server into $template"
+    rm -rf "$template"
+    mkdir -p "$template"
+    # From the cache directory: the installer drops its own .log file next to the working directory.
+    (cd "$cache_dir" && java -jar "$installer" --installServer "$template") > "$cache_dir/install.log" 2>&1 \
+        || { tail -40 "$cache_dir/install.log" >&2; exit 1; }
+    touch "$template/.installed"
+fi
+
+args_file="libraries/net/neoforged/neoforge/$neo_version/unix_args.txt"
+[[ -f "$template/$args_file" ]] || { echo "missing $args_file in the installed server" >&2; exit 1; }
+
+# ------------------------------------------------------------------------- run
+
+# run <mode> <extra jvm properties...>
+run() {
+    local mode="$1"
+    shift
+    local dir="$out_dir/$mode"
+
+    log "preparing $mode server in $dir"
+    rm -rf "$dir"
+    mkdir -p "$dir"
+    cp -a "$template/." "$dir/"
+    rm -f "$dir/.installed"
+    mkdir -p "$dir/mods"
+    cp "$mod_jar" "$dir/mods/"
+
+    echo "eula=true" > "$dir/eula.txt"
+    cat > "$dir/server.properties" <<'PROPERTIES'
+level-name=world
+level-type=minecraft:flat
+level-seed=e2e
+online-mode=false
+max-players=1
+view-distance=10
+simulation-distance=10
+spawn-protection=0
+sync-chunk-writes=false
+enable-jmx-monitoring=false
+enable-status=false
+max-tick-time=-1
+PROPERTIES
+
+    local fifo="$dir/console-in"
+    mkfifo "$fifo"
+    (
+        cd "$dir"
+        exec java -Xmx2G "$@" "@$args_file" --nogui < console-in > console.log 2>&1
+    ) &
+    local server=$!
+    # Opening the write end unblocks the server's read end; keeping it open keeps stdin alive.
+    exec 9> "$fifo"
+
+    log "waiting for $mode server to finish loading"
+    local waited=0
+    until grep -q 'Done (' "$dir/console.log" 2>/dev/null; do
+        if ! kill -0 "$server" 2>/dev/null; then
+            exec 9>&-
+            tail -40 "$dir/console.log" >&2
+            echo "$mode server exited before it finished loading" >&2
+            exit 1
+        fi
+        sleep 1
+        waited=$((waited + 1))
+        if [[ $waited -ge $boot_timeout ]]; then
+            exec 9>&-
+            kill "$server" 2>/dev/null || true
+            echo "$mode server did not start within ${boot_timeout}s" >&2
+            exit 1
+        fi
+    done
+
+    say() { printf '%s\n' "$1" >&9; log "  > $1"; sleep "${2:-2}"; }
+
+    # A leaking chunk loader, simulated with the one permanent ticket every vanilla server can make.
+    # The pause covers generating the 4x4 square, which has never been visited before.
+    say "forceload add $force_from $force_to" 5
+    say "chunkinspector status"
+    say "chunkinspector why 102 102"
+    say "chunkinspector snapshot full" 5
+    # Long enough for at least one periodic snapshot (interval is 200 ticks = 10s) to also land.
+    say "chunkinspector status" 12
+    say "stop" 0
+
+    log "waiting for $mode server to shut down"
+    wait "$server" || true
+    exec 9>&-
+    rm -f "$fifo"
+
+    local summaries="$dir/world/chunk-summaries"
+    [[ -d "$summaries" ]] || { tail -60 "$dir/console.log" >&2; echo "no $summaries produced" >&2; exit 1; }
+    log "$mode done: $(ls "$summaries" | wc -l) files in chunk-summaries"
+}
+
+mkdir -p "$out_dir"
+
+# Tier one: nothing injected into the game at all.
+run light \
+    -Dchunkinspector.interval=200 \
+    -Dchunkinspector.retention=5
+
+# Tier two: the opt-in expensive analysis.
+run deep \
+    -Dchunkinspector.deep=true \
+    -Dchunkinspector.attribution=true \
+    -Dchunkinspector.interval=200 \
+    -Dchunkinspector.retention=5
+
+log "all servers completed"
@@ -0,0 +1,18 @@
+org.gradle.jvmargs=-Xmx3G
+org.gradle.daemon=true
+org.gradle.parallel=true
+org.gradle.caching=true
+
+# Environment
+minecraft_version=1.21.1
+neo_version=21.1.247
+parchment_minecraft_version=1.21.1
+
+# Mod
+mod_id=chunkinspector
+mod_name=Chunk Inspector
+mod_license=MIT
+mod_version=1.0.0
+mod_group_id=dev.chunkinspector
+mod_authors=ChunkInspector Contributors
+mod_description=Diagnoses why chunks stay loaded on heavily-modded servers by tracking chunk tickets and writing reports to <level>/chunk-summaries.
@@ -0,0 +1,7 @@
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.10.2-bin.zip
+networkTimeout=10000
+validateDistributionUrl=true
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists
@@ -0,0 +1,252 @@
+#!/bin/sh
+
+#
+# Copyright © 2015-2021 the original authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+##############################################################################
+#
+#   Gradle start up script for POSIX generated by Gradle.
+#
+#   Important for running:
+#
+#   (1) You need a POSIX-compliant shell to run this script. If your /bin/sh is
+#       noncompliant, but you have some other compliant shell such as ksh or
+#       bash, then to run this script, type that shell name before the whole
+#       command line, like:
+#
+#           ksh Gradle
+#
+#       Busybox and similar reduced shells will NOT work, because this script
+#       requires all of these POSIX shell features:
+#         * functions;
+#         * expansions «$var», «${var}», «${var:-default}», «${var+SET}»,
+#           «${var#prefix}», «${var%suffix}», and «$( cmd )»;
+#         * compound commands having a testable exit status, especially «case»;
+#         * various built-in commands including «command», «set», and «ulimit».
+#
+#   Important for patching:
+#
+#   (2) This script targets any POSIX shell, so it avoids extensions provided
+#       by Bash, Ksh, etc; in particular arrays are avoided.
+#
+#       The "traditional" practice of packing multiple parameters into a
+#       space-separated string is a well documented source of bugs and security
+#       problems, so this is (mostly) avoided, by progressively accumulating
+#       options in "$@", and eventually passing that to Java.
+#
+#       Where the inherited environment variables (DEFAULT_JVM_OPTS, JAVA_OPTS,
+#       and GRADLE_OPTS) rely on word-splitting, this is performed explicitly;
+#       see the in-line comments for details.
+#
+#       There are tweaks for specific operating systems such as AIX, CygWin,
+#       Darwin, MinGW, and NonStop.
+#
+#   (3) This script is generated from the Groovy template
+#       https://github.com/gradle/gradle/blob/HEAD/platforms/jvm/plugins-application/src/main/resources/org/gradle/api/internal/plugins/unixStartScript.txt
+#       within the Gradle project.
+#
+#       You can find Gradle at https://github.com/gradle/gradle/.
+#
+##############################################################################
+
+# Attempt to set APP_HOME
+
+# Resolve links: $0 may be a link
+app_path=$0
+
+# Need this for daisy-chained symlinks.
+while
+    APP_HOME=${app_path%"${app_path##*/}"}  # leaves a trailing /; empty if no leading path
+    [ -h "$app_path" ]
+do
+    ls=$( ls -ld "$app_path" )
+    link=${ls#*' -> '}
+    case $link in             #(
+      /*)   app_path=$link ;; #(
+      *)    app_path=$APP_HOME$link ;;
+    esac
+done
+
+# This is normally unused
+# shellcheck disable=SC2034
+APP_BASE_NAME=${0##*/}
+# Discard cd standard output in case $CDPATH is set (https://github.com/gradle/gradle/issues/25036)
+APP_HOME=$( cd -P "${APP_HOME:-./}" > /dev/null && printf '%s
+' "$PWD" ) || exit
+
+# Use the maximum available, or set MAX_FD != -1 to use that value.
+MAX_FD=maximum
+
+warn () {
+    echo "$*"
+} >&2
+
+die () {
+    echo
+    echo "$*"
+    echo
+    exit 1
+} >&2
+
+# OS specific support (must be 'true' or 'false').
+cygwin=false
+msys=false
+darwin=false
+nonstop=false
+case "$( uname )" in                #(
+  CYGWIN* )         cygwin=true  ;; #(
+  Darwin* )         darwin=true  ;; #(
+  MSYS* | MINGW* )  msys=true    ;; #(
+  NONSTOP* )        nonstop=true ;;
+esac
+
+CLASSPATH=$APP_HOME/gradle/wrapper/gradle-wrapper.jar
+
+
+# Determine the Java command to use to start the JVM.
+if [ -n "$JAVA_HOME" ] ; then
+    if [ -x "$JAVA_HOME/jre/sh/java" ] ; then
+        # IBM's JDK on AIX uses strange locations for the executables
+        JAVACMD=$JAVA_HOME/jre/sh/java
+    else
+        JAVACMD=$JAVA_HOME/bin/java
+    fi
+    if [ ! -x "$JAVACMD" ] ; then
+        die "ERROR: JAVA_HOME is set to an invalid directory: $JAVA_HOME
+
+Please set the JAVA_HOME variable in your environment to match the
+location of your Java installation."
+    fi
+else
+    JAVACMD=java
+    if ! command -v java >/dev/null 2>&1
+    then
+        die "ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
+
+Please set the JAVA_HOME variable in your environment to match the
+location of your Java installation."
+    fi
+fi
+
+# Increase the maximum file descriptors if we can.
+if ! "$cygwin" && ! "$darwin" && ! "$nonstop" ; then
+    case $MAX_FD in #(
+      max*)
+        # In POSIX sh, ulimit -H is undefined. That's why the result is checked to see if it worked.
+        # shellcheck disable=SC2039,SC3045
+        MAX_FD=$( ulimit -H -n ) ||
+            warn "Could not query maximum file descriptor limit"
+    esac
+    case $MAX_FD in  #(
+      '' | soft) :;; #(
+      *)
+        # In POSIX sh, ulimit -n is undefined. That's why the result is checked to see if it worked.
+        # shellcheck disable=SC2039,SC3045
+        ulimit -n "$MAX_FD" ||
+            warn "Could not set maximum file descriptor limit to $MAX_FD"
+    esac
+fi
+
+# Collect all arguments for the java command, stacking in reverse order:
+#   * args from the command line
+#   * the main class name
+#   * -classpath
+#   * -D...appname settings
+#   * --module-path (only if needed)
+#   * DEFAULT_JVM_OPTS, JAVA_OPTS, and GRADLE_OPTS environment variables.
+
+# For Cygwin or MSYS, switch paths to Windows format before running java
+if "$cygwin" || "$msys" ; then
+    APP_HOME=$( cygpath --path --mixed "$APP_HOME" )
+    CLASSPATH=$( cygpath --path --mixed "$CLASSPATH" )
+
+    JAVACMD=$( cygpath --unix "$JAVACMD" )
+
+    # Now convert the arguments - kludge to limit ourselves to /bin/sh
+    for arg do
+        if
+            case $arg in                                #(
+              -*)   false ;;                            # don't mess with options #(
+              /?*)  t=${arg#/} t=/${t%%/*}              # looks like a POSIX filepath
+                    [ -e "$t" ] ;;                      #(
+              *)    false ;;
+            esac
+        then
+            arg=$( cygpath --path --ignore --mixed "$arg" )
+        fi
+        # Roll the args list around exactly as many times as the number of
+        # args, so each arg winds up back in the position where it started, but
+        # possibly modified.
+        #
+        # NB: a `for` loop captures its iteration list before it begins, so
+        # changing the positional parameters here affects neither the number of
+        # iterations, nor the values presented in `arg`.
+        shift                   # remove old arg
+        set -- "$@" "$arg"      # push replacement arg
+    done
+fi
+
+
+# Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+DEFAULT_JVM_OPTS='"-Xmx64m" "-Xms64m"'
+
+# Collect all arguments for the java command:
+#   * DEFAULT_JVM_OPTS, JAVA_OPTS, JAVA_OPTS, and optsEnvironmentVar are not allowed to contain shell fragments,
+#     and any embedded shellness will be escaped.
+#   * For example: A user cannot expect ${Hostname} to be expanded, as it is an environment variable and will be
+#     treated as '${Hostname}' itself on the command line.
+
+set -- \
+        "-Dorg.gradle.appname=$APP_BASE_NAME" \
+        -classpath "$CLASSPATH" \
+        org.gradle.wrapper.GradleWrapperMain \
+        "$@"
+
+# Stop when "xargs" is not available.
+if ! command -v xargs >/dev/null 2>&1
+then
+    die "xargs is not available"
+fi
+
+# Use "xargs" to parse quoted args.
+#
+# With -n1 it outputs one arg per line, with the quotes and backslashes removed.
+#
+# In Bash we could simply go:
+#
+#   readarray ARGS < <( xargs -n1 <<<"$var" ) &&
+#   set -- "${ARGS[@]}" "$@"
+#
+# but POSIX shell has neither arrays nor command substitution, so instead we
+# post-process each arg (as a line of input to sed) to backslash-escape any
+# character that might be a shell metacharacter, then use eval to reverse
+# that process (while maintaining the separation between arguments), and wrap
+# the whole thing up as a single "set" statement.
+#
+# This will of course break if any of these variables contains a newline or
+# an unmatched quote.
+#
+
+eval "set -- $(
+        printf '%s\n' "$DEFAULT_JVM_OPTS $JAVA_OPTS $GRADLE_OPTS" |
+        xargs -n1 |
+        sed ' s~[^-[:alnum:]+,./:=@_]~\\&~g; ' |
+        tr '\n' ' '
+    )" '"$@"'
+
+exec "$JAVACMD" "$@"
@@ -0,0 +1,94 @@
+@rem
+@rem Copyright 2015 the original author or authors.
+@rem
+@rem Licensed under the Apache License, Version 2.0 (the "License");
+@rem you may not use this file except in compliance with the License.
+@rem You may obtain a copy of the License at
+@rem
+@rem      https://www.apache.org/licenses/LICENSE-2.0
+@rem
+@rem Unless required by applicable law or agreed to in writing, software
+@rem distributed under the License is distributed on an "AS IS" BASIS,
+@rem WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+@rem See the License for the specific language governing permissions and
+@rem limitations under the License.
+@rem
+@rem SPDX-License-Identifier: Apache-2.0
+@rem
+
+@if "%DEBUG%"=="" @echo off
+@rem ##########################################################################
+@rem
+@rem  Gradle startup script for Windows
+@rem
+@rem ##########################################################################
+
+@rem Set local scope for the variables with windows NT shell
+if "%OS%"=="Windows_NT" setlocal
+
+set DIRNAME=%~dp0
+if "%DIRNAME%"=="" set DIRNAME=.
+@rem This is normally unused
+set APP_BASE_NAME=%~n0
+set APP_HOME=%DIRNAME%
+
+@rem Resolve any "." and ".." in APP_HOME to make it shorter.
+for %%i in ("%APP_HOME%") do set APP_HOME=%%~fi
+
+@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+set DEFAULT_JVM_OPTS="-Xmx64m" "-Xms64m"
+
+@rem Find java.exe
+if defined JAVA_HOME goto findJavaFromJavaHome
+
+set JAVA_EXE=java.exe
+%JAVA_EXE% -version >NUL 2>&1
+if %ERRORLEVEL% equ 0 goto execute
+
+echo. 1>&2
+echo ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH. 1>&2
+echo. 1>&2
+echo Please set the JAVA_HOME variable in your environment to match the 1>&2
+echo location of your Java installation. 1>&2
+
+goto fail
+
+:findJavaFromJavaHome
+set JAVA_HOME=%JAVA_HOME:"=%
+set JAVA_EXE=%JAVA_HOME%/bin/java.exe
+
+if exist "%JAVA_EXE%" goto execute
+
+echo. 1>&2
+echo ERROR: JAVA_HOME is set to an invalid directory: %JAVA_HOME% 1>&2
+echo. 1>&2
+echo Please set the JAVA_HOME variable in your environment to match the 1>&2
+echo location of your Java installation. 1>&2
+
+goto fail
+
+:execute
+@rem Setup the command line
+
+set CLASSPATH=%APP_HOME%\gradle\wrapper\gradle-wrapper.jar
+
+
+@rem Execute Gradle
+"%JAVA_EXE%" %DEFAULT_JVM_OPTS% %JAVA_OPTS% %GRADLE_OPTS% "-Dorg.gradle.appname=%APP_BASE_NAME%" -classpath "%CLASSPATH%" org.gradle.wrapper.GradleWrapperMain %*
+
+:end
+@rem End local scope for the variables with windows NT shell
+if %ERRORLEVEL% equ 0 goto mainEnd
+
+:fail
+rem Set variable GRADLE_EXIT_CONSOLE if you need the _script_ return code instead of
+rem the _cmd.exe /c_ return code!
+set EXIT_CODE=%ERRORLEVEL%
+if %EXIT_CODE% equ 0 set EXIT_CODE=1
+if not ""=="%GRADLE_EXIT_CONSOLE%" exit %EXIT_CODE%
+exit /b %EXIT_CODE%
+
+:mainEnd
+if "%OS%"=="Windows_NT" endlocal
+
+:omega
@@ -0,0 +1,13 @@
+pluginManagement {
+    repositories {
+        mavenCentral()
+        gradlePluginPortal()
+        maven { name = 'NeoForged'; url = 'https://maven.neoforged.net/releases' }
+    }
+}
+
+plugins {
+    id 'org.gradle.toolchains.foojay-resolver-convention' version '0.8.0'
+}
+
+rootProject.name = 'chunkinspector'
@@ -0,0 +1,241 @@
+package dev.chunkinspector.e2e;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import dev.chunkinspector.report.InspectionReport;
+import dev.chunkinspector.report.InspectionReport.Hotspot;
+import dev.chunkinspector.report.InspectionReport.LevelReport;
+import dev.chunkinspector.report.InspectionReport.OriginEntry;
+import dev.chunkinspector.report.InspectionReport.TicketEntry;
+import dev.chunkinspector.report.InspectionReport.TypeBreakdown;
+import java.util.List;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
+
+/**
+ * Asserts on two real NeoForge dedicated servers that {@code e2e/run-servers.sh} has already run to
+ * completion: one with the default configuration, one with {@code -Dchunkinspector.deep=true}. Both
+ * were driven through the same session — force-load a 4x4 square of chunks, ask the mod about it,
+ * then shut down.
+ */
+class ChunkInspectorE2ETest {
+
+    /** Matches the forceload issued by the script: chunks (100, 100) through (103, 103). */
+    private static final int FORCED_FROM = 100;
+    private static final int FORCED_TO = 103;
+    private static final int FORCED_CHUNKS = 16;
+
+    private static final Pattern BOUNDS = Pattern.compile("(-?\\d+), (-?\\d+) \\.\\. (-?\\d+), (-?\\d+)");
+
+    private static final ServerRun LIGHT = ServerRun.of("light");
+    private static final ServerRun DEEP = ServerRun.of("deep");
+
+    static List<ServerRun> bothModes() {
+        return List.of(LIGHT, DEEP);
+    }
+
+    private static TypeBreakdown byType(LevelReport level, String type) {
+        return level.byType().stream()
+                .filter(t -> t.type().equals(type))
+                .findFirst()
+                .orElseThrow(() -> new AssertionError("no " + type + " tickets in " + level.byType()));
+    }
+
+    // ------------------------------------------------------- the reports exist and are well formed
+
+    @ParameterizedTest
+    @MethodSource("bothModes")
+    void reportsLandInTheLevelDirectoryUnderChunkSummaries(ServerRun run) {
+        assertThat(run.summaries()).isDirectory();
+        assertThat(run.summaries().getParent().getFileName()).hasToString("world");
+
+        InspectionReport latest = run.latest();
+        assertThat(latest.schema()).isEqualTo(InspectionReport.SCHEMA);
+        assertThat(latest.generatedAt()).isNotBlank();
+        assertThat(latest.levels()).extracting(LevelReport::dimension)
+                .contains("minecraft:overworld", "minecraft:the_nether", "minecraft:the_end");
+        assertThat(run.digest()).contains("Chunk Inspector", "minecraft:overworld");
+    }
+
+    @ParameterizedTest
+    @MethodSource("bothModes")
+    void everyTriggerProducesItsOwnSnapshotAndTheLastOneIsTheShutdown(ServerRun run) {
+        assertThat(run.snapshots()).extracting(InspectionReport::trigger)
+                .contains("periodic", "command", "shutdown");
+        // latest.json must mirror the newest snapshot, which is always the one taken on shutdown.
+        assertThat(run.latest().trigger()).isEqualTo("shutdown");
+    }
+
+    @ParameterizedTest
+    @MethodSource("bothModes")
+    void oldSnapshotsArePrunedToTheConfiguredRetention(ServerRun run) {
+        assertThat(run.snapshotFiles()).hasSizeLessThanOrEqualTo(5); // -Dchunkinspector.retention=5
+        assertThat(run.snapshotFiles()).isNotEmpty();
+    }
+
+    @ParameterizedTest
+    @MethodSource("bothModes")
+    void theServerStartedCleanlyWithNoMixinOrModErrors(ServerRun run) {
+        String console = run.console();
+        assertThat(console).contains("Chunk Inspector active");
+        assertThat(console).doesNotContain(
+                "Mixin apply failed",
+                "Mixin prepare failed",
+                "org.spongepowered.asm.mixin.throwables",
+                "Failed to create mod instance",
+                "Chunk summary failed");
+    }
+
+    // -------------------------------------------------------------- the leak is actually diagnosed
+
+    @ParameterizedTest
+    @MethodSource("bothModes")
+    void theForceloadedChunksAreCountedAsPermanentTickets(ServerRun run) {
+        LevelReport overworld = ServerRun.level(run.latest(), "minecraft:overworld");
+
+        assertThat(overworld.totals().forcedChunks()).isEqualTo(FORCED_CHUNKS);
+        assertThat(overworld.totals().permanentTickets()).isGreaterThanOrEqualTo(FORCED_CHUNKS);
+        assertThat(overworld.totals().loadedChunks()).isGreaterThanOrEqualTo(FORCED_CHUNKS);
+
+        TypeBreakdown forced = byType(overworld, "forced");
+        assertThat(forced.tickets()).isEqualTo(FORCED_CHUNKS);
+        assertThat(forced.chunks()).isEqualTo(FORCED_CHUNKS);
+        assertThat(forced.expiring()).as("a forceload never expires on its own").isFalse();
+    }
+
+    @ParameterizedTest
+    @MethodSource("bothModes")
+    void everyForcedTicketIsFlaggedAsASuspectAndNamesItsChunk(ServerRun run) {
+        List<TicketEntry> forced = ServerRun.level(run.latest(), "minecraft:overworld").suspects().stream()
+                .filter(s -> s.type().equals("forced"))
+                .toList();
+
+        assertThat(forced).hasSize(FORCED_CHUNKS);
+        assertThat(forced).allSatisfy(entry -> {
+            assertThat(entry.reason()).contains("permanent ticket");
+            assertThat(entry.owner()).matches("chunk 10[0-3], 10[0-3]");
+            assertThat(entry.chunk()).matches("10[0-3], 10[0-3]");
+            assertThat(entry.blockPos()).isNotBlank();
+        });
+        assertThat(run.digest()).contains("most suspicious tickets", "forced");
+    }
+
+    @ParameterizedTest
+    @MethodSource("bothModes")
+    void theSpawnChunksAreRecognisedAsNormalAndNeverFlagged(ServerRun run) {
+        LevelReport overworld = ServerRun.level(run.latest(), "minecraft:overworld");
+
+        assertThat(byType(overworld, "start").tickets()).isPositive();
+        assertThat(overworld.suspects()).extracting(TicketEntry::type).doesNotContain("start", "player");
+    }
+
+    @ParameterizedTest
+    @MethodSource("bothModes")
+    void theForceloadedSquareShowsUpAsItsOwnClusterOfLoadedChunks(ServerRun run) {
+        List<Hotspot> hotspots = ServerRun.level(run.latest(), "minecraft:overworld").hotspots();
+
+        assertThat(hotspots).isNotEmpty();
+        assertThat(hotspots).anySatisfy(hotspot -> {
+            Matcher bounds = BOUNDS.matcher(hotspot.bounds());
+            assertThat(bounds.matches()).as("parsable bounds in %s", hotspot).isTrue();
+            assertThat(Integer.parseInt(bounds.group(1))).isLessThanOrEqualTo(FORCED_FROM);
+            assertThat(Integer.parseInt(bounds.group(2))).isLessThanOrEqualTo(FORCED_FROM);
+            assertThat(Integer.parseInt(bounds.group(3))).isGreaterThanOrEqualTo(FORCED_TO);
+            assertThat(Integer.parseInt(bounds.group(4))).isGreaterThanOrEqualTo(FORCED_TO);
+            assertThat(hotspot.chunks()).isGreaterThanOrEqualTo(FORCED_CHUNKS);
+            assertThat(hotspot.causes()).anyMatch(cause -> cause.startsWith("forced"));
+        });
+    }
+
+    @ParameterizedTest
+    @MethodSource("bothModes")
+    void theShutdownSnapshotAttributesEveryLoadedChunkToATicket(ServerRun run) {
+        LevelReport overworld = ServerRun.level(run.latest(), "minecraft:overworld");
+
+        assertThat(overworld.attributionRan()).isTrue();
+        assertThat(overworld.unattributed())
+                .as("every loaded chunk should be explained by some ticket")
+                .isEmpty();
+        assertThat(byType(overworld, "forced").influence())
+                .as("the forceload keeps at least its own chunks loaded")
+                .isGreaterThanOrEqualTo(FORCED_CHUNKS);
+    }
+
+    // ------------------------------------------------------------------------------ the commands
+
+    @ParameterizedTest
+    @MethodSource("bothModes")
+    void theCommandsAnswerOnTheConsole(ServerRun run) {
+        String console = run.console();
+
+        assertThat(console).contains("minecraft:overworld: "); // /chunkinspector status
+        assertThat(console).contains("Chunk 102, 102 is at level"); // /chunkinspector why
+        assertThat(console).contains("forced from chunk 10"); // ... naming the forceload
+        assertThat(console).containsPattern("Wrote .*chunk-summaries.summary-.*\\.json"); // ... snapshot
+    }
+
+    // ------------------------------------------------------------------------- the two cost tiers
+
+    @Test
+    void withoutDeepModeMinecraftIsNeverInstrumentedAndNoCallSiteIsNamed() {
+        assertThat(LIGHT.console()).contains("deep mode off, leaving untouched Minecraft's classes");
+        assertThat(LIGHT.latest().settings().deep()).isFalse();
+
+        assertThat(LIGHT.latest().levels()).allSatisfy(level -> {
+            assertThat(level.origins()).isEmpty();
+            assertThat(level.suspects()).allSatisfy(suspect -> assertThat(suspect.origin()).isNull());
+        });
+        // The overworld genuinely has suspects, so the check above is not vacuous everywhere.
+        assertThat(ServerRun.level(LIGHT.latest(), "minecraft:overworld").suspects()).isNotEmpty();
+        assertThat(LIGHT.digest()).contains("-Dchunkinspector.deep=true");
+    }
+
+    @Test
+    void deepModeNamesTheCodeThatCreatedEachTicket() {
+        assertThat(DEEP.console()).contains("deep mode on, instrumenting Minecraft's classes");
+        assertThat(DEEP.latest().settings().deep()).isTrue();
+
+        LevelReport overworld = ServerRun.level(DEEP.latest(), "minecraft:overworld");
+        assertThat(overworld.origins()).isNotEmpty();
+        assertThat(overworld.origins()).allSatisfy(origin -> {
+            assertThat(origin.id()).matches("[0-9a-f]{12}");
+            assertThat(origin.adds()).isPositive();
+            assertThat(origin.live()).isPositive();
+            assertThat(origin.types()).isNotEmpty();
+            assertThat(origin.stack()).isNotEmpty();
+            assertThat(origin.stack()).noneMatch(frame -> frame.startsWith("dev.chunkinspector."));
+        });
+
+        // The forceload was issued by the command, so its origin must lead back to ServerLevel.
+        OriginEntry forcedOrigin = overworld.origins().stream()
+                .filter(o -> o.types().contains("forced"))
+                .findFirst()
+                .orElseThrow(() -> new AssertionError("no call site recorded for forced tickets"));
+        assertThat(forcedOrigin.stack().getFirst()).startsWith("net.minecraft.server.level.ServerLevel.setChunkForced");
+        assertThat(forcedOrigin.live()).isEqualTo(FORCED_CHUNKS);
+        assertThat(forcedOrigin.mod()).isNotBlank();
+
+        // ... and every flagged forced ticket must cross-reference it.
+        assertThat(overworld.suspects()).filteredOn(s -> s.type().equals("forced"))
+                .extracting(TicketEntry::origin)
+                .containsOnly(forcedOrigin.id());
+
+        assertThat(DEEP.digest()).contains("ticket call sites", "net.minecraft.server.level.ServerLevel.setChunkForced");
+    }
+
+    @Test
+    void deepModeDoesNotChangeWhatTheLightModeAlreadyFoundCorrectly() {
+        LevelReport light = ServerRun.level(LIGHT.latest(), "minecraft:overworld");
+        LevelReport deep = ServerRun.level(DEEP.latest(), "minecraft:overworld");
+
+        // Two independent servers ran the same script, so the ticket picture must match. That is the
+        // evidence that the cheap tier loses nothing but the call stacks.
+        assertThat(deep.totals().forcedChunks()).isEqualTo(light.totals().forcedChunks());
+        assertThat(byType(deep, "forced").chunks()).isEqualTo(byType(light, "forced").chunks());
+        assertThat(byType(deep, "start").tickets()).isEqualTo(byType(light, "start").tickets());
+        assertThat(deep.suspects()).hasSameSizeAs(light.suspects());
+    }
+}
@@ -0,0 +1,87 @@
+package dev.chunkinspector.e2e;
+
+import com.google.gson.Gson;
+import dev.chunkinspector.report.InspectionReport;
+import dev.chunkinspector.report.InspectionReport.LevelReport;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.List;
+import java.util.stream.Stream;
+
+/**
+ * One finished server session, as left on disk by {@code e2e/run-servers.sh}.
+ *
+ * <p>Reading the reports back through the production {@link InspectionReport} records rather than
+ * through raw JSON means the test also proves the file a server writes is a file this mod can read.
+ */
+record ServerRun(String mode, Path directory) {
+
+    private static final Gson GSON = new Gson();
+
+    static ServerRun of(String mode) {
+        Path root = Path.of(System.getProperty("e2e.root", "build/e2e"));
+        return new ServerRun(mode, root.resolve(mode));
+    }
+
+    Path summaries() {
+        return directory.resolve("world").resolve("chunk-summaries");
+    }
+
+    InspectionReport latest() {
+        return read(summaries().resolve("latest.json"));
+    }
+
+    String digest() {
+        return text(summaries().resolve("latest.txt"));
+    }
+
+    String console() {
+        return text(directory.resolve("console.log"));
+    }
+
+    List<Path> snapshotFiles() {
+        try (Stream<Path> files = Files.list(summaries())) {
+            return files.filter(p -> p.getFileName().toString().startsWith("summary-")).sorted().toList();
+        } catch (IOException e) {
+            throw new UncheckedIOException(e);
+        }
+    }
+
+    List<InspectionReport> snapshots() {
+        return snapshotFiles().stream().map(ServerRun::read).toList();
+    }
+
+    /** The overworld report, which is where the scripted session does its damage. */
+    LevelReport overworld() {
+        return level(latest(), "minecraft:overworld");
+    }
+
+    static LevelReport level(InspectionReport report, String dimension) {
+        return report.levels().stream()
+                .filter(l -> l.dimension().equals(dimension))
+                .findFirst()
+                .orElseThrow(() -> new AssertionError("no report for " + dimension + " in " + report.levels().stream()
+                        .map(LevelReport::dimension)
+                        .toList()));
+    }
+
+    private static InspectionReport read(Path file) {
+        return GSON.fromJson(text(file), InspectionReport.class);
+    }
+
+    private static String text(Path file) {
+        try {
+            return Files.readString(file, StandardCharsets.UTF_8);
+        } catch (IOException e) {
+            throw new UncheckedIOException("expected " + file.toAbsolutePath() + " to exist", e);
+        }
+    }
+
+    @Override
+    public String toString() {
+        return mode;
+    }
+}
@@ -0,0 +1,68 @@
+package dev.chunkinspector;
+
+import dev.chunkinspector.origin.OriginRegistry;
+import net.neoforged.bus.api.IEventBus;
+import net.neoforged.bus.api.SubscribeEvent;
+import net.neoforged.fml.common.Mod;
+import net.neoforged.neoforge.common.NeoForge;
+import net.neoforged.neoforge.event.RegisterCommandsEvent;
+import net.neoforged.neoforge.event.server.ServerStartedEvent;
+import net.neoforged.neoforge.event.server.ServerStoppingEvent;
+import net.neoforged.neoforge.event.tick.ServerTickEvent;
+
+/**
+ * Entry point.
+ *
+ * <p>Two tiers of instrumentation:
+ * <ul>
+ *   <li><b>Always on.</b> Nothing is injected into the game. An access transformer widens a handful
+ *       of ticket fields, and a snapshot walks them on demand. Many tickets already name their
+ *       owner — a NeoForge forced chunk carries the requesting mod's id in its key — so this alone
+ *       often identifies the culprit.</li>
+ *   <li><b>{@code -Dchunkinspector.deep=true}.</b> Adds the stack trace behind every ticket, for the
+ *       cases where the key is opaque. Only then are the instrumenting Mixins applied at all.</li>
+ * </ul>
+ */
+@Mod(ChunkInspector.MOD_ID)
+public final class ChunkInspector {
+
+    public static final String MOD_ID = "chunkinspector";
+
+    private static final OriginRegistry ORIGINS = new OriginRegistry();
+    private static final InspectorService SERVICE = new InspectorService(ORIGINS);
+
+    public ChunkInspector(IEventBus modBus) {
+        if (InspectorConfig.enabled()) {
+            NeoForge.EVENT_BUS.register(this);
+        }
+    }
+
+    /** Used by the deep-mode Mixin; static because a Mixin has no other way to reach the mod. */
+    public static OriginRegistry origins() {
+        return ORIGINS;
+    }
+
+    public static InspectorService service() {
+        return SERVICE;
+    }
+
+    @SubscribeEvent
+    void onServerStarted(ServerStartedEvent event) {
+        SERVICE.serverStarted(event.getServer());
+    }
+
+    @SubscribeEvent
+    void onServerTick(ServerTickEvent.Post event) {
+        SERVICE.serverTick(event.getServer());
+    }
+
+    @SubscribeEvent
+    void onServerStopping(ServerStoppingEvent event) {
+        SERVICE.serverStopping(event.getServer());
+    }
+
+    @SubscribeEvent
+    void onRegisterCommands(RegisterCommandsEvent event) {
+        InspectorCommand.register(event.getDispatcher());
+    }
+}
@@ -0,0 +1,135 @@
+package dev.chunkinspector;
+
+import com.mojang.brigadier.CommandDispatcher;
+import com.mojang.brigadier.arguments.IntegerArgumentType;
+import com.mojang.brigadier.context.CommandContext;
+import dev.chunkinspector.analysis.ChunkExplainer;
+import dev.chunkinspector.analysis.ChunkKey;
+import dev.chunkinspector.analysis.LevelAnalysis;
+import dev.chunkinspector.game.LevelCapture;
+import dev.chunkinspector.report.InspectionReport.LevelReport;
+import java.util.List;
+import lombok.experimental.UtilityClass;
+import net.minecraft.ChatFormatting;
+import net.minecraft.commands.CommandSourceStack;
+import net.minecraft.commands.Commands;
+import net.minecraft.core.SectionPos;
+import net.minecraft.network.chat.Component;
+import net.minecraft.network.chat.MutableComponent;
+
+/**
+ * {@code /chunkinspector} — operator tooling for the two questions that come up in practice:
+ * "what is loading everything?" and "why is this particular chunk loaded?".
+ */
+@UtilityClass
+public class InspectorCommand {
+
+    private final int PERMISSION_LEVEL = 2;
+    private final int SHOWN_TYPES = 6;
+    private final int SHOWN_SUSPECTS = 5;
+
+    public void register(CommandDispatcher<CommandSourceStack> dispatcher) {
+        dispatcher.register(Commands.literal(ChunkInspector.MOD_ID)
+                .requires(source -> source.hasPermission(PERMISSION_LEVEL))
+                .then(Commands.literal("snapshot")
+                        .executes(context -> snapshot(context, InspectorConfig.attributionInSnapshots()))
+                        .then(Commands.literal("full").executes(context -> snapshot(context, true))))
+                .then(Commands.literal("status").executes(InspectorCommand::status))
+                .then(Commands.literal("why")
+                        .executes(context -> why(context, currentChunk(context)))
+                        .then(Commands.argument("chunkX", IntegerArgumentType.integer())
+                                .then(Commands.argument("chunkZ", IntegerArgumentType.integer())
+                                        .executes(context -> why(context, ChunkKey.of(
+                                                IntegerArgumentType.getInteger(context, "chunkX"),
+                                                IntegerArgumentType.getInteger(context, "chunkZ"))))))));
+    }
+
+    private int snapshot(CommandContext<CommandSourceStack> context, boolean attribution) {
+        CommandSourceStack source = context.getSource();
+        source.sendSuccess(() -> literal("Capturing chunk summary" + (attribution ? " with full attribution" : "") + "…"), true);
+        ChunkInspector.service()
+                .snapshot(source.getServer(), "command", attribution)
+                .whenComplete((path, error) -> source.getServer().execute(() -> {
+                    if (error != null) {
+                        source.sendFailure(literal("Chunk summary failed: " + error.getMessage()));
+                    } else {
+                        source.sendSuccess(() -> literal("Wrote " + path), true);
+                    }
+                }));
+        return 1;
+    }
+
+    private int status(CommandContext<CommandSourceStack> context) {
+        CommandSourceStack source = context.getSource();
+        LevelCapture capture = LevelCapture.of(source.getLevel());
+        LevelReport report = LevelAnalysis.analyse(
+                capture.dimension(),
+                capture.gameTime(),
+                capture.tickets(),
+                capture.chunks(),
+                capture.maxLevel(),
+                false,
+                ChunkInspector.origins().snapshot());
+
+        var totals = report.totals();
+        send(source, ChatFormatting.AQUA, capture.dimension() + ": " + totals.loadedChunks() + " loaded, "
+                + totals.entityTicking() + " entity-ticking, " + totals.tickets() + " tickets ("
+                + totals.permanentTickets() + " permanent)");
+        report.byType().stream().limit(SHOWN_TYPES).forEach(type ->
+                send(source, ChatFormatting.GRAY, "  " + type.type() + ": " + type.tickets() + " tickets on "
+                        + type.chunks() + " chunks, level>=" + type.minLevel()
+                        + (type.expiring() ? " (expires)" : "")));
+        List<?> suspects = report.suspects();
+        if (suspects.isEmpty()) {
+            send(source, ChatFormatting.GREEN, "  no permanent non-vanilla tickets");
+        } else {
+            report.suspects().stream().limit(SHOWN_SUSPECTS).forEach(suspect ->
+                    send(source, ChatFormatting.YELLOW, "  " + suspect.type() + " @ " + suspect.chunk()
+                            + " — " + suspect.owner() + " — " + suspect.reason()));
+        }
+        send(source, ChatFormatting.DARK_GRAY, "Run /" + ChunkInspector.MOD_ID + " snapshot full for the complete report.");
+        return 1;
+    }
+
+    private int why(CommandContext<CommandSourceStack> context, long chunk) {
+        CommandSourceStack source = context.getSource();
+        LevelCapture capture = LevelCapture.of(source.getLevel());
+        ChunkExplainer.Explanation explanation = ChunkExplainer.explain(capture.tickets(), chunk, capture.maxLevel());
+
+        if (explanation.contributors().isEmpty()) {
+            send(source, ChatFormatting.GREEN, "Chunk " + ChunkKey.format(chunk) + " is not held by any ticket.");
+            return 0;
+        }
+        send(source, ChatFormatting.AQUA, "Chunk " + ChunkKey.format(chunk) + " is at level " + explanation.level() + ":");
+        explanation.contributors().forEach(contribution -> {
+            var ticket = contribution.ticket();
+            send(source, ChatFormatting.GRAY, "  %s from %s at chunk %s (distance %d, level %d) — %s%s".formatted(
+                    contribution.contributed() == explanation.level() ? "▶ " + ticket.type() : "  " + ticket.type(),
+                    ticket.owner(),
+                    ChunkKey.format(ticket.chunk()),
+                    contribution.distance(),
+                    contribution.contributed(),
+                    ticket.expiring() ? "expires" : "permanent",
+                    ticket.originId() == null ? "" : ", origin " + ticket.originId()));
+        });
+        if (!InspectorConfig.deep()) {
+            send(source, ChatFormatting.DARK_GRAY, "Restart with -Dchunkinspector.deep=true to see which code added these.");
+        }
+        return explanation.contributors().size();
+    }
+
+    private long currentChunk(CommandContext<CommandSourceStack> context) {
+        var position = context.getSource().getPosition();
+        return ChunkKey.of(
+                SectionPos.blockToSectionCoord(position.x),
+                SectionPos.blockToSectionCoord(position.z));
+    }
+
+    private void send(CommandSourceStack source, ChatFormatting colour, String text) {
+        source.sendSuccess(() -> literal(text).withStyle(colour), false);
+    }
+
+    private MutableComponent literal(String text) {
+        return Component.literal(text);
+    }
+}
@@ -0,0 +1,89 @@
+package dev.chunkinspector;
+
+import lombok.experimental.UtilityClass;
+
+/**
+ * All tuning is done through system properties so that a server administrator can change behaviour
+ * without a config file round-trip and, more importantly, so that the values are known before the
+ * Mixin subsystem decides which transformations to apply.
+ *
+ * <p>The expensive instrumentation ({@link #deep()}) is opt-in: when it is off the corresponding
+ * Mixins are never applied to Minecraft's classes at all, so the cost is exactly zero.
+ */
+@UtilityClass
+public class InspectorConfig {
+
+    public static final String PREFIX = "chunkinspector.";
+
+    /** Master switch. When false the mod loads but installs no hooks and writes no reports. */
+    public boolean enabled() {
+        return flag("enabled", true);
+    }
+
+    /**
+     * Enables the expensive instrumentation: every ticket addition is attributed to the code that
+     * requested it by walking the call stack. Costs a stack walk per ticket add.
+     */
+    public boolean deep() {
+        return enabled() && flag("deep", false);
+    }
+
+    /** Ticks between automatic snapshots. {@code 0} disables periodic snapshots. */
+    public int intervalTicks() {
+        return positiveOrZero("interval", 600);
+    }
+
+    /**
+     * Whether periodic snapshots also run the chunk-to-ticket attribution pass, which is
+     * {@code O(sum of ticket influence areas)}. Always available on demand via the command.
+     */
+    public boolean attributionInSnapshots() {
+        return flag("attribution", false);
+    }
+
+    /** How many timestamped snapshot files to keep per level before the oldest are deleted. */
+    public int retention() {
+        return positiveOrZero("retention", 48);
+    }
+
+    /** Maximum number of stack frames retained per captured origin. */
+    public int stackDepth() {
+        return Math.clamp(intProperty("stackDepth", 24), 4, 256);
+    }
+
+    /** Upper bound on distinct call sites remembered, so a pathological server cannot OOM. */
+    public int maxOrigins() {
+        return Math.clamp(intProperty("maxOrigins", 8192), 64, 1 << 20);
+    }
+
+    /**
+     * Only capture a stack for one in {@code N} ticket additions. {@code 1} (the default) captures
+     * everything; raise it on servers where ticket churn is itself the bottleneck.
+     */
+    public int sampleRate() {
+        return Math.max(1, intProperty("sampleRate", 1));
+    }
+
+    /** Emit a plain-text digest next to the JSON report. */
+    public boolean textReport() {
+        return flag("textReport", true);
+    }
+
+    public boolean flag(String key, boolean fallback) {
+        String raw = System.getProperty(PREFIX + key);
+        return raw == null || raw.isBlank() ? fallback : Boolean.parseBoolean(raw);
+    }
+
+    private int intProperty(String key, int fallback) {
+        try {
+            String raw = System.getProperty(PREFIX + key);
+            return raw == null || raw.isBlank() ? fallback : Integer.parseInt(raw.trim());
+        } catch (NumberFormatException e) {
+            return fallback;
+        }
+    }
+
+    private int positiveOrZero(String key, int fallback) {
+        return Math.max(0, intProperty(key, fallback));
+    }
+}
@@ -0,0 +1,147 @@
+package dev.chunkinspector;
+
+import dev.chunkinspector.analysis.LevelAnalysis;
+import dev.chunkinspector.game.LevelCapture;
+import dev.chunkinspector.origin.Origin;
+import dev.chunkinspector.origin.OriginRegistry;
+import dev.chunkinspector.report.InspectionReport;
+import dev.chunkinspector.report.ReportWriter;
+import java.nio.file.Path;
+import java.time.Instant;
+import java.time.ZoneOffset;
+import java.time.format.DateTimeFormatter;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import lombok.extern.slf4j.Slf4j;
+import net.minecraft.server.MinecraftServer;
+import net.minecraft.server.level.ServerLevel;
+import net.minecraft.world.level.storage.LevelResource;
+
+/**
+ * Owns the snapshot lifecycle: capture on the server thread, analyse and write off it.
+ *
+ * <p>Splitting the work this way is what lets the periodic snapshot run on a busy server without
+ * being felt — the main thread only copies two collections, and the propagation pass and the JSON
+ * serialisation happen on a background thread against immutable data.
+ */
+@Slf4j
+public final class InspectorService {
+
+    private static final DateTimeFormatter STAMP =
+            DateTimeFormatter.ofPattern("yyyyMMdd-HHmmss").withZone(ZoneOffset.UTC);
+
+    private final OriginRegistry origins;
+    private final int intervalTicks = InspectorConfig.intervalTicks();
+
+    private ReportWriter writer;
+    private ExecutorService reporter;
+    private int ticksSinceSnapshot;
+
+    public InspectorService(OriginRegistry origins) {
+        this.origins = origins;
+    }
+
+    public void serverStarted(MinecraftServer server) {
+        Path levelDirectory = server.getWorldPath(LevelResource.ROOT).normalize();
+        writer = new ReportWriter(levelDirectory, InspectorConfig.retention());
+        reporter = Executors.newSingleThreadExecutor(runnable -> {
+            Thread thread = new Thread(runnable, "chunkinspector-report");
+            thread.setDaemon(true);
+            return thread;
+        });
+        log.info("Chunk Inspector active: reports in {}, deep={}, interval={} ticks",
+                writer.directory(), InspectorConfig.deep(), intervalTicks);
+    }
+
+    public void serverTick(MinecraftServer server) {
+        if (intervalTicks <= 0 || writer == null) {
+            return;
+        }
+        if (++ticksSinceSnapshot >= intervalTicks) {
+            ticksSinceSnapshot = 0;
+            snapshot(server, "periodic", InspectorConfig.attributionInSnapshots());
+        }
+    }
+
+    public void serverStopping(MinecraftServer server) {
+        if (writer == null) {
+            return;
+        }
+        // A shutdown snapshot is the one people come back to, so it is worth the full analysis and
+        // worth blocking briefly for.
+        CompletableFuture<Path> pending = snapshot(server, "shutdown", true);
+        reporter.shutdown();
+        try {
+            pending.get(30, TimeUnit.SECONDS);
+            reporter.awaitTermination(10, TimeUnit.SECONDS);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        } catch (Exception e) {
+            log.warn("Shutdown chunk summary did not complete", e);
+        }
+        writer = null;
+    }
+
+    /** Captures on the calling (server) thread and completes once the report has been written. */
+    public CompletableFuture<Path> snapshot(MinecraftServer server, String trigger, boolean attribution) {
+        if (writer == null) {
+            return CompletableFuture.failedFuture(new IllegalStateException("no level directory yet"));
+        }
+        List<LevelCapture> captures = capture(server);
+        Collection<Origin> originSnapshot = origins.snapshot();
+        Instant now = Instant.now();
+        ReportWriter target = writer;
+
+        return CompletableFuture.supplyAsync(() -> {
+            try {
+                InspectionReport report = assemble(captures, originSnapshot, trigger, attribution, now);
+                return target.write(report, STAMP.format(now), InspectorConfig.textReport());
+            } catch (Exception e) {
+                throw new IllegalStateException("failed to write chunk summary", e);
+            }
+        }, reporter).whenComplete((path, error) -> {
+            if (error != null) {
+                log.error("Chunk summary failed", error);
+            } else {
+                log.info("Wrote chunk summary {}", path);
+            }
+        });
+    }
+
+    /** Must run on the server thread. */
+    public List<LevelCapture> capture(MinecraftServer server) {
+        var captures = new ArrayList<LevelCapture>();
+        for (ServerLevel level : server.getAllLevels()) {
+            captures.add(LevelCapture.of(level));
+        }
+        return List.copyOf(captures);
+    }
+
+    static InspectionReport assemble(
+            List<LevelCapture> captures, Collection<Origin> origins, String trigger, boolean attribution, Instant now) {
+        return new InspectionReport(
+                InspectionReport.SCHEMA,
+                now.toString(),
+                trigger,
+                new InspectionReport.Settings(
+                        InspectorConfig.deep(),
+                        attribution,
+                        InspectorConfig.intervalTicks(),
+                        InspectorConfig.sampleRate()),
+                captures.stream()
+                        .map(capture -> LevelAnalysis.analyse(
+                                capture.dimension(),
+                                capture.gameTime(),
+                                capture.tickets(),
+                                capture.chunks(),
+                                capture.maxLevel(),
+                                attribution,
+                                origins))
+                        .toList());
+    }
+}
@@ -0,0 +1,51 @@
+package dev.chunkinspector.analysis;
+
+import java.util.Comparator;
+import java.util.List;
+import lombok.experimental.UtilityClass;
+
+/**
+ * Answers the one question an administrator actually asks: "why is <em>this</em> chunk loaded?".
+ *
+ * <p>A single chunk needs no propagation pass — a ticket at distance {@code d} contributes level
+ * {@code ticketLevel + d} — so this is a linear scan over the tickets and cheap enough to run from a
+ * command at any time, even on a server with deep mode off.
+ */
+@UtilityClass
+public class ChunkExplainer {
+
+    private final int MAX_CONTRIBUTORS = 10;
+
+    /**
+     * @param ticket      the contributing ticket
+     * @param distance    Chebyshev distance from the ticket to the chunk in question
+     * @param contributed the level this ticket imposes on that chunk
+     */
+    public record Contribution(TicketRecord ticket, int distance, int contributed) {}
+
+    /**
+     * @param chunk        the chunk that was asked about
+     * @param level        its resulting level, or {@code Integer.MAX_VALUE} if nothing reaches it
+     * @param contributors every ticket that reaches it, strongest first
+     */
+    public record Explanation(long chunk, int level, List<Contribution> contributors) {
+
+        public boolean loaded(int maxLevel) {
+            return level <= maxLevel;
+        }
+    }
+
+    public Explanation explain(List<TicketRecord> tickets, long chunk, int maxLevel) {
+        List<Contribution> contributors = tickets.stream()
+                .map(ticket -> {
+                    int distance = ChunkKey.distance(ticket.chunk(), chunk);
+                    return new Contribution(ticket, distance, Math.max(0, ticket.level()) + distance);
+                })
+                .filter(c -> c.contributed() <= maxLevel)
+                .sorted(Comparator.comparingInt(Contribution::contributed))
+                .limit(MAX_CONTRIBUTORS)
+                .toList();
+        int level = contributors.isEmpty() ? Integer.MAX_VALUE : contributors.getFirst().contributed();
+        return new Explanation(chunk, level, contributors);
+    }
+}
@@ -0,0 +1,38 @@
+package dev.chunkinspector.analysis;
+
+import lombok.experimental.UtilityClass;
+
+/**
+ * Packing identical to {@code net.minecraft.world.level.ChunkPos}, restated here so that the
+ * analysis layer stays free of Minecraft types and can be unit tested on its own.
+ */
+@UtilityClass
+public class ChunkKey {
+
+    public long of(int x, int z) {
+        return (long) x & 0xFFFF_FFFFL | ((long) z & 0xFFFF_FFFFL) << 32;
+    }
+
+    public int x(long key) {
+        return (int) (key & 0xFFFF_FFFFL);
+    }
+
+    public int z(long key) {
+        return (int) (key >>> 32 & 0xFFFF_FFFFL);
+    }
+
+    /** Chebyshev distance, which is how chunk ticket levels propagate. */
+    public int distance(long a, long b) {
+        return Math.max(Math.abs(x(a) - x(b)), Math.abs(z(a) - z(b)));
+    }
+
+    /** {@code "12, -34"} — the form players type into {@code /tp}-adjacent commands. */
+    public String format(long key) {
+        return x(key) + ", " + z(key);
+    }
+
+    /** Block coordinates of the chunk centre, at a height that is safe to teleport to. */
+    public String formatBlock(long key) {
+        return (x(key) << 4) + 8 + " ~ " + ((z(key) << 4) + 8);
+    }
+}
@@ -0,0 +1,23 @@
+package dev.chunkinspector.analysis;
+
+/**
+ * One chunk the server is currently holding.
+ *
+ * @param chunk      packed chunk position
+ * @param level      the chunk's effective ticket level
+ * @param fullStatus {@code INACCESSIBLE}, {@code FULL}, {@code BLOCK_TICKING} or {@code ENTITY_TICKING}
+ */
+public record ChunkRecord(long chunk, int level, String fullStatus) {
+
+    public boolean loaded(int maxLevel) {
+        return level <= maxLevel;
+    }
+
+    public boolean entityTicking() {
+        return "ENTITY_TICKING".equals(fullStatus);
+    }
+
+    public boolean blockTicking() {
+        return entityTicking() || "BLOCK_TICKING".equals(fullStatus);
+    }
+}
@@ -0,0 +1,377 @@
+package dev.chunkinspector.analysis;
+
+import dev.chunkinspector.origin.Origin;
+import dev.chunkinspector.report.InspectionReport.Hotspot;
+import dev.chunkinspector.report.InspectionReport.LevelReport;
+import dev.chunkinspector.report.InspectionReport.OriginEntry;
+import dev.chunkinspector.report.InspectionReport.TicketEntry;
+import dev.chunkinspector.report.InspectionReport.Totals;
+import dev.chunkinspector.report.InspectionReport.TypeBreakdown;
+import it.unimi.dsi.fastutil.longs.LongArrayList;
+import it.unimi.dsi.fastutil.longs.LongIterator;
+import it.unimi.dsi.fastutil.longs.LongOpenHashSet;
+import it.unimi.dsi.fastutil.objects.Object2IntMap;
+import it.unimi.dsi.fastutil.objects.Object2IntOpenHashMap;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Comparator;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.SequencedMap;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+/**
+ * Turns a flat capture of one dimension's tickets and chunks into the report a human reads.
+ *
+ * <p>Deliberately free of Minecraft types: everything it needs arrives as {@link TicketRecord} and
+ * {@link ChunkRecord}, which keeps the interesting logic — attribution, clustering, suspicion
+ * ranking — directly unit testable.
+ */
+public final class LevelAnalysis {
+
+    /** Ticket types that are part of normal operation and never worth flagging. */
+    private static final List<String> BENIGN_TYPES = List.of("player", "start", "unknown", "post_teleport", "portal");
+
+    private static final int MAX_SUSPECTS = 25;
+    private static final int MAX_HOTSPOTS = 15;
+    private static final int MAX_OWNERS_PER_TYPE = 8;
+    private static final int MAX_CAUSES_PER_HOTSPOT = 5;
+    private static final int MAX_UNATTRIBUTED = 64;
+    private static final int MIN_HOTSPOT_CHUNKS = 4;
+    private static final int NO_INFLUENCE = -1;
+
+    private final String dimension;
+    private final long gameTime;
+    private final List<TicketRecord> tickets;
+    private final List<ChunkRecord> chunks;
+    private final int maxLevel;
+    private final Collection<Origin> origins;
+
+    private LevelAnalysis(
+            String dimension, long gameTime, List<TicketRecord> tickets, List<ChunkRecord> chunks, int maxLevel, Collection<Origin> origins) {
+        this.dimension = dimension;
+        this.gameTime = gameTime;
+        this.tickets = tickets;
+        this.chunks = chunks;
+        this.maxLevel = maxLevel;
+        this.origins = origins;
+    }
+
+    /**
+     * @param maxLevel       {@code ChunkLevel.MAX_LEVEL}; chunks above this are not loaded
+     * @param attribution    run the propagation pass that maps every loaded chunk to a ticket
+     * @param origins        deep-mode call sites, empty when deep mode is off
+     */
+    public static LevelReport analyse(
+            String dimension,
+            long gameTime,
+            List<TicketRecord> tickets,
+            List<ChunkRecord> chunks,
+            int maxLevel,
+            boolean attribution,
+            Collection<Origin> origins) {
+        return new LevelAnalysis(dimension, gameTime, tickets, chunks, maxLevel, origins).build(attribution);
+    }
+
+    private LevelReport build(boolean attribution) {
+        Attribution result = attribution ? attribute() : Attribution.none(tickets.size());
+        return new LevelReport(
+                dimension,
+                gameTime,
+                totals(),
+                breakdowns(result),
+                suspects(result),
+                hotspots(result),
+                originEntries(),
+                result.unattributed(),
+                attribution);
+    }
+
+    // ------------------------------------------------------------------ totals
+
+    private Totals totals() {
+        var ticketedChunks = new LongOpenHashSet();
+        int permanent = 0;
+        int forced = 0;
+        for (TicketRecord ticket : tickets) {
+            ticketedChunks.add(ticket.chunk());
+            if (!ticket.expiring()) {
+                permanent++;
+            }
+            if ("forced".equals(ticket.type())) {
+                forced++;
+            }
+        }
+        int loaded = 0;
+        int blockTicking = 0;
+        int entityTicking = 0;
+        for (ChunkRecord chunk : chunks) {
+            if (!"INACCESSIBLE".equals(chunk.fullStatus())) {
+                loaded++;
+            }
+            if (chunk.blockTicking()) {
+                blockTicking++;
+            }
+            if (chunk.entityTicking()) {
+                entityTicking++;
+            }
+        }
+        return new Totals(chunks.size(), loaded, blockTicking, entityTicking, tickets.size(), ticketedChunks.size(), permanent, forced);
+    }
+
+    // ------------------------------------------------------------- attribution
+
+    /**
+     * @param influencePerTicket loaded chunks won by each ticket, indexed like {@link #tickets}
+     * @param winnerPerChunk     loaded chunk -> ticket index, only populated when attribution ran
+     * @param unattributed       loaded chunks no ticket explains, formatted for the report
+     * @param ran                whether the propagation pass was performed at all
+     */
+    private record Attribution(int[] influencePerTicket, LevelPropagation.Result winnerPerChunk, List<String> unattributed, boolean ran) {
+
+        static Attribution none(int ticketCount) {
+            return new Attribution(new int[ticketCount], null, List.of(), false);
+        }
+
+        int influenceOf(int ticketIndex) {
+            return ran ? influencePerTicket[ticketIndex] : NO_INFLUENCE;
+        }
+    }
+
+    private Attribution attribute() {
+        List<LevelPropagation.Source> sources = tickets.stream()
+                .map(t -> new LevelPropagation.Source(t.chunk(), t.level()))
+                .toList();
+        LevelPropagation.Result propagated = LevelPropagation.solve(sources, maxLevel);
+
+        int[] influence = new int[tickets.size()];
+        var unattributed = new ArrayList<String>();
+        for (ChunkRecord chunk : chunks) {
+            if ("INACCESSIBLE".equals(chunk.fullStatus())) {
+                continue;
+            }
+            int winner = propagated.winnerOf(chunk.chunk());
+            if (winner == LevelPropagation.Result.NONE) {
+                if (unattributed.size() < MAX_UNATTRIBUTED) {
+                    unattributed.add(ChunkKey.format(chunk.chunk()));
+                }
+                continue;
+            }
+            influence[winner]++;
+        }
+        return new Attribution(influence, propagated, List.copyOf(unattributed), true);
+    }
+
+    // ------------------------------------------------------------- by type
+
+    private List<TypeBreakdown> breakdowns(Attribution attribution) {
+        SequencedMap<String, TypeAccumulator> byType = new LinkedHashMap<>();
+        for (int i = 0; i < tickets.size(); i++) {
+            TicketRecord ticket = tickets.get(i);
+            byType.computeIfAbsent(ticket.type(), TypeAccumulator::new).accept(ticket, attribution.influenceOf(i));
+        }
+        return byType.values().stream()
+                .map(TypeAccumulator::toBreakdown)
+                .sorted(Comparator.comparingInt(TypeBreakdown::influence).reversed()
+                        .thenComparing(Comparator.comparingInt(TypeBreakdown::chunks).reversed()))
+                .toList();
+    }
+
+    /** Mutable fold state for one ticket type; kept private so the report stays immutable. */
+    private static final class TypeAccumulator {
+        private final String type;
+        private final LongOpenHashSet chunks = new LongOpenHashSet();
+        private final SequencedMap<String, Boolean> owners = new LinkedHashMap<>();
+        private int tickets;
+        private int minLevel = Integer.MAX_VALUE;
+        private int influence = NO_INFLUENCE;
+        private long oldest;
+        /** Only claim a type expires on its own when every one of its tickets does. */
+        private boolean expiring = true;
+
+        TypeAccumulator(String type) {
+            this.type = type;
+        }
+
+        void accept(TicketRecord ticket, int ticketInfluence) {
+            tickets++;
+            chunks.add(ticket.chunk());
+            minLevel = Math.min(minLevel, ticket.level());
+            oldest = Math.max(oldest, ticket.ageTicks());
+            expiring &= ticket.expiring();
+            if (owners.size() < MAX_OWNERS_PER_TYPE) {
+                owners.putIfAbsent(ticket.owner(), Boolean.TRUE);
+            }
+            if (ticketInfluence != NO_INFLUENCE) {
+                influence = Math.max(influence, 0) + ticketInfluence;
+            }
+        }
+
+        TypeBreakdown toBreakdown() {
+            return new TypeBreakdown(type, tickets, chunks.size(), minLevel, influence, oldest, expiring, List.copyOf(owners.keySet()));
+        }
+    }
+
+    // ------------------------------------------------------------- suspects
+
+    private List<TicketEntry> suspects(Attribution attribution) {
+        record Scored(TicketRecord ticket, int influence) {}
+        return IntStream.range(0, tickets.size())
+                .mapToObj(i -> new Scored(tickets.get(i), attribution.influenceOf(i)))
+                .filter(s -> suspicious(s.ticket()))
+                .sorted(Comparator.comparingInt(Scored::influence).reversed()
+                        .thenComparing(Comparator.comparingLong((Scored s) -> s.ticket().ageTicks()).reversed()))
+                .limit(MAX_SUSPECTS)
+                .map(s -> new TicketEntry(
+                        ChunkKey.format(s.ticket().chunk()),
+                        ChunkKey.formatBlock(s.ticket().chunk()),
+                        s.ticket().type(),
+                        s.ticket().level(),
+                        s.ticket().ageTicks(),
+                        s.ticket().owner(),
+                        s.ticket().originId(),
+                        describe(s.ticket(), s.influence())))
+                .toList();
+    }
+
+    /**
+     * A ticket is worth reporting when nothing will ever remove it for you: it has no timeout and
+     * it is not one of the types the game manages itself.
+     */
+    private static boolean suspicious(TicketRecord ticket) {
+        return !ticket.expiring() && !BENIGN_TYPES.contains(ticket.type());
+    }
+
+    private static String describe(TicketRecord ticket, int influence) {
+        var reason = new StringBuilder("permanent ticket, alive ").append(formatTicks(ticket.ageTicks()));
+        if (influence != NO_INFLUENCE) {
+            reason.append(", keeping ").append(influence).append(influence == 1 ? " chunk" : " chunks").append(" loaded");
+        }
+        if (ticket.level() <= 31) {
+            reason.append("; entity-ticking, so mobs and machines run here");
+        }
+        return reason.toString();
+    }
+
+    static String formatTicks(long ticks) {
+        long seconds = ticks / 20;
+        if (seconds < 60) {
+            return seconds + "s";
+        }
+        if (seconds < 3600) {
+            return seconds / 60 + "m" + seconds % 60 + "s";
+        }
+        return seconds / 3600 + "h" + seconds % 3600 / 60 + "m";
+    }
+
+    // ------------------------------------------------------------- hotspots
+
+    /**
+     * Finds 8-connected clusters of loaded chunks. A leaking chunk loader shows up as one blob,
+     * which tells an administrator where to fly to far more directly than a list of coordinates.
+     */
+    private List<Hotspot> hotspots(Attribution attribution) {
+        var loaded = new LongOpenHashSet();
+        for (ChunkRecord chunk : chunks) {
+            if (!"INACCESSIBLE".equals(chunk.fullStatus())) {
+                loaded.add(chunk.chunk());
+            }
+        }
+
+        var clusters = new ArrayList<Hotspot>();
+        var visited = new LongOpenHashSet(loaded.size());
+        var stack = new LongArrayList();
+        for (LongIterator seeds = loaded.iterator(); seeds.hasNext(); ) {
+            long seed = seeds.nextLong();
+            if (!visited.add(seed)) {
+                continue;
+            }
+            stack.add(seed);
+            var members = new LongArrayList();
+            while (!stack.isEmpty()) {
+                long chunk = stack.removeLong(stack.size() - 1);
+                members.add(chunk);
+                int x = ChunkKey.x(chunk);
+                int z = ChunkKey.z(chunk);
+                for (int dx = -1; dx <= 1; dx++) {
+                    for (int dz = -1; dz <= 1; dz++) {
+                        long neighbour = ChunkKey.of(x + dx, z + dz);
+                        if ((dx != 0 || dz != 0) && loaded.contains(neighbour) && visited.add(neighbour)) {
+                            stack.add(neighbour);
+                        }
+                    }
+                }
+            }
+            if (members.size() >= MIN_HOTSPOT_CHUNKS) {
+                clusters.add(toHotspot(members, attribution));
+            }
+        }
+        return clusters.stream()
+                .sorted(Comparator.comparingInt(Hotspot::chunks).reversed())
+                .limit(MAX_HOTSPOTS)
+                .toList();
+    }
+
+    private Hotspot toHotspot(LongArrayList members, Attribution attribution) {
+        int minX = Integer.MAX_VALUE;
+        int minZ = Integer.MAX_VALUE;
+        int maxX = Integer.MIN_VALUE;
+        int maxZ = Integer.MIN_VALUE;
+        Object2IntMap<String> causes = new Object2IntOpenHashMap<>();
+        var memberSet = attribution.ran() ? null : new LongOpenHashSet(members);
+
+        for (int i = 0; i < members.size(); i++) {
+            long chunk = members.getLong(i);
+            minX = Math.min(minX, ChunkKey.x(chunk));
+            maxX = Math.max(maxX, ChunkKey.x(chunk));
+            minZ = Math.min(minZ, ChunkKey.z(chunk));
+            maxZ = Math.max(maxZ, ChunkKey.z(chunk));
+            if (attribution.ran()) {
+                int winner = attribution.winnerPerChunk().winnerOf(chunk);
+                if (winner != LevelPropagation.Result.NONE) {
+                    causes.mergeInt(tickets.get(winner).type(), 1, Integer::sum);
+                }
+            }
+        }
+        if (memberSet != null) {
+            // Without the propagation pass, fall back to whichever tickets sit inside the cluster.
+            for (TicketRecord ticket : tickets) {
+                if (memberSet.contains(ticket.chunk())) {
+                    causes.mergeInt(ticket.type(), 1, Integer::sum);
+                }
+            }
+        }
+
+        long centre = ChunkKey.of((minX + maxX) / 2, (minZ + maxZ) / 2);
+        List<String> topCauses = causes.object2IntEntrySet().stream()
+                .sorted(Comparator.comparingInt(Object2IntMap.Entry<String>::getIntValue).reversed())
+                .limit(MAX_CAUSES_PER_HOTSPOT)
+                .map(e -> e.getKey() + " x" + e.getIntValue())
+                .toList();
+        return new Hotspot(
+                members.size(),
+                minX + ", " + minZ + " .. " + maxX + ", " + maxZ,
+                ChunkKey.format(centre),
+                ChunkKey.formatBlock(centre),
+                topCauses);
+    }
+
+    // ------------------------------------------------------------- origins
+
+    private List<OriginEntry> originEntries() {
+        if (origins.isEmpty()) {
+            return List.of();
+        }
+        Map<String, Long> live = tickets.stream()
+                .filter(t -> t.originId() != null)
+                .collect(Collectors.groupingBy(TicketRecord::originId, Collectors.counting()));
+        return origins.stream()
+                .filter(o -> live.containsKey(o.id()))
+                .map(o -> new OriginEntry(o.id(), o.mod(), o.adds(), live.get(o.id()).intValue(), o.types(), o.stack()))
+                .sorted(Comparator.comparingInt(OriginEntry::live).reversed()
+                        .thenComparing(Comparator.comparingLong(OriginEntry::adds).reversed()))
+                .toList();
+    }
+}
@@ -0,0 +1,124 @@
+package dev.chunkinspector.analysis;
+
+import it.unimi.dsi.fastutil.longs.Long2IntMap;
+import it.unimi.dsi.fastutil.longs.Long2IntOpenHashMap;
+import it.unimi.dsi.fastutil.longs.LongArrayList;
+import java.util.List;
+
+/**
+ * Replays vanilla's chunk-ticket level propagation while remembering <em>which</em> ticket won each
+ * chunk. That is the whole point of the mod: the game knows a chunk is loaded at level 33, but it
+ * throws away the reason, and the reason is what an administrator needs.
+ *
+ * <p>The rules are taken verbatim from {@code ChunkTracker}: a ticket at chunk {@code C} with level
+ * {@code L} gives every chunk at Chebyshev distance {@code d} a level of {@code L + d}, the lowest
+ * level wins, and propagation stops once the level exceeds {@code ChunkLevel.MAX_LEVEL}.
+ *
+ * <p>Implemented as Dial's algorithm — a bucket queue indexed by level — so the whole pass is
+ * {@code O(loaded chunks)} rather than {@code O(sum of ticket areas)}. That is what makes the
+ * "expensive" analysis affordable enough to also offer it on a timer.
+ */
+public final class LevelPropagation {
+
+    private LevelPropagation() {}
+
+    /**
+     * A ticket reduced to what propagation cares about.
+     *
+     * @param chunk packed chunk position, see {@link ChunkKey}
+     * @param level the ticket level; lower loads more
+     */
+    public record Source(long chunk, int level) {}
+
+    /**
+     * @param level  chunk -> resulting ticket level, absent when the chunk is not reachable
+     * @param winner chunk -> index into the source list that produced that level
+     */
+    public record Result(Long2IntMap level, Long2IntMap winner) {
+
+        public static final int NONE = -1;
+
+        /** The propagated level, or {@code Integer.MAX_VALUE} when nothing reaches this chunk. */
+        public int levelOf(long chunk) {
+            return level.containsKey(chunk) ? level.get(chunk) : Integer.MAX_VALUE;
+        }
+
+        /** Index of the source responsible for {@code chunk}, or {@link #NONE}. */
+        public int winnerOf(long chunk) {
+            return winner.containsKey(chunk) ? winner.get(chunk) : NONE;
+        }
+
+        public int reachedChunks() {
+            return level.size();
+        }
+    }
+
+    /**
+     * @param sources  every live ticket, in a stable order; indices are reported back in the result
+     * @param maxLevel the highest level still considered loaded, i.e. {@code ChunkLevel.MAX_LEVEL}
+     */
+    public static Result solve(List<Source> sources, int maxLevel) {
+        var level = new Long2IntOpenHashMap();
+        var winner = new Long2IntOpenHashMap();
+        level.defaultReturnValue(Integer.MAX_VALUE);
+        winner.defaultReturnValue(Result.NONE);
+
+        // One bucket per level. Chunks only ever move to a strictly higher bucket, so a single
+        // forward sweep suffices and no priority queue is needed.
+        var buckets = new LongArrayList[maxLevel + 1];
+
+        for (int i = 0; i < sources.size(); i++) {
+            Source source = sources.get(i);
+            relax(level, winner, buckets, maxLevel, source.chunk(), source.level(), i);
+        }
+
+        for (int current = 0; current <= maxLevel; current++) {
+            LongArrayList bucket = buckets[current];
+            if (bucket == null) {
+                continue;
+            }
+            for (int i = 0; i < bucket.size(); i++) {
+                long chunk = bucket.getLong(i);
+                if (level.get(chunk) != current) {
+                    continue; // superseded by a stronger ticket after being queued
+                }
+                int owner = winner.get(chunk);
+                int x = ChunkKey.x(chunk);
+                int z = ChunkKey.z(chunk);
+                for (int dx = -1; dx <= 1; dx++) {
+                    for (int dz = -1; dz <= 1; dz++) {
+                        if (dx == 0 && dz == 0) {
+                            continue;
+                        }
+                        relax(level, winner, buckets, maxLevel, ChunkKey.of(x + dx, z + dz), current + 1, owner);
+                    }
+                }
+            }
+            buckets[current] = null; // release as we go; a big world holds a lot of chunks
+        }
+
+        return new Result(level, winner);
+    }
+
+    private static void relax(
+            Long2IntOpenHashMap level,
+            Long2IntOpenHashMap winner,
+            LongArrayList[] buckets,
+            int maxLevel,
+            long chunk,
+            int newLevel,
+            int owner) {
+        // Vanilla clamps levels into [0, maxLevel]; a region ticket with a radius above 33 would
+        // otherwise produce a negative level and a negative bucket index.
+        int clamped = Math.max(0, newLevel);
+        if (clamped > maxLevel || clamped >= level.get(chunk)) {
+            return;
+        }
+        level.put(chunk, clamped);
+        winner.put(chunk, owner);
+        if (buckets[clamped] == null) {
+            buckets[clamped] = new LongArrayList();
+        }
+        buckets[clamped].add(chunk);
+    }
+}
@@ -0,0 +1,33 @@
+package dev.chunkinspector.analysis;
+
+/**
+ * One live chunk ticket, flattened out of Minecraft's data structures so the analysis can run
+ * without touching the server.
+ *
+ * @param chunk    packed chunk position
+ * @param type     the {@code TicketType} name, e.g. {@code player} or {@code neoforge:block}
+ * @param level    ticket level; lower loads more, 33 is a fully loaded chunk
+ * @param timeout  ticks after which the game drops the ticket on its own, {@code 0} means never
+ * @param ageTicks how long the ticket has been alive
+ * @param owner    rendered ticket key — for many types this identifies the culprit outright
+ * @param originId deep-mode call site id; {@code null} outside deep mode
+ */
+public record TicketRecord(
+        long chunk,
+        String type,
+        int level,
+        long timeout,
+        long ageTicks,
+        String owner,
+        String originId) {
+
+    /** Tickets that expire on their own cannot be the cause of a permanently loaded chunk. */
+    public boolean expiring() {
+        return timeout > 0;
+    }
+
+    /** Player view/simulation tickets are expected and are never reported as suspects. */
+    public boolean fromPlayer() {
+        return "player".equals(type);
+    }
+}
@@ -0,0 +1,64 @@
+package dev.chunkinspector.game;
+
+import dev.chunkinspector.analysis.ChunkRecord;
+import dev.chunkinspector.analysis.TicketRecord;
+import dev.chunkinspector.origin.TicketOrigin;
+import java.util.ArrayList;
+import java.util.List;
+import net.minecraft.server.level.ChunkHolder;
+import net.minecraft.server.level.ChunkLevel;
+import net.minecraft.server.level.ChunkMap;
+import net.minecraft.server.level.DistanceManager;
+import net.minecraft.server.level.ServerLevel;
+import net.minecraft.server.level.Ticket;
+
+/**
+ * A plain copy of one dimension's ticket and chunk state.
+ *
+ * <p>Taking a copy is the whole trick behind "low cost": the main thread only walks two collections
+ * and allocates records — no analysis, no I/O, no locks — and everything expensive then happens on
+ * a background thread against this immutable snapshot.
+ *
+ * @param dimension the dimension id, e.g. {@code minecraft:overworld}
+ * @param maxLevel  {@code ChunkLevel.MAX_LEVEL}, captured so the analysis needs no game classes
+ */
+public record LevelCapture(
+        String dimension,
+        long gameTime,
+        List<TicketRecord> tickets,
+        List<ChunkRecord> chunks,
+        int maxLevel) {
+
+    /** Must run on the server thread. */
+    public static LevelCapture of(ServerLevel level) {
+        ChunkMap chunkMap = level.getChunkSource().chunkMap;
+        DistanceManager distances = chunkMap.getDistanceManager();
+        long now = distances.ticketTickCounter;
+
+        var tickets = new ArrayList<TicketRecord>();
+        distances.tickets.long2ObjectEntrySet().forEach(entry -> {
+            for (Ticket<?> ticket : entry.getValue()) {
+                tickets.add(new TicketRecord(
+                        entry.getLongKey(),
+                        String.valueOf(ticket.getType()),
+                        ticket.getTicketLevel(),
+                        ticket.getType().timeout(),
+                        Math.max(0, now - ticket.createdTick),
+                        TicketKeys.render(ticket.key),
+                        TicketOrigin.of(ticket)));
+            }
+        });
+
+        var chunks = new ArrayList<ChunkRecord>();
+        for (ChunkHolder holder : chunkMap.getChunks()) {
+            chunks.add(new ChunkRecord(holder.getPos().toLong(), holder.getTicketLevel(), holder.getFullStatus().name()));
+        }
+
+        return new LevelCapture(
+                level.dimension().location().toString(),
+                level.getGameTime(),
+                List.copyOf(tickets),
+                List.copyOf(chunks),
+                ChunkLevel.MAX_LEVEL);
+    }
+}
@@ -0,0 +1,31 @@
+package dev.chunkinspector.game;
+
+import dev.chunkinspector.mixin.TicketOwnerAccessor;
+import lombok.experimental.UtilityClass;
+import net.minecraft.core.BlockPos;
+import net.minecraft.util.Unit;
+import net.minecraft.world.entity.Entity;
+import net.minecraft.world.level.ChunkPos;
+
+/**
+ * Renders a ticket's key into something a human can act on.
+ *
+ * <p>This matters more than it looks: for a NeoForge forced chunk the key already contains the id of
+ * the mod that requested it, so a leaking chunk loader is identified without any instrumentation at
+ * all. Only when the key is opaque does deep mode become necessary.
+ */
+@UtilityClass
+public class TicketKeys {
+
+    public String render(Object key) {
+        return switch (key) {
+            case null -> "-";
+            case ChunkPos pos -> "chunk " + pos.x + ", " + pos.z;
+            case BlockPos pos -> "block " + pos.toShortString();
+            case Unit ignored -> "-";
+            case TicketOwnerAccessor owner -> owner.chunkinspector$controller() + " -> " + render(owner.chunkinspector$owner());
+            case Entity entity -> entity.getType().toShortString() + " " + entity.getStringUUID();
+            default -> String.valueOf(key);
+        };
+    }
+}
@@ -0,0 +1,35 @@
+package dev.chunkinspector.mixin;
+
+import dev.chunkinspector.ChunkInspector;
+import dev.chunkinspector.origin.TicketOrigin;
+import net.minecraft.server.level.DistanceManager;
+import net.minecraft.server.level.Ticket;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+/**
+ * The single choke point for chunk tickets: {@code addRegionTicket}, {@code addTicket},
+ * {@code updateChunkForced} and the player ticket tracker all funnel through this method.
+ *
+ * <p>Stamping the <em>incoming</em> ticket at HEAD is deliberate. If an equal ticket already exists,
+ * {@code SortedArraySet.addOrGet} keeps the old one and discards ours, so the stamp that survives is
+ * always the one from the call that genuinely created the ticket — which is the call an
+ * administrator is looking for.
+ *
+ * <p>Only applied when deep mode is enabled — see {@link InspectorMixinPlugin}.
+ */
+@Mixin(DistanceManager.class)
+public abstract class DistanceManagerMixin {
+
+    @Inject(method = "addTicket(JLnet/minecraft/server/level/Ticket;)V", at = @At("HEAD"))
+    private void chunkinspector$recordOrigin(long chunk, Ticket<?> ticket, CallbackInfo ci) {
+        // Ticket is final, so javac rejects `instanceof TicketOrigin`; TicketMixin has already made
+        // every instance implement it by the time this runs.
+        TicketOrigin holder = (TicketOrigin) (Object) ticket;
+        if (holder.chunkinspector$origin() == null) {
+            holder.chunkinspector$origin(ChunkInspector.origins().record(String.valueOf(ticket.getType())));
+        }
+    }
+}
@@ -0,0 +1,60 @@
+package dev.chunkinspector.mixin;
+
+import dev.chunkinspector.InspectorConfig;
+import java.util.List;
+import java.util.Set;
+import lombok.extern.slf4j.Slf4j;
+import org.objectweb.asm.tree.ClassNode;
+import org.spongepowered.asm.mixin.extensibility.IMixinConfigPlugin;
+import org.spongepowered.asm.mixin.extensibility.IMixinInfo;
+
+/**
+ * Keeps the "cheap by default" promise honest.
+ *
+ * <p>The census that runs on every server only reads fields that an access transformer has already
+ * widened, so it costs nothing until a snapshot is taken. The attribution hooks are a different
+ * matter — they run inside {@code addTicket}. Rather than leave a disabled branch in a hot method,
+ * this plugin declines to apply those Mixins at all unless {@code -Dchunkinspector.deep=true} was
+ * passed, so with deep mode off Minecraft's bytecode is untouched.
+ */
+@Slf4j
+public class InspectorMixinPlugin implements IMixinConfigPlugin {
+
+    /** Mixins that instrument the game rather than merely read it. */
+    private static final Set<String> DEEP_ONLY = Set.of(
+            "dev.chunkinspector.mixin.DistanceManagerMixin",
+            "dev.chunkinspector.mixin.TicketMixin");
+
+    private boolean deep;
+
+    @Override
+    public void onLoad(String mixinPackage) {
+        this.deep = InspectorConfig.deep();
+        log.info("Chunk Inspector: deep mode {}, {} Minecraft's classes",
+                deep ? "on" : "off", deep ? "instrumenting" : "leaving untouched");
+    }
+
+    @Override
+    public boolean shouldApplyMixin(String targetClassName, String mixinClassName) {
+        return deep || !DEEP_ONLY.contains(mixinClassName);
+    }
+
+    @Override
+    public String getRefMapperConfig() {
+        return null;
+    }
+
+    @Override
+    public void acceptTargets(Set<String> myTargets, Set<String> otherTargets) {}
+
+    @Override
+    public List<String> getMixins() {
+        return null;
+    }
+
+    @Override
+    public void preApply(String targetClassName, ClassNode targetClass, String mixinClassName, IMixinInfo mixinInfo) {}
+
+    @Override
+    public void postApply(String targetClassName, ClassNode targetClass, String mixinClassName, IMixinInfo mixinInfo) {}
+}
@@ -0,0 +1,28 @@
+package dev.chunkinspector.mixin;
+
+import dev.chunkinspector.origin.TicketOrigin;
+import net.minecraft.server.level.Ticket;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Unique;
+
+/**
+ * Adds a single reference field to every ticket so that deep mode can attribute it back to a call
+ * site without an external map. Only applied when deep mode is enabled — see
+ * {@link InspectorMixinPlugin}.
+ */
+@Mixin(Ticket.class)
+public abstract class TicketMixin implements TicketOrigin {
+
+    @Unique
+    private String chunkinspector$originId;
+
+    @Override
+    public String chunkinspector$origin() {
+        return this.chunkinspector$originId;
+    }
+
+    @Override
+    public void chunkinspector$origin(String id) {
+        this.chunkinspector$originId = id;
+    }
+}
@@ -0,0 +1,22 @@
+package dev.chunkinspector.mixin;
+
+import net.minecraft.resources.ResourceLocation;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.gen.Accessor;
+
+/**
+ * NeoForge's forced-chunk tickets carry a {@code TicketOwner} key that already names the mod that
+ * asked for the chunk — the single most useful piece of attribution available, and free. The class
+ * is package private with no getters, so this accessor exposes it.
+ *
+ * <p>Purely an accessor: no game code is modified, so this one is applied even outside deep mode.
+ */
+@Mixin(targets = "net.neoforged.neoforge.common.world.chunk.ForcedChunkManager$TicketOwner", remap = false)
+public interface TicketOwnerAccessor {
+
+    @Accessor("id")
+    ResourceLocation chunkinspector$controller();
+
+    @Accessor("owner")
+    Comparable<?> chunkinspector$owner();
+}
@@ -0,0 +1,54 @@
+package dev.chunkinspector.origin;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import lombok.experimental.UtilityClass;
+
+/**
+ * Best-effort "which mod owns this class". NeoForge loads every mod as a named JPMS module, so the
+ * module name is both the cheapest and the most accurate answer available; the remaining branches
+ * only exist for classes that arrive on the classpath some other way.
+ */
+@UtilityClass
+public class ModAttribution {
+
+    private final Map<String, String> CACHE = new ConcurrentHashMap<>();
+
+    private final String UNKNOWN = "unknown";
+
+    public String of(Class<?> type) {
+        if (type == null) {
+            return UNKNOWN;
+        }
+        return CACHE.computeIfAbsent(type.getName(), ignored -> resolve(type));
+    }
+
+    private String resolve(Class<?> type) {
+        String module = type.getModule() == null ? null : type.getModule().getName();
+        if (module != null && !module.isBlank() && !module.startsWith("java.") && !module.startsWith("jdk.")) {
+            return module;
+        }
+        return fromCodeSource(type);
+    }
+
+    /** Falls back to the jar file name, which is still enough for a human to identify the mod. */
+    private String fromCodeSource(Class<?> type) {
+        try {
+            var domain = type.getProtectionDomain();
+            var source = domain == null ? null : domain.getCodeSource();
+            if (source == null || source.getLocation() == null) {
+                return UNKNOWN;
+            }
+            // Trailing slash included: mods can be loaded from an exploded directory, where the last
+            // path segment is still the most identifiable thing available.
+            String path = source.getLocation().getPath();
+            while (path.endsWith("/")) {
+                path = path.substring(0, path.length() - 1);
+            }
+            String file = path.substring(path.lastIndexOf('/') + 1);
+            return file.isBlank() ? UNKNOWN : file;
+        } catch (RuntimeException e) {
+            return UNKNOWN;
+        }
+    }
+}
@@ -0,0 +1,14 @@
+package dev.chunkinspector.origin;
+
+import java.util.List;
+
+/**
+ * An immutable snapshot of one call site that creates chunk tickets.
+ *
+ * @param id    short stable hash of {@link #stack()}, stamped onto every ticket created here
+ * @param mod   best-effort owning mod id
+ * @param adds  how many tickets this site has created since the server started
+ * @param types the ticket types seen from this site
+ * @param stack the trimmed stack trace, outermost caller last
+ */
+public record Origin(String id, String mod, long adds, List<String> types, List<String> stack) {}
@@ -0,0 +1,125 @@
+package dev.chunkinspector.origin;
+
+import dev.chunkinspector.InspectorConfig;
+import java.lang.StackWalker.StackFrame;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.atomic.LongAdder;
+
+/**
+ * Deep-mode only: remembers the distinct call sites that create chunk tickets.
+ *
+ * <p>Each site gets a short id which is stamped straight onto the {@code Ticket} object, so there is
+ * no side table keyed by ticket and therefore nothing to leak when the game drops a ticket without
+ * telling us — which it does, in {@code purgeStaleTickets} and {@code removeTicketsOnClosing}.
+ */
+public final class OriginRegistry {
+
+    /**
+     * Frames belonging to these prefixes are the plumbing between the caller and the ticket system;
+     * dropping them puts the responsible mod's own frame at the top of every trace.
+     */
+    private static final List<String> PLUMBING = List.of(
+            "dev.chunkinspector.",
+            "net.minecraft.server.level.DistanceManager",
+            "net.minecraft.server.level.ChunkMap",
+            "net.minecraft.server.level.ServerChunkCache",
+            "net.minecraft.server.level.TicketType",
+            "java.",
+            "jdk.");
+
+    private static final StackWalker WALKER =
+            StackWalker.getInstance(Set.of(StackWalker.Option.RETAIN_CLASS_REFERENCE), 64);
+
+    private final Map<String, Site> sites = new ConcurrentHashMap<>();
+    private final AtomicLong sampleCounter = new AtomicLong();
+    private final int stackDepth = InspectorConfig.stackDepth();
+    private final int maxOrigins = InspectorConfig.maxOrigins();
+    private final int sampleRate = InspectorConfig.sampleRate();
+
+    /**
+     * Captures the current call stack and returns the id to stamp on the ticket, or {@code null}
+     * when this add was not sampled, produced no usable frames, or the registry is already full.
+     */
+    public String record(String ticketType) {
+        if (sampleRate > 1 && sampleCounter.incrementAndGet() % sampleRate != 0) {
+            return null;
+        }
+        Captured captured = capture();
+        if (captured == null) {
+            return null;
+        }
+        String id = idOf(captured.stack());
+        Site site = sites.get(id);
+        if (site == null) {
+            if (sites.size() >= maxOrigins) {
+                return null;
+            }
+            site = sites.computeIfAbsent(id, ignored -> new Site(captured.mod(), captured.stack()));
+        }
+        site.adds.increment();
+        site.types.add(ticketType);
+        return id;
+    }
+
+    public List<Origin> snapshot() {
+        return sites.entrySet().stream()
+                .map(e -> new Origin(
+                        e.getKey(),
+                        e.getValue().mod,
+                        e.getValue().adds.sum(),
+                        List.copyOf(e.getValue().types),
+                        e.getValue().stack))
+                .toList();
+    }
+
+    public boolean isEmpty() {
+        return sites.isEmpty();
+    }
+
+    private record Captured(String mod, List<String> stack) {}
+
+    /** One walk yields both the rendered trace and the owning mod of its topmost frame. */
+    private Captured capture() {
+        return WALKER.walk(frames -> {
+            List<StackFrame> kept = frames
+                    .filter(f -> PLUMBING.stream().noneMatch(prefix -> f.getClassName().startsWith(prefix)))
+                    .limit(stackDepth)
+                    .toList();
+            if (kept.isEmpty()) {
+                return null;
+            }
+            return new Captured(
+                    ModAttribution.of(kept.getFirst().getDeclaringClass()),
+                    kept.stream().map(f -> f.getClassName() + "." + f.getMethodName() + ":" + f.getLineNumber()).toList());
+        });
+    }
+
+    /** A 48-bit FNV-1a fold: short enough to read in a report, wide enough not to collide. */
+    static String idOf(List<String> stack) {
+        long hash = 0xcbf29ce484222325L;
+        for (String frame : stack) {
+            for (int i = 0; i < frame.length(); i++) {
+                hash = (hash ^ frame.charAt(i)) * 0x100000001b3L;
+            }
+            hash = (hash ^ '\n') * 0x100000001b3L;
+        }
+        return "%012x".formatted(hash & 0xFFFF_FFFF_FFFFL);
+    }
+
+    /** Mutable per-site counters; only ever read through {@link #snapshot()}. */
+    private static final class Site {
+        private final String mod;
+        private final List<String> stack;
+        private final LongAdder adds = new LongAdder();
+        private final Set<String> types = ConcurrentHashMap.newKeySet();
+
+        Site(String mod, List<String> stack) {
+            this.mod = mod;
+            this.stack = stack;
+        }
+    }
+}
@@ -0,0 +1,18 @@
+package dev.chunkinspector.origin;
+
+/**
+ * Implemented on {@code net.minecraft.server.level.Ticket} by the deep-mode Mixin so a ticket can
+ * carry the id of the call site that created it. Outside deep mode the Mixin is never applied and
+ * the cast simply fails, which the caller treats as "no attribution available".
+ */
+public interface TicketOrigin {
+
+    String chunkinspector$origin();
+
+    void chunkinspector$origin(String id);
+
+    /** Null-safe read for a ticket that may or may not have been instrumented. */
+    static String of(Object ticket) {
+        return ticket instanceof TicketOrigin holder ? holder.chunkinspector$origin() : null;
+    }
+}
@@ -0,0 +1,136 @@
+package dev.chunkinspector.report;
+
+import java.util.List;
+
+/**
+ * The serialised form of a single inspection. Everything below is a plain record tree so that Gson
+ * can write it without any custom adapters, and so that the unit tests can build one by hand.
+ *
+ * @param schema      bumped whenever the shape changes incompatibly
+ * @param generatedAt ISO-8601 wall-clock time the snapshot was taken
+ * @param trigger     what caused the snapshot ({@code periodic}, {@code command}, {@code shutdown})
+ * @param settings    the effective configuration, so a report is self-describing
+ * @param levels      one entry per loaded dimension
+ */
+public record InspectionReport(
+        int schema,
+        String generatedAt,
+        String trigger,
+        Settings settings,
+        List<LevelReport> levels) {
+
+    public static final int SCHEMA = 1;
+
+    /** Effective configuration at the time of the snapshot. */
+    public record Settings(boolean deep, boolean attribution, int intervalTicks, int sampleRate) {}
+
+    /**
+     * @param dimension        e.g. {@code minecraft:overworld}
+     * @param gameTime         the level's game time
+     * @param totals           headline counters
+     * @param byType           per-{@code TicketType} aggregation, descending by chunk count
+     * @param suspects         individual tickets that look like leaks, most suspicious first
+     * @param hotspots         contiguous clusters of loaded chunks, largest first
+     * @param origins          deep-mode call sites that created tickets, most tickets first
+     * @param unattributed     loaded chunks no ticket can explain (only when attribution ran)
+     * @param attributionRan   whether the expensive chunk-to-ticket pass was performed
+     */
+    public record LevelReport(
+            String dimension,
+            long gameTime,
+            Totals totals,
+            List<TypeBreakdown> byType,
+            List<TicketEntry> suspects,
+            List<Hotspot> hotspots,
+            List<OriginEntry> origins,
+            List<String> unattributed,
+            boolean attributionRan) {}
+
+    /**
+     * @param trackedChunks      every chunk the chunk map holds, including partially generated ones
+     * @param loadedChunks       chunks at {@code FULL} status or better, i.e. really in memory
+     * @param blockTicking       chunks receiving block ticks
+     * @param entityTicking      chunks receiving entity ticks — the expensive ones
+     * @param tickets            total live tickets
+     * @param ticketedChunks     chunks that carry at least one ticket
+     * @param permanentTickets   tickets with no timeout; only these can leak forever
+     * @param forcedChunks       vanilla {@code /forceload} tickets
+     */
+    public record Totals(
+            int trackedChunks,
+            int loadedChunks,
+            int blockTicking,
+            int entityTicking,
+            int tickets,
+            int ticketedChunks,
+            int permanentTickets,
+            int forcedChunks) {}
+
+    /**
+     * @param type            the {@code TicketType} name, e.g. {@code minecraft:player}
+     * @param tickets         how many live tickets of this type exist
+     * @param chunks          how many distinct chunks carry one
+     * @param minLevel        the strongest (numerically lowest) level seen — lower loads more
+     * @param influence       chunks kept loaded by this type, or {@code -1} if attribution was skipped
+     * @param oldestAgeTicks  age of the longest-lived ticket of this type
+     * @param expiring        whether this type has a timeout, i.e. it cannot leak forever
+     * @param owners          distinct ticket keys, truncated; useful for spotting a runaway mod
+     */
+    public record TypeBreakdown(
+            String type,
+            int tickets,
+            int chunks,
+            int minLevel,
+            int influence,
+            long oldestAgeTicks,
+            boolean expiring,
+            List<String> owners) {}
+
+    /**
+     * A single ticket that is worth a human look.
+     *
+     * @param chunk    {@code "x, z"} chunk coordinates
+     * @param blockPos the block coordinates of that chunk's corner, for {@code /tp}
+     * @param owner    rendered ticket key
+     * @param origin   deep-mode call site id, or {@code null}
+     * @param reason   why the ticket was flagged
+     */
+    public record TicketEntry(
+            String chunk,
+            String blockPos,
+            String type,
+            int level,
+            long ageTicks,
+            String owner,
+            String origin,
+            String reason) {}
+
+    /**
+     * A rectangular cluster of loaded chunks, which usually corresponds to one culprit.
+     *
+     * @param chunks   number of loaded chunks in the cluster
+     * @param bounds   {@code "minX, minZ .. maxX, maxZ"} in chunk coordinates
+     * @param center   representative chunk, {@code "x, z"}
+     * @param blockPos block coordinates of the center, for {@code /tp}
+     * @param causes   ticket types responsible, descending by contribution
+     */
+    public record Hotspot(int chunks, String bounds, String center, String blockPos, List<String> causes) {}
+
+    /**
+     * A deep-mode call site.
+     *
+     * @param id      short stable hash, cross-referenced from {@link TicketEntry#origin()}
+     * @param mod     best-effort mod id that owns the top non-Minecraft frame
+     * @param adds    how many tickets this site has added since the server started
+     * @param live    how many of those tickets are still alive
+     * @param types   ticket types created here
+     * @param stack   the trimmed stack trace
+     */
+    public record OriginEntry(
+            String id,
+            String mod,
+            long adds,
+            int live,
+            List<String> types,
+            List<String> stack) {}
+}
@@ -0,0 +1,105 @@
+package dev.chunkinspector.report;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import java.io.IOException;
+import java.io.Writer;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardCopyOption;
+import java.util.Comparator;
+import java.util.List;
+import java.util.stream.Stream;
+
+/**
+ * Persists reports under {@code <levelDir>/chunk-summaries}.
+ *
+ * <p>Writes go through a temporary file and an atomic move so that a half-written {@code latest.json}
+ * can never be observed by whatever tooling an administrator points at the directory.
+ */
+public final class ReportWriter {
+
+    public static final String DIRECTORY = "chunk-summaries";
+
+    private static final String PREFIX = "summary-";
+    private static final String JSON = ".json";
+    private static final Gson GSON = new GsonBuilder().setPrettyPrinting().create();
+
+    private final Path directory;
+    private final int retention;
+
+    public ReportWriter(Path levelDirectory, int retention) {
+        this.directory = levelDirectory.resolve(DIRECTORY);
+        this.retention = retention;
+    }
+
+    public Path directory() {
+        return directory;
+    }
+
+    /**
+     * Writes the timestamped snapshot plus the {@code latest} aliases and prunes old snapshots.
+     *
+     * @return the timestamped file that was written
+     */
+    public Path write(InspectionReport report, String stamp, boolean alsoText) throws IOException {
+        Files.createDirectories(directory);
+
+        Path snapshot = free(PREFIX + stamp + "-" + report.trigger());
+        writeAtomic(snapshot, writer -> GSON.toJson(report, writer));
+        writeAtomic(directory.resolve("latest" + JSON), writer -> GSON.toJson(report, writer));
+        if (alsoText) {
+            writeAtomic(directory.resolve("latest.txt"), writer -> writer.write(TextDigest.render(report)));
+        }
+        prune();
+        return snapshot;
+    }
+
+    /**
+     * The name a snapshot should take, made unique if one is already there. The stamp only resolves
+     * to the second, so a periodic snapshot and one asked for by hand really can land together; all
+     * writes go through a single thread, which is what makes this check-then-create safe.
+     */
+    private Path free(String name) {
+        Path candidate = directory.resolve(name + JSON);
+        for (int attempt = 2; Files.exists(candidate); attempt++) {
+            candidate = directory.resolve(name + "-" + attempt + JSON);
+        }
+        return candidate;
+    }
+
+    /** Keeps the {@code retention} newest snapshots; {@code 0} keeps everything. */
+    private void prune() throws IOException {
+        if (retention <= 0) {
+            return;
+        }
+        try (Stream<Path> files = Files.list(directory)) {
+            List<Path> snapshots = files
+                    .filter(p -> p.getFileName().toString().startsWith(PREFIX))
+                    .filter(p -> p.getFileName().toString().endsWith(JSON))
+                    .sorted(Comparator.comparing(p -> p.getFileName().toString()))
+                    .toList();
+            for (int i = 0; i < snapshots.size() - retention; i++) {
+                Files.deleteIfExists(snapshots.get(i));
+            }
+        }
+    }
+
+    private void writeAtomic(Path target, ContentWriter content) throws IOException {
+        Path temporary = target.resolveSibling(target.getFileName() + ".tmp");
+        try (Writer writer = Files.newBufferedWriter(temporary, StandardCharsets.UTF_8)) {
+            content.writeTo(writer);
+        }
+        try {
+            Files.move(temporary, target, StandardCopyOption.REPLACE_EXISTING, StandardCopyOption.ATOMIC_MOVE);
+        } catch (java.nio.file.AtomicMoveNotSupportedException e) {
+            Files.move(temporary, target, StandardCopyOption.REPLACE_EXISTING);
+        }
+    }
+
+    @FunctionalInterface
+    private interface ContentWriter {
+        void writeTo(Writer writer) throws IOException;
+    }
+}
@@ -0,0 +1,102 @@
+package dev.chunkinspector.report;
+
+import dev.chunkinspector.report.InspectionReport.LevelReport;
+import java.util.List;
+import lombok.experimental.UtilityClass;
+
+/**
+ * Renders a report as the summary an administrator actually reads first. The JSON alongside it stays
+ * the machine-readable source of truth; this is the version you can page through over SSH.
+ */
+@UtilityClass
+public class TextDigest {
+
+    private final int TOP_TYPES = 12;
+    private final int TOP_SUSPECTS = 10;
+    private final int TOP_HOTSPOTS = 8;
+    private final int TOP_ORIGINS = 8;
+
+    public String render(InspectionReport report) {
+        var out = new StringBuilder(4096);
+        out.append("Chunk Inspector — ").append(report.generatedAt()).append("  (").append(report.trigger()).append(")\n");
+        out.append("deep=").append(report.settings().deep())
+                .append("  attribution=").append(report.settings().attribution())
+                .append("  interval=").append(report.settings().intervalTicks()).append(" ticks\n");
+
+        for (LevelReport level : report.levels()) {
+            renderLevel(out, level);
+        }
+        if (!report.settings().deep()) {
+            out.append("\nNo call site could be named for the tickets above. Restart with ")
+                    .append("-Dchunkinspector.deep=true to record the stack trace behind every ticket.\n");
+        }
+        return out.toString();
+    }
+
+    private void renderLevel(StringBuilder out, LevelReport level) {
+        var totals = level.totals();
+        out.append("\n══ ").append(level.dimension()).append(" ══  game time ").append(level.gameTime()).append('\n');
+        out.append("  chunks: ").append(totals.loadedChunks()).append(" loaded")
+                .append(", ").append(totals.blockTicking()).append(" block-ticking")
+                .append(", ").append(totals.entityTicking()).append(" entity-ticking")
+                .append(" (").append(totals.trackedChunks()).append(" tracked)\n");
+        out.append("  tickets: ").append(totals.tickets()).append(" on ").append(totals.ticketedChunks()).append(" chunks")
+                .append(", ").append(totals.permanentTickets()).append(" never expire")
+                .append(", ").append(totals.forcedChunks()).append(" from /forceload\n");
+
+        section(out, "by ticket type", level.byType().stream().limit(TOP_TYPES).map(t ->
+                "%-28s %5d tickets  %5d chunks  level>=%d  %s  oldest %s%s".formatted(
+                        t.type(),
+                        t.tickets(),
+                        t.chunks(),
+                        t.minLevel(),
+                        t.influence() < 0 ? "influence n/a" : "keeps %d loaded".formatted(t.influence()),
+                        formatTicks(t.oldestAgeTicks()),
+                        t.expiring() ? "  (expires)" : "")).toList());
+
+        section(out, "most suspicious tickets", level.suspects().stream().limit(TOP_SUSPECTS).map(s ->
+                "chunk %-14s %-24s level %2d  %s%s\n      -> %s".formatted(
+                        s.chunk(),
+                        s.type(),
+                        s.level(),
+                        s.owner(),
+                        s.origin() == null ? "" : "  origin " + s.origin(),
+                        s.reason())).toList());
+
+        section(out, "loaded chunk clusters", level.hotspots().stream().limit(TOP_HOTSPOTS).map(h ->
+                "%5d chunks  centre %-14s  bounds %-28s  %s".formatted(
+                        h.chunks(), h.center(), h.bounds(), String.join(", ", h.causes()))).toList());
+
+        section(out, "ticket call sites", level.origins().stream().limit(TOP_ORIGINS).map(o ->
+                "%s  mod=%-24s live=%-5d adds=%-7d %s\n      %s".formatted(
+                        o.id(),
+                        o.mod(),
+                        o.live(),
+                        o.adds(),
+                        String.join(",", o.types()),
+                        String.join("\n      ", o.stack()))).toList());
+
+        if (level.attributionRan() && !level.unattributed().isEmpty()) {
+            section(out, "loaded but explained by no ticket", level.unattributed());
+        }
+    }
+
+    private void section(StringBuilder out, String title, List<String> lines) {
+        if (lines.isEmpty()) {
+            return;
+        }
+        out.append("\n  ").append(title).append('\n');
+        lines.forEach(line -> out.append("    ").append(line).append('\n'));
+    }
+
+    private String formatTicks(long ticks) {
+        long seconds = ticks / 20;
+        if (seconds < 60) {
+            return seconds + "s";
+        }
+        if (seconds < 3600) {
+            return seconds / 60 + "m";
+        }
+        return seconds / 3600 + "h" + seconds % 3600 / 60 + "m";
+    }
+}
@@ -0,0 +1,7 @@
+# Read-only widening. The census reads these fields directly instead of injecting into the game,
+# which is what keeps the always-on mode free: nothing is transformed on any hot path.
+public net.minecraft.server.level.DistanceManager tickets
+public net.minecraft.server.level.DistanceManager ticketTickCounter
+public net.minecraft.server.level.Ticket key
+public net.minecraft.server.level.Ticket createdTick
+public net.minecraft.server.level.ChunkMap getChunks()Ljava/lang/Iterable;
@@ -0,0 +1,15 @@
+{
+  "required": true,
+  "minVersion": "0.8.5",
+  "package": "dev.chunkinspector.mixin",
+  "compatibilityLevel": "JAVA_21",
+  "plugin": "dev.chunkinspector.mixin.InspectorMixinPlugin",
+  "injectors": {
+    "defaultRequire": 1
+  },
+  "mixins": [
+    "DistanceManagerMixin",
+    "TicketMixin",
+    "TicketOwnerAccessor"
+  ]
+}
@@ -0,0 +1,27 @@
+modLoader = "javafml"
+loaderVersion = "[4,)"
+license = "${mod_license}"
+
+[[mods]]
+modId = "${mod_id}"
+version = "${mod_version}"
+displayName = "${mod_name}"
+authors = "${mod_authors}"
+description = '''${mod_description}'''
+
+[[mixins]]
+config = "${mod_id}.mixins.json"
+
+[[dependencies.${mod_id}]]
+modId = "neoforge"
+type = "required"
+versionRange = "[21.1.0,)"
+ordering = "NONE"
+side = "BOTH"
+
+[[dependencies.${mod_id}]]
+modId = "minecraft"
+type = "required"
+versionRange = "[1.21.1,1.22)"
+ordering = "NONE"
+side = "BOTH"
@@ -0,0 +1,97 @@
+package dev.chunkinspector;
+
+import dev.chunkinspector.analysis.ChunkKey;
+import dev.chunkinspector.analysis.ChunkRecord;
+import dev.chunkinspector.analysis.LevelAnalysis;
+import dev.chunkinspector.analysis.TicketRecord;
+import dev.chunkinspector.origin.Origin;
+import dev.chunkinspector.report.InspectionReport;
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * A small, hand-built world used by most tests: a player standing at the origin, and a mod that has
+ * quietly forced a 5x5 block of chunks a hundred chunks away and never let go. That is exactly the
+ * situation the mod exists to diagnose, so the same fixture serves the analysis, report and digest
+ * tests.
+ */
+public final class Fixtures {
+
+    /** {@code ChunkLevel.MAX_LEVEL} for 1.21.1: 33 plus the generation radius around a full chunk. */
+    public static final int MAX_LEVEL = 41;
+
+    public static final String LEAK_ORIGIN = "a1b2c3d4e5f6";
+
+    private Fixtures() {}
+
+    public static TicketRecord ticket(
+            int x, int z, String type, int level, long timeout, long age, String owner, String origin) {
+        return new TicketRecord(ChunkKey.of(x, z), type, level, timeout, age, owner, origin);
+    }
+
+    /** The player's own view ticket — always present, never a suspect. */
+    public static TicketRecord playerTicket() {
+        return ticket(0, 0, "player", 31, 0, 1_000, "chunk 0, 0", null);
+    }
+
+    /** A NeoForge forced-chunk ticket carrying the requesting mod's id, as the real key does. */
+    public static TicketRecord leakTicket() {
+        return ticket(100, 100, "neoforge:block", 31, 0, 72_000, "leakymod:loader -> block 1600, 64, 1600", LEAK_ORIGIN);
+    }
+
+    /** A vanilla {@code /forceload} on the same chunk, so ties have to be broken. */
+    public static TicketRecord forcedTicket() {
+        return ticket(100, 100, "forced", 31, 0, 5, "chunk 100, 100", null);
+    }
+
+    /** Expires by itself, so it can never be the cause of a permanently loaded chunk. */
+    public static TicketRecord portalTicket() {
+        return ticket(50, 50, "portal", 30, 300, 10, "block 800, 64, 800", null);
+    }
+
+    public static List<TicketRecord> tickets() {
+        return List.of(playerTicket(), leakTicket(), forcedTicket(), portalTicket());
+    }
+
+    public static List<ChunkRecord> chunks() {
+        var chunks = new ArrayList<ChunkRecord>();
+        chunks.addAll(square(0, 0, 1, 33, "FULL"));
+        chunks.addAll(square(100, 100, 2, 31, "ENTITY_TICKING"));
+        // A chunk the server still tracks but no longer keeps in memory: it must not count as loaded.
+        chunks.add(new ChunkRecord(ChunkKey.of(500, 500), MAX_LEVEL, "INACCESSIBLE"));
+        return List.copyOf(chunks);
+    }
+
+    /** A {@code (2 * radius + 1)}-square of chunks centred on {@code (x, z)}. */
+    public static List<ChunkRecord> square(int x, int z, int radius, int level, String fullStatus) {
+        var chunks = new ArrayList<ChunkRecord>();
+        for (int dx = -radius; dx <= radius; dx++) {
+            for (int dz = -radius; dz <= radius; dz++) {
+                chunks.add(new ChunkRecord(ChunkKey.of(x + dx, z + dz), level, fullStatus));
+            }
+        }
+        return chunks;
+    }
+
+    public static List<Origin> origins() {
+        return List.of(new Origin(
+                LEAK_ORIGIN,
+                "leakymod",
+                42,
+                List.of("neoforge:block"),
+                List.of("leakymod.machine.QuarryBlockEntity.tick:88", "leakymod.machine.QuarryBlockEntity.serverTick:41")));
+    }
+
+    public static InspectionReport.LevelReport levelReport(boolean attribution) {
+        return LevelAnalysis.analyse("minecraft:overworld", 123_456, tickets(), chunks(), MAX_LEVEL, attribution, origins());
+    }
+
+    public static InspectionReport report(boolean deep, boolean attribution) {
+        return new InspectionReport(
+                InspectionReport.SCHEMA,
+                "2026-07-31T10:15:30Z",
+                "command",
+                new InspectionReport.Settings(deep, attribution, 600, 1),
+                List.of(levelReport(attribution)));
+    }
+}
@@ -0,0 +1,91 @@
+package dev.chunkinspector;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.List;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+
+class InspectorConfigTest {
+
+    private static final List<String> KEYS = List.of(
+            "enabled", "deep", "interval", "attribution", "retention", "stackDepth", "maxOrigins", "sampleRate", "textReport");
+
+    @AfterEach
+    void clearProperties() {
+        KEYS.forEach(key -> System.clearProperty(InspectorConfig.PREFIX + key));
+    }
+
+    private static void set(String key, String value) {
+        System.setProperty(InspectorConfig.PREFIX + key, value);
+    }
+
+    @Test
+    void defaultsAreTheCheapConfiguration() {
+        assertThat(InspectorConfig.enabled()).isTrue();
+        assertThat(InspectorConfig.deep()).isFalse();
+        assertThat(InspectorConfig.attributionInSnapshots()).isFalse();
+        assertThat(InspectorConfig.intervalTicks()).isEqualTo(600);
+        assertThat(InspectorConfig.retention()).isEqualTo(48);
+        assertThat(InspectorConfig.sampleRate()).isEqualTo(1);
+        assertThat(InspectorConfig.textReport()).isTrue();
+    }
+
+    @Test
+    void deepModeRequiresTheModToBeEnabledAtAll() {
+        set("deep", "true");
+        assertThat(InspectorConfig.deep()).isTrue();
+
+        set("enabled", "false");
+        assertThat(InspectorConfig.deep()).isFalse();
+    }
+
+    @Test
+    void blankAndMissingValuesFallBackToTheDefault() {
+        set("interval", "");
+        assertThat(InspectorConfig.intervalTicks()).isEqualTo(600);
+
+        set("retention", "   ");
+        assertThat(InspectorConfig.retention()).isEqualTo(48);
+    }
+
+    @Test
+    void unparseableNumbersDoNotBreakAServerStartup() {
+        set("interval", "every-so-often");
+        set("maxOrigins", "lots");
+
+        assertThat(InspectorConfig.intervalTicks()).isEqualTo(600);
+        assertThat(InspectorConfig.maxOrigins()).isEqualTo(8192);
+    }
+
+    @Test
+    void intervalsAndRetentionMayBeZeroButNeverNegative() {
+        set("interval", "0");
+        set("retention", "-5");
+
+        assertThat(InspectorConfig.intervalTicks()).isZero();
+        assertThat(InspectorConfig.retention()).isZero();
+    }
+
+    @Test
+    void boundedValuesAreClampedIntoAWorkableRange() {
+        set("stackDepth", "1");
+        assertThat(InspectorConfig.stackDepth()).isEqualTo(4);
+        set("stackDepth", "10000");
+        assertThat(InspectorConfig.stackDepth()).isEqualTo(256);
+
+        set("maxOrigins", "1");
+        assertThat(InspectorConfig.maxOrigins()).isEqualTo(64);
+
+        set("sampleRate", "0");
+        assertThat(InspectorConfig.sampleRate()).isEqualTo(1);
+        set("sampleRate", "64");
+        assertThat(InspectorConfig.sampleRate()).isEqualTo(64);
+    }
+
+    @Test
+    void valuesAreTrimmedBecauseServerScriptsAreWrittenByHand() {
+        set("interval", " 40 ");
+        assertThat(InspectorConfig.intervalTicks()).isEqualTo(40);
+    }
+}
@@ -0,0 +1,81 @@
+package dev.chunkinspector.analysis;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import dev.chunkinspector.Fixtures;
+import dev.chunkinspector.analysis.ChunkExplainer.Explanation;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class ChunkExplainerTest {
+
+    private static final int MAX = 33;
+
+    @Test
+    void aTicketExplainsEveryChunkWithinItsReach() {
+        var tickets = List.of(Fixtures.ticket(0, 0, "forced", 31, 0, 100, "chunk 0, 0", null));
+
+        Explanation here = ChunkExplainer.explain(tickets, ChunkKey.of(0, 0), MAX);
+        assertThat(here.level()).isEqualTo(31);
+        assertThat(here.loaded(MAX)).isTrue();
+        assertThat(here.contributors()).singleElement().satisfies(c -> {
+            assertThat(c.distance()).isZero();
+            assertThat(c.contributed()).isEqualTo(31);
+            assertThat(c.ticket().type()).isEqualTo("forced");
+        });
+
+        Explanation edge = ChunkExplainer.explain(tickets, ChunkKey.of(2, -2), MAX);
+        assertThat(edge.level()).isEqualTo(33);
+        assertThat(edge.contributors()).singleElement().satisfies(c -> assertThat(c.distance()).isEqualTo(2));
+    }
+
+    @Test
+    void aChunkOutOfEveryTicketsReachIsExplainedByNothing() {
+        var tickets = List.of(Fixtures.ticket(0, 0, "forced", 31, 0, 100, "chunk 0, 0", null));
+
+        Explanation explanation = ChunkExplainer.explain(tickets, ChunkKey.of(3, 0), MAX);
+
+        assertThat(explanation.contributors()).isEmpty();
+        assertThat(explanation.level()).isEqualTo(Integer.MAX_VALUE);
+        assertThat(explanation.loaded(MAX)).isFalse();
+    }
+
+    @Test
+    void contributorsAreOrderedByHowMuchTheyActuallyContribute() {
+        var tickets = List.of(
+                Fixtures.ticket(3, 0, "far", 31, 0, 1, "far", null), // 31 + 3 = 34, out of reach
+                Fixtures.ticket(2, 0, "weak", 31, 0, 1, "weak", null), // 31 + 2 = 33
+                Fixtures.ticket(0, 0, "strong", 30, 0, 1, "strong", null)); // 30 + 0 = 30
+
+        Explanation explanation = ChunkExplainer.explain(tickets, ChunkKey.of(0, 0), MAX);
+
+        assertThat(explanation.contributors()).extracting(c -> c.ticket().type()).containsExactly("strong", "weak");
+        assertThat(explanation.level()).isEqualTo(30);
+    }
+
+    @Test
+    void negativeTicketLevelsAreClampedTheSameWayPropagationClampsThem() {
+        var tickets = List.of(Fixtures.ticket(0, 0, "huge_region", -5, 0, 1, "-", null));
+
+        assertThat(ChunkExplainer.explain(tickets, ChunkKey.of(0, 0), MAX).level()).isZero();
+        assertThat(ChunkExplainer.explain(tickets, ChunkKey.of(10, 0), MAX).level()).isEqualTo(10);
+    }
+
+    @Test
+    void agreesWithTheFullPropagationPass() {
+        // The explainer is a shortcut for one chunk; if it ever disagreed with the propagation the
+        // command and the report would tell an administrator two different stories.
+        var tickets = Fixtures.tickets();
+        var sources = tickets.stream().map(t -> new LevelPropagation.Source(t.chunk(), t.level())).toList();
+        var propagated = LevelPropagation.solve(sources, Fixtures.MAX_LEVEL);
+
+        for (int x = -3; x <= 3; x++) {
+            for (int z = -3; z <= 3; z++) {
+                long chunk = ChunkKey.of(x, z);
+                assertThat(ChunkExplainer.explain(tickets, chunk, Fixtures.MAX_LEVEL).level())
+                        .as("chunk %s", ChunkKey.format(chunk))
+                        .isEqualTo(propagated.levelOf(chunk));
+            }
+        }
+    }
+}
@@ -0,0 +1,48 @@
+package dev.chunkinspector.analysis;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+class ChunkKeyTest {
+
+    @ParameterizedTest
+    @CsvSource({
+        "0, 0",
+        "1, 2",
+        "-1, -1",
+        "1875000, -1875000",
+        "2147483647, -2147483648",
+    })
+    void roundTripsIncludingNegativeCoordinates(int x, int z) {
+        long key = ChunkKey.of(x, z);
+        assertThat(ChunkKey.x(key)).isEqualTo(x);
+        assertThat(ChunkKey.z(key)).isEqualTo(z);
+    }
+
+    @Test
+    void packsExactlyLikeChunkPos() {
+        // ChunkPos.asLong puts x in the low word and z in the high word; a mismatch here would make
+        // every key we read out of the game meaningless.
+        assertThat(ChunkKey.of(1, 2)).isEqualTo(0x0000_0002_0000_0001L);
+        assertThat(ChunkKey.of(-1, 0)).isEqualTo(0x0000_0000_FFFF_FFFFL);
+    }
+
+    @Test
+    void distanceIsChebyshevBecauseThatIsHowLevelsPropagate() {
+        long origin = ChunkKey.of(0, 0);
+        assertThat(ChunkKey.distance(origin, ChunkKey.of(3, 1))).isEqualTo(3);
+        assertThat(ChunkKey.distance(origin, ChunkKey.of(-4, 4))).isEqualTo(4);
+        assertThat(ChunkKey.distance(origin, origin)).isZero();
+        assertThat(ChunkKey.distance(ChunkKey.of(10, 10), ChunkKey.of(7, 12))).isEqualTo(3);
+    }
+
+    @Test
+    void formatsCoordinatesAPlayerCanType() {
+        assertThat(ChunkKey.format(ChunkKey.of(12, -34))).isEqualTo("12, -34");
+        assertThat(ChunkKey.formatBlock(ChunkKey.of(0, 0))).isEqualTo("8 ~ 8");
+        assertThat(ChunkKey.formatBlock(ChunkKey.of(100, -3))).isEqualTo("1608 ~ -40");
+    }
+}
@@ -0,0 +1,178 @@
+package dev.chunkinspector.analysis;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import dev.chunkinspector.Fixtures;
+import dev.chunkinspector.report.InspectionReport.Hotspot;
+import dev.chunkinspector.report.InspectionReport.LevelReport;
+import dev.chunkinspector.report.InspectionReport.TypeBreakdown;
+import java.util.ArrayList;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class LevelAnalysisTest {
+
+    private static TypeBreakdown byType(LevelReport report, String type) {
+        return report.byType().stream()
+                .filter(t -> t.type().equals(type))
+                .findFirst()
+                .orElseThrow(() -> new AssertionError("no breakdown for " + type + " in " + report.byType()));
+    }
+
+    @Test
+    void totalsSeparateTrackedFromActuallyLoadedChunks() {
+        var totals = Fixtures.levelReport(false).totals();
+
+        assertThat(totals.trackedChunks()).isEqualTo(35); // 3x3 + 5x5 + one inaccessible
+        assertThat(totals.loadedChunks()).isEqualTo(34);
+        assertThat(totals.entityTicking()).isEqualTo(25);
+        assertThat(totals.blockTicking()).isEqualTo(25); // entity-ticking implies block-ticking
+        assertThat(totals.tickets()).isEqualTo(4);
+        assertThat(totals.ticketedChunks()).isEqualTo(3); // two tickets share chunk 100, 100
+        assertThat(totals.permanentTickets()).isEqualTo(3); // the portal ticket expires
+        assertThat(totals.forcedChunks()).isEqualTo(1);
+    }
+
+    @Test
+    void typeBreakdownRecordsOwnersAndWhetherTheTypeExpires() {
+        LevelReport report = Fixtures.levelReport(false);
+
+        assertThat(report.byType()).extracting(TypeBreakdown::type)
+                .containsExactlyInAnyOrder("player", "neoforge:block", "forced", "portal");
+
+        TypeBreakdown leak = byType(report, "neoforge:block");
+        assertThat(leak.tickets()).isEqualTo(1);
+        assertThat(leak.chunks()).isEqualTo(1);
+        assertThat(leak.minLevel()).isEqualTo(31);
+        assertThat(leak.oldestAgeTicks()).isEqualTo(72_000);
+        assertThat(leak.expiring()).isFalse();
+        // The NeoForge ticket key names the mod outright — no instrumentation needed.
+        assertThat(leak.owners()).containsExactly("leakymod:loader -> block 1600, 64, 1600");
+
+        assertThat(byType(report, "portal").expiring()).isTrue();
+    }
+
+    @Test
+    void aTypeOnlyCountsAsExpiringWhenEveryTicketOfItDoes() {
+        var tickets = List.of(
+                Fixtures.ticket(0, 0, "mixed", 31, 300, 10, "a", null),
+                Fixtures.ticket(1, 0, "mixed", 31, 0, 10, "b", null));
+
+        LevelReport report = LevelAnalysis.analyse("d", 0, tickets, List.of(), Fixtures.MAX_LEVEL, false, List.of());
+
+        assertThat(byType(report, "mixed").expiring()).isFalse();
+    }
+
+    @Test
+    void suspectsAreThePermanentNonVanillaTicketsOldestFirst() {
+        LevelReport report = Fixtures.levelReport(false);
+
+        assertThat(report.suspects()).extracting("type").containsExactly("neoforge:block", "forced");
+        var leak = report.suspects().getFirst();
+        assertThat(leak.chunk()).isEqualTo("100, 100");
+        assertThat(leak.blockPos()).isEqualTo("1608 ~ 1608");
+        assertThat(leak.origin()).isEqualTo(Fixtures.LEAK_ORIGIN);
+        assertThat(leak.reason()).contains("permanent ticket", "1h0m", "entity-ticking");
+    }
+
+    @Test
+    void attributionRanksTypesByTheChunksTheyActuallyKeepLoaded() {
+        LevelReport report = Fixtures.levelReport(true);
+
+        assertThat(report.attributionRan()).isTrue();
+        assertThat(byType(report, "neoforge:block").influence()).isEqualTo(25);
+        assertThat(byType(report, "player").influence()).isEqualTo(9);
+        // The forced ticket sits on the same chunk but was added second, so it wins nothing; the
+        // portal ticket is nowhere near a loaded chunk.
+        assertThat(byType(report, "forced").influence()).isZero();
+        assertThat(byType(report, "portal").influence()).isZero();
+
+        assertThat(report.byType()).extracting(TypeBreakdown::type).startsWith("neoforge:block", "player");
+        assertThat(report.suspects().getFirst().reason()).contains("keeping 25 chunks loaded");
+    }
+
+    @Test
+    void withoutAttributionInfluenceIsReportedAsUnknownRatherThanZero() {
+        LevelReport report = Fixtures.levelReport(false);
+
+        assertThat(report.attributionRan()).isFalse();
+        assertThat(report.byType()).allSatisfy(type -> assertThat(type.influence()).isEqualTo(-1));
+        assertThat(report.suspects().getFirst().reason()).doesNotContain("keeping");
+        assertThat(report.unattributed()).isEmpty();
+    }
+
+    @Test
+    void hotspotsGroupLoadedChunksIntoClustersLargestFirst() {
+        List<Hotspot> hotspots = Fixtures.levelReport(true).hotspots();
+
+        assertThat(hotspots).hasSize(2);
+        Hotspot leak = hotspots.getFirst();
+        assertThat(leak.chunks()).isEqualTo(25);
+        assertThat(leak.bounds()).isEqualTo("98, 98 .. 102, 102");
+        assertThat(leak.center()).isEqualTo("100, 100");
+        assertThat(leak.blockPos()).isEqualTo("1608 ~ 1608");
+        assertThat(leak.causes()).containsExactly("neoforge:block x25");
+
+        assertThat(hotspots.get(1).chunks()).isEqualTo(9);
+        assertThat(hotspots.get(1).causes()).containsExactly("player x9");
+    }
+
+    @Test
+    void withoutAttributionHotspotCausesFallBackToTheTicketsInsideTheCluster() {
+        List<Hotspot> hotspots = Fixtures.levelReport(false).hotspots();
+
+        assertThat(hotspots.getFirst().causes())
+                .containsExactlyInAnyOrder("neoforge:block x1", "forced x1");
+    }
+
+    @Test
+    void clustersSmallerThanFourChunksAreNotWorthReporting() {
+        var chunks = new ArrayList<>(Fixtures.square(0, 0, 1, 33, "FULL")); // 9 chunks
+        chunks.addAll(Fixtures.square(900, 900, 0, 33, "FULL")); // a single lonely chunk
+
+        LevelReport report =
+                LevelAnalysis.analyse("d", 0, List.of(), chunks, Fixtures.MAX_LEVEL, false, List.of());
+
+        assertThat(report.hotspots()).extracting(Hotspot::chunks).containsExactly(9);
+    }
+
+    @Test
+    void chunksNoTicketExplainsAreListedSeparately() {
+        var chunks = new ArrayList<>(Fixtures.chunks());
+        chunks.add(new ChunkRecord(ChunkKey.of(9999, 9999), 33, "FULL"));
+
+        LevelReport report = LevelAnalysis.analyse(
+                "d", 0, Fixtures.tickets(), chunks, Fixtures.MAX_LEVEL, true, List.of());
+
+        // Everything else is explained; only the orphan is left over.
+        assertThat(report.unattributed()).containsExactly("9999, 9999");
+    }
+
+    @Test
+    void originsAreReportedOnlyWhileTheirTicketsAreStillAlive() {
+        LevelReport withLiveTicket = Fixtures.levelReport(false);
+        assertThat(withLiveTicket.origins()).hasSize(1);
+        var origin = withLiveTicket.origins().getFirst();
+        assertThat(origin.id()).isEqualTo(Fixtures.LEAK_ORIGIN);
+        assertThat(origin.mod()).isEqualTo("leakymod");
+        assertThat(origin.live()).isEqualTo(1);
+        assertThat(origin.adds()).isEqualTo(42);
+
+        LevelReport ticketGone = LevelAnalysis.analyse(
+                "d", 0, List.of(Fixtures.playerTicket()), Fixtures.chunks(), Fixtures.MAX_LEVEL, false, Fixtures.origins());
+        assertThat(ticketGone.origins()).isEmpty();
+    }
+
+    @Test
+    void anEmptyLevelProducesAnEmptyReportRatherThanFailing() {
+        LevelReport report =
+                LevelAnalysis.analyse("minecraft:the_end", 0, List.of(), List.of(), Fixtures.MAX_LEVEL, true, List.of());
+
+        assertThat(report.totals().trackedChunks()).isZero();
+        assertThat(report.byType()).isEmpty();
+        assertThat(report.suspects()).isEmpty();
+        assertThat(report.hotspots()).isEmpty();
+        assertThat(report.unattributed()).isEmpty();
+        assertThat(report.attributionRan()).isTrue();
+    }
+}
@@ -0,0 +1,100 @@
+package dev.chunkinspector.analysis;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import dev.chunkinspector.analysis.LevelPropagation.Result;
+import dev.chunkinspector.analysis.LevelPropagation.Source;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class LevelPropagationTest {
+
+    private static final int MAX = 33;
+
+    private static Result solve(Source... sources) {
+        return LevelPropagation.solve(List.of(sources), MAX);
+    }
+
+    @Test
+    void noSourcesReachNothing() {
+        Result result = LevelPropagation.solve(List.of(), MAX);
+        assertThat(result.reachedChunks()).isZero();
+        assertThat(result.levelOf(ChunkKey.of(0, 0))).isEqualTo(Integer.MAX_VALUE);
+        assertThat(result.winnerOf(ChunkKey.of(0, 0))).isEqualTo(Result.NONE);
+    }
+
+    @Test
+    void levelGrowsWithChebyshevDistance() {
+        Result result = solve(new Source(ChunkKey.of(0, 0), 31));
+
+        assertThat(result.levelOf(ChunkKey.of(0, 0))).isEqualTo(31);
+        assertThat(result.levelOf(ChunkKey.of(1, 1))).isEqualTo(32); // diagonal is distance 1
+        assertThat(result.levelOf(ChunkKey.of(2, 0))).isEqualTo(33);
+        assertThat(result.levelOf(ChunkKey.of(-2, 2))).isEqualTo(33);
+        assertThat(result.levelOf(ChunkKey.of(3, 0))).isEqualTo(Integer.MAX_VALUE); // 34 > MAX
+        // A ticket at level 31 with MAX 33 covers a 5x5 square and nothing more.
+        assertThat(result.reachedChunks()).isEqualTo(25);
+    }
+
+    @Test
+    void everyReachedChunkNamesItsSource() {
+        Result result = solve(new Source(ChunkKey.of(7, -7), 32));
+
+        assertThat(result.winnerOf(ChunkKey.of(7, -7))).isZero();
+        assertThat(result.winnerOf(ChunkKey.of(8, -6))).isZero();
+        assertThat(result.winnerOf(ChunkKey.of(9, -7))).isEqualTo(Result.NONE);
+    }
+
+    @Test
+    void theStrongerTicketWinsEachChunkIndependently() {
+        // A weak ticket at the origin and a strong one four chunks east. Chunks in between belong to
+        // whichever actually explains them, which is the whole point of tracking a winner at all.
+        Result result = solve(new Source(ChunkKey.of(0, 0), 32), new Source(ChunkKey.of(4, 0), 30));
+
+        assertThat(result.levelOf(ChunkKey.of(0, 0))).isEqualTo(32);
+        assertThat(result.winnerOf(ChunkKey.of(0, 0))).isZero();
+
+        assertThat(result.levelOf(ChunkKey.of(2, 0))).isEqualTo(32); // 30 + 2 beats 32 + 2
+        assertThat(result.winnerOf(ChunkKey.of(2, 0))).isEqualTo(1);
+
+        assertThat(result.levelOf(ChunkKey.of(4, 0))).isEqualTo(30);
+        assertThat(result.winnerOf(ChunkKey.of(4, 0))).isEqualTo(1);
+    }
+
+    @Test
+    void tiesGoToTheFirstSourceSoResultsAreDeterministic() {
+        Result result = solve(new Source(ChunkKey.of(0, 0), 31), new Source(ChunkKey.of(0, 0), 31));
+
+        assertThat(result.winnerOf(ChunkKey.of(0, 0))).isZero();
+        assertThat(result.winnerOf(ChunkKey.of(1, 0))).isZero();
+    }
+
+    @Test
+    void ticketsWeakerThanMaxLevelReachNothing() {
+        Result result = solve(new Source(ChunkKey.of(0, 0), MAX + 1));
+        assertThat(result.reachedChunks()).isZero();
+    }
+
+    @Test
+    void negativeLevelsAreClampedLikeVanilla() {
+        // A region ticket with a radius above 33 produces a negative level; before clamping this
+        // indexed a bucket array out of bounds.
+        Result result = solve(new Source(ChunkKey.of(0, 0), -5));
+
+        assertThat(result.levelOf(ChunkKey.of(0, 0))).isZero();
+        assertThat(result.levelOf(ChunkKey.of(1, 0))).isEqualTo(1);
+        assertThat(result.levelOf(ChunkKey.of(MAX, 0))).isEqualTo(MAX);
+        assertThat(result.levelOf(ChunkKey.of(MAX + 1, 0))).isEqualTo(Integer.MAX_VALUE);
+    }
+
+    @Test
+    void aChunkQueuedThenSupersededKeepsTheStrongerAnswer() {
+        // Source 0 is discovered first and queues (5, 0) at level 33; source 1 later claims the same
+        // chunk at level 31. The stale queue entry must not overwrite it.
+        Result result = solve(new Source(ChunkKey.of(0, 0), 28), new Source(ChunkKey.of(5, 0), 31));
+
+        assertThat(result.levelOf(ChunkKey.of(5, 0))).isEqualTo(31);
+        assertThat(result.winnerOf(ChunkKey.of(5, 0))).isEqualTo(1);
+        assertThat(result.levelOf(ChunkKey.of(4, 0))).isEqualTo(32);
+    }
+}
@@ -0,0 +1,35 @@
+package dev.chunkinspector.origin;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import example.mod.LeakyChunkLoader;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class ModAttributionTest {
+
+    @Test
+    void platformModulesAreNeverReportedAsMods() {
+        // "java.base" would be a technically correct module name and a useless answer, so the JDK's
+        // own modules are rejected and the lookup falls through instead.
+        assertThat(List.class.getModule().getName()).isEqualTo("java.base");
+        assertThat(ModAttribution.of(List.class)).isEqualTo("unknown");
+    }
+
+    @Test
+    void classpathClassesFallBackToTheirCodeSource() {
+        // Loaded from an exploded directory, so this exercises the trailing-slash path that a mod
+        // loaded from a folder rather than a jar would also take.
+        assertThat(ModAttribution.of(LeakyChunkLoader.class)).isNotBlank().isNotEqualTo("unknown");
+    }
+
+    @Test
+    void aMissingClassIsNotAnError() {
+        assertThat(ModAttribution.of(null)).isEqualTo("unknown");
+    }
+
+    @Test
+    void repeatedLookupsAreCachedAndStable() {
+        assertThat(ModAttribution.of(LeakyChunkLoader.class)).isEqualTo(ModAttribution.of(LeakyChunkLoader.class));
+    }
+}
@@ -0,0 +1,87 @@
+package dev.chunkinspector.origin;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import example.mod.LeakyChunkLoader;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Lives in {@code dev.chunkinspector.origin} on purpose: the registry drops frames from its own
+ * package, so the test's own frames disappear from the capture and the recorded stack is exactly
+ * what a real mod's would be.
+ */
+class OriginRegistryTest {
+
+    @Test
+    void idsAreTwelveHexCharactersAndDependOnTheWholeStack() {
+        String a = OriginRegistry.idOf(List.of("com.example.Foo.bar:12"));
+        String b = OriginRegistry.idOf(List.of("com.example.Foo.bar:13"));
+        String c = OriginRegistry.idOf(List.of("com.example.Foo.bar:12", "com.example.Foo.baz:99"));
+
+        assertThat(a).matches("[0-9a-f]{12}");
+        assertThat(a).isEqualTo(OriginRegistry.idOf(List.of("com.example.Foo.bar:12")));
+        assertThat(a).isNotEqualTo(b).isNotEqualTo(c);
+        assertThat(OriginRegistry.idOf(List.of())).matches("[0-9a-f]{12}");
+    }
+
+    @Test
+    void frameOrderMattersSoTwoPathsIntoTheSameMethodStaySeparate() {
+        assertThat(OriginRegistry.idOf(List.of("a:1", "b:2")))
+                .isNotEqualTo(OriginRegistry.idOf(List.of("b:2", "a:1")));
+    }
+
+    @Test
+    void repeatedAddsFromOneCallSiteFoldIntoOneOrigin() {
+        var registry = new OriginRegistry();
+        assertThat(registry.isEmpty()).isTrue();
+
+        List<String> ids = new LeakyChunkLoader(registry).forceLoad("neoforge:block", 3);
+
+        assertThat(ids).hasSize(3).containsOnly(ids.getFirst());
+        assertThat(registry.snapshot()).singleElement().satisfies(origin -> {
+            assertThat(origin.id()).isEqualTo(ids.getFirst());
+            assertThat(origin.adds()).isEqualTo(3);
+            assertThat(origin.types()).containsExactly("neoforge:block");
+            assertThat(origin.stack()).isNotEmpty();
+            // Our own frames and the ticket plumbing are gone; the mod's frame is on top.
+            assertThat(origin.stack().getFirst()).startsWith("example.mod.LeakyChunkLoader.forceLoad:");
+            assertThat(origin.stack()).noneMatch(frame -> frame.startsWith("dev.chunkinspector."));
+            assertThat(origin.mod()).isNotBlank();
+        });
+    }
+
+    @Test
+    void distinctCallSitesGetDistinctOrigins() {
+        var registry = new OriginRegistry();
+        var loader = new LeakyChunkLoader(registry);
+
+        String fromLoop = loader.forceLoad("forced", 1).getFirst();
+        String fromOnce = loader.loadOnce("forced");
+
+        assertThat(fromLoop).isNotEqualTo(fromOnce);
+        assertThat(registry.snapshot()).hasSize(2).extracting(Origin::id).containsExactlyInAnyOrder(fromLoop, fromOnce);
+    }
+
+    @Test
+    void oneCallSiteCreatingSeveralTicketTypesRecordsThemAll() {
+        var registry = new OriginRegistry();
+        var loader = new LeakyChunkLoader(registry);
+
+        loader.loadOnce("neoforge:block");
+        loader.loadOnce("neoforge:entity");
+
+        assertThat(registry.snapshot()).singleElement().satisfies(origin -> {
+            assertThat(origin.adds()).isEqualTo(2);
+            assertThat(origin.types()).containsExactlyInAnyOrder("neoforge:block", "neoforge:entity");
+        });
+    }
+
+    @Test
+    void aFreshRegistryContributesNothingToAReport() {
+        var registry = new OriginRegistry();
+
+        assertThat(registry.isEmpty()).isTrue();
+        assertThat(registry.snapshot()).isEmpty();
+    }
+}
@@ -0,0 +1,120 @@
+package dev.chunkinspector.report;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.google.gson.Gson;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParser;
+import dev.chunkinspector.Fixtures;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.List;
+import java.util.stream.Stream;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+class ReportWriterTest {
+
+    private static List<String> names(Path directory) throws IOException {
+        try (Stream<Path> files = Files.list(directory)) {
+            return files.map(p -> p.getFileName().toString()).sorted().toList();
+        }
+    }
+
+    @Test
+    void writesUnderChunkSummariesInsideTheLevelDirectory(@TempDir Path levelDirectory) throws IOException {
+        var writer = new ReportWriter(levelDirectory, 8);
+
+        Path written = writer.write(Fixtures.report(false, true), "20260731-101530", true);
+
+        assertThat(writer.directory()).isEqualTo(levelDirectory.resolve("chunk-summaries"));
+        assertThat(written).isEqualTo(writer.directory().resolve("summary-20260731-101530-command.json"));
+        assertThat(names(writer.directory()))
+                .containsExactly("latest.json", "latest.txt", "summary-20260731-101530-command.json");
+    }
+
+    @Test
+    void theJsonRoundTripsBackIntoTheSameReport(@TempDir Path levelDirectory) throws IOException {
+        var writer = new ReportWriter(levelDirectory, 8);
+        InspectionReport original = Fixtures.report(true, true);
+
+        Path written = writer.write(original, "20260731-101530", false);
+
+        InspectionReport parsed =
+                new Gson().fromJson(Files.readString(written, StandardCharsets.UTF_8), InspectionReport.class);
+        assertThat(parsed).isEqualTo(original);
+    }
+
+    @Test
+    void latestAlwaysMirrorsTheNewestSnapshot(@TempDir Path levelDirectory) throws IOException {
+        var writer = new ReportWriter(levelDirectory, 8);
+
+        writer.write(Fixtures.report(false, false), "20260731-101530", true);
+        writer.write(Fixtures.report(true, true), "20260731-101630", true);
+
+        JsonObject latest = JsonParser
+                .parseString(Files.readString(writer.directory().resolve("latest.json"), StandardCharsets.UTF_8))
+                .getAsJsonObject();
+        assertThat(latest.getAsJsonObject("settings").get("deep").getAsBoolean()).isTrue();
+        assertThat(latest.get("schema").getAsInt()).isEqualTo(InspectionReport.SCHEMA);
+        assertThat(Files.readString(writer.directory().resolve("latest.txt"))).contains("minecraft:overworld");
+    }
+
+    @Test
+    void onlyTheNewestSnapshotsAreKept(@TempDir Path levelDirectory) throws IOException {
+        var writer = new ReportWriter(levelDirectory, 2);
+
+        for (int minute = 0; minute < 5; minute++) {
+            writer.write(Fixtures.report(false, false), "20260731-1015%02d".formatted(minute), false);
+        }
+
+        assertThat(names(writer.directory())).containsExactly(
+                "latest.json", "summary-20260731-101503-command.json", "summary-20260731-101504-command.json");
+    }
+
+    @Test
+    void twoSnapshotsInTheSameSecondBothSurvive(@TempDir Path levelDirectory) throws IOException {
+        var writer = new ReportWriter(levelDirectory, 8);
+
+        Path first = writer.write(Fixtures.report(false, false), "20260731-101530", false);
+        Path second = writer.write(Fixtures.report(true, true), "20260731-101530", false);
+
+        assertThat(second).isNotEqualTo(first);
+        assertThat(names(writer.directory())).containsExactly(
+                "latest.json", "summary-20260731-101530-command-2.json", "summary-20260731-101530-command.json");
+    }
+
+    @Test
+    void retentionOfZeroKeepsEverything(@TempDir Path levelDirectory) throws IOException {
+        var writer = new ReportWriter(levelDirectory, 0);
+
+        for (int minute = 0; minute < 3; minute++) {
+            writer.write(Fixtures.report(false, false), "20260731-1015%02d".formatted(minute), false);
+        }
+
+        assertThat(names(writer.directory())).hasSize(4); // three snapshots plus latest.json
+    }
+
+    @Test
+    void noTemporaryFilesSurviveAWrite(@TempDir Path levelDirectory) throws IOException {
+        var writer = new ReportWriter(levelDirectory, 8);
+
+        writer.write(Fixtures.report(true, true), "20260731-101530", true);
+
+        assertThat(names(writer.directory())).noneMatch(name -> name.endsWith(".tmp"));
+    }
+
+    @Test
+    void overwritingLatestRepeatedlyNeverLeavesItTruncated(@TempDir Path levelDirectory) throws IOException {
+        var writer = new ReportWriter(levelDirectory, 4);
+        Path latest = levelDirectory.resolve("chunk-summaries").resolve("latest.json");
+
+        for (int i = 0; i < 4; i++) {
+            writer.write(Fixtures.report(false, true), "20260731-10150%d".formatted(i), true);
+            assertThat(new Gson().fromJson(Files.readString(latest), InspectionReport.class))
+                    .isEqualTo(Fixtures.report(false, true));
+        }
+    }
+}
@@ -0,0 +1,60 @@
+package dev.chunkinspector.report;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import dev.chunkinspector.Fixtures;
+import dev.chunkinspector.analysis.LevelAnalysis;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class TextDigestTest {
+
+    @Test
+    void showsTheLeakWithoutTheAdministratorHavingToOpenTheJson() {
+        String digest = TextDigest.render(Fixtures.report(true, true));
+
+        assertThat(digest).contains(
+                "minecraft:overworld",
+                "34 loaded",
+                "25 entity-ticking",
+                "by ticket type",
+                "most suspicious tickets",
+                "loaded chunk clusters",
+                "ticket call sites");
+        // The three things an administrator needs: what, where, and who.
+        assertThat(digest).contains("neoforge:block");
+        assertThat(digest).contains("98, 98 .. 102, 102");
+        assertThat(digest).contains("leakymod");
+    }
+
+    @Test
+    void influenceIsShownAsUnavailableRatherThanZeroWhenAttributionWasSkipped() {
+        String digest = TextDigest.render(Fixtures.report(true, false));
+
+        assertThat(digest).contains("influence n/a").doesNotContain("keeps 0 loaded");
+    }
+
+    @Test
+    void withoutDeepModeItSaysHowToGetCallSites() {
+        assertThat(TextDigest.render(Fixtures.report(false, true)))
+                .contains("-Dchunkinspector.deep=true");
+        assertThat(TextDigest.render(Fixtures.report(true, true)))
+                .doesNotContain("-Dchunkinspector.deep=true");
+    }
+
+    @Test
+    void sectionsWithNothingInThemAreOmittedEntirely() {
+        var empty = new InspectionReport(
+                InspectionReport.SCHEMA,
+                "2026-07-31T10:15:30Z",
+                "periodic",
+                new InspectionReport.Settings(true, true, 600, 1),
+                List.of(LevelAnalysis.analyse(
+                        "minecraft:the_nether", 0, List.of(), List.of(), Fixtures.MAX_LEVEL, true, List.of())));
+
+        String digest = TextDigest.render(empty);
+
+        assertThat(digest).contains("minecraft:the_nether", "0 loaded");
+        assertThat(digest).doesNotContain("by ticket type", "loaded chunk clusters", "ticket call sites");
+    }
+}
@@ -0,0 +1,33 @@
+package example.mod;
+
+import dev.chunkinspector.origin.OriginRegistry;
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Stands in for a third-party mod that adds chunk tickets. It deliberately lives outside the
+ * {@code dev.chunkinspector} package, because the registry filters our own frames out of every
+ * capture — a test calling the registry directly would leave nothing to attribute.
+ */
+public final class LeakyChunkLoader {
+
+    private final OriginRegistry registry;
+
+    public LeakyChunkLoader(OriginRegistry registry) {
+        this.registry = registry;
+    }
+
+    /** Every add comes from the same line, so all of them must fold into one call site. */
+    public List<String> forceLoad(String ticketType, int times) {
+        var ids = new ArrayList<String>();
+        for (int i = 0; i < times; i++) {
+            ids.add(registry.record(ticketType));
+        }
+        return ids;
+    }
+
+    /** A second, distinct call site in the same class. */
+    public String loadOnce(String ticketType) {
+        return registry.record(ticketType);
+    }
+}