package client

import (
	"encoding/json"
	"fmt"
	"os"
	"strconv"
	"strings"
	"time"
)

// Protocol constants (mirror of the Java Protocol class; see PROTOCOL.md).
const (
	IntentRedapricot = 17
	ProtocolVersion  = 767 // arbitrary; the hub ignores it

	MagicControl = 0x01
	MagicWorker  = 0x02

	CIDLen = 16

	CtlSessionReady   = 0x00
	CtlRegister       = 0x01
	CtlUnregister     = 0x02
	CtlRegisterAck    = 0x03
	CtlControlRequest = 0x04
	CtlPing           = 0x05
	CtlPong           = 0x06

	MuxSyn  = 0x00
	MuxData = 0x01
	MuxFin  = 0x02
	MuxRst  = 0x03
	MuxWnd  = 0x04
	MuxPing = 0x05
	MuxPong = 0x06
	// MuxResume reattaches a parked stream to this conn (CID + our accepted
	// offset); MuxResumeAck carries the hub's accepted offset and a fresh CID.
	MuxResume    = 0x07
	MuxResumeAck = 0x08

	// MuxCtlSid is the reserved stream id carrying connection-scoped mux frames
	// (PING/PONG). Real streams are numbered from 1.
	MuxCtlSid = 0

	// RST reason codes (optional trailing byte; absence means "unspecified").
	// Distinguishing them matters for resume: an unknown stream is terminal,
	// while "already bound" means a racing attempt won and this one must leave
	// the stream alone rather than tear down a player the hub just rebound.
	RstUnspecified     = 0x00
	RstUnknownStream   = 0x01
	RstAlreadyBound    = 0x02
	RstResumeAbandoned = 0x03
	RstFlowControl     = 0x04
	RstDialFailed      = 0x05

	FrameError = 0x7F

	// SaturationThreshold caps how many streams share one worker conn once the
	// pool has grown to maxConn. Below maxConn the pool grows first (§7.1), so a
	// single connection is never a shared point of failure for every player.
	SaturationThreshold = 8

	// Session-establishment feature flags (trailing VarInt on the Rekey message).
	FlagStreamFC = 0x01
	// FlagWorkerHeartbeat enables mux-level PING/PONG on worker conns. Without
	// it a worker conn whose path is silently blackholed (NAT/conntrack drop,
	// firewall) is never detected: the read loop parks forever, the dead conn
	// stays in the pool, and no player can be served until the client restarts.
	FlagWorkerHeartbeat = 0x02
	// FlagStreamResume enables stream resumption (PROTOCOL.md §7.5): a worker
	// conn drop parks its streams instead of killing them, the hub hangs the
	// player sockets, and the client reattaches each stream byte-exactly over a
	// fresh conn. Negotiated, so either side may decline and get today's
	// behaviour (immediate teardown) unchanged.
	FlagStreamResume = 0x04

	// Per-stream flow-control window bounds (bytes). The advertised window is the
	// receiver's promise of how much un-credited DATA it will buffer per stream.
	DefaultStreamWindow = 256 * 1024
	MinStreamWindow     = 32 * 1024
	MaxStreamWindow     = 8 << 20

	// DataChunkSize caps a single DATA frame's payload so no stream monopolizes
	// the shared worker connection for long.
	DataChunkSize = 32 * 1024
)

// Egress bandwidth shaping (see shaper.go and docs/architecture.md §6). These
// are entirely client-local: nothing here appears on the wire.
const (
	// MinBandwidth floors a configured cap. Below this the tunnel cannot carry a
	// Minecraft session at all, so such a value is a unit typo ("20bps" for
	// "20mbps") and is rejected rather than silently clamped.
	MinBandwidth = 8 * 1024

	// ShaperBurstSeconds is how much transmission time the token bucket banks
	// while idle. Big enough to absorb a chunk-load spike; small enough that
	// releasing it cannot overrun the physical uplink and rebuild the standing
	// queue the cap exists to prevent.
	ShaperBurstSeconds = 0.2

	// MinShaperBurst must exceed DataChunkSize: a request larger than the bucket
	// could never be afforded and would park forever.
	MinShaperBurst = 64 * 1024
	MaxShaperBurst = 4 << 20

	// ShaperSliceSeconds bounds how long one stream holds the link before the
	// scheduler can switch, by sizing the send chunk to that much transmission
	// time. Above ~13 Mbps this yields DataChunkSize and nothing changes.
	ShaperSliceSeconds = 0.02
	MinShaperChunk     = 4 * 1024
)

// Timeouts. Every tunnel socket is covered by one of these: without them a
// silently dropped path (no FIN/RST) leaves the client parked forever.
const (
	// HandshakeTimeout bounds session establishment end to end — the TCP dial,
	// the Rekey write and the SessionReady read. A hub that accepts the socket
	// but never answers must not park the caller (and, for the pool, every other
	// player behind it) indefinitely.
	HandshakeTimeout = 15 * time.Second

	// TCPKeepAlivePeriod asks the kernel to probe idle tunnel sockets, so a peer
	// that becomes unreachable is detected even when no frames are in flight.
	TCPKeepAlivePeriod = 30 * time.Second

	// MissedHeartbeats is how many ping intervals may pass with no reply before
	// a session is declared dead and dropped.
	MissedHeartbeats = 3

	// MinPingIntervalMs floors the configured ping interval so the derived
	// heartbeat timeout can never be short enough to cause spurious drops.
	// Applied in LoadConfig, i.e. to configs that come from disk.
	MinPingIntervalMs = 1000

	// DefaultResumeGraceMs is how long a parked stream keeps trying to reattach
	// before giving up and closing the destination.
	//
	// Chosen against the backend, not the tunnel: a hung player stops answering
	// the game server's KeepAlive, and vanilla disconnects a silent client at
	// 30s. A longer grace would resume sessions the backend then kicks anyway.
	DefaultResumeGraceMs = 15000

	// MinResumeGraceMs floors the grace so it can always fit at least one dial;
	// a grace shorter than HandshakeTimeout could never complete an attempt.
	MinResumeGraceMs = 2000

	// ResumeRetryDelay paces reattach attempts after a failure. Short, because
	// the player is hanging for the whole grace period.
	ResumeRetryDelay = 500 * time.Millisecond

	// maxControlBackoff caps the control-session reconnect delay. The hub holds
	// this client's routes only for its own registration grace, so a backoff that
	// grew past that would strand players it is hanging on our behalf.
	maxControlBackoff = 10 * time.Second

	// ResumeAckTimeout bounds the wait for RESUME_ACK on a conn that completed
	// its handshake but then went quiet, so a wedged hub does not consume the
	// entire grace budget in one attempt.
	ResumeAckTimeout = 10 * time.Second
)

// heartbeatTimeout is how long a session may go without a reply before it is
// considered dead, derived from the configured ping interval.
func (c *Config) heartbeatTimeout() time.Duration {
	return c.pingInterval() * MissedHeartbeats
}

// pingInterval is the configured heartbeat period.
func (c *Config) pingInterval() time.Duration {
	return time.Duration(c.PingIntervalMs) * time.Millisecond
}

// Mapping routes a registered pattern to a real destination.
type Mapping struct {
	Pattern       string `json:"pattern"`
	Destination   string `json:"destination"`
	ProxyProtocol bool   `json:"proxyProtocol"`
	// VelocitySecret, when non-empty, answers the destination's Velocity
	// modern-forwarding login query (velocity:player_info) with this secret,
	// forwarding the player's real IP, username and UUID (see velocity.go).
	VelocitySecret string `json:"velocitySecret"`
}

// Config is the client configuration (PROTOCOL.md §9.2).
type Config struct {
	Server            string `json:"server"`
	PSK               string `json:"psk"`
	MaxConn           int    `json:"maxConn"`
	PingIntervalMs    int    `json:"pingIntervalMs"`
	StreamWindowBytes int    `json:"streamWindowBytes"` // per-stream receive window; 0 = default
	// MaxBandwidth caps what the client sends to the hub, aggregated over every
	// stream on every worker conn — the direction that carries the game server's
	// output to the players, and the one a residential uplink runs out of first.
	// Empty means no limit. See parseBandwidth for the accepted syntax.
	MaxBandwidth string `json:"maxBandwidth"`
	// StreamResume enables stream resumption (PROTOCOL.md §7.5). A pointer so an
	// absent key means "on" while an explicit false disables it: with it off the
	// client never offers the flag, allocates no retransmit buffers, and behaves
	// exactly as a pre-resume client.
	StreamResume *bool `json:"streamResume"`
	// ResumeGraceMs bounds how long a parked stream keeps trying to reattach.
	// Clamped below the hub's advertised grace so the client always gives up
	// first and the hub is never left holding a player nobody will claim.
	ResumeGraceMs int `json:"resumeGraceMs"`
	// StatsIntervalMs enables the periodic performance summary; 0 (the default)
	// disables it and costs nothing.
	StatsIntervalMs int       `json:"statsIntervalMs"`
	Mappings        []Mapping `json:"mappings"`
}

// resumeEnabled reports whether stream resumption is configured on.
func (c *Config) resumeEnabled() bool { return c.StreamResume == nil || *c.StreamResume }

// resumeGrace is how long a parked stream may keep trying to reattach.
func (c *Config) resumeGrace() time.Duration {
	ms := c.ResumeGraceMs
	if ms <= 0 {
		ms = DefaultResumeGraceMs
	}
	if ms < MinResumeGraceMs {
		ms = MinResumeGraceMs
	}
	return time.Duration(ms) * time.Millisecond
}

// bandwidthUnits maps a rate suffix to its value in bytes per second. Bit units
// are decimal because that is what ISPs quote; byte units are binary to match
// streamWindowBytes. Ordered longest-suffix-first so "kbps" is not read as
// "bps", nor "gb/s" as "b/s".
var bandwidthUnits = []struct {
	suffix string
	mul    float64
}{
	{"gbps", 1e9 / 8}, {"gbit", 1e9 / 8},
	{"mbps", 1e6 / 8}, {"mbit", 1e6 / 8},
	{"kbps", 1e3 / 8}, {"kbit", 1e3 / 8},
	{"gb/s", 1 << 30}, {"mb/s", 1 << 20}, {"kb/s", 1 << 10},
	{"bps", 1.0 / 8},
	{"b/s", 1},
}

// parseBandwidth converts a human-readable rate to bytes per second. The empty
// string means "no limit" and yields 0.
//
//	"20mbps"  20 megabits/s  = 2500000 B/s
//	"512kbps" 512 kilobits/s =   64000 B/s
//	"2MB/s"   2 mebibytes/s  = 2097152 B/s
//	"1500000" a bare number is already bytes per second
func parseBandwidth(s string) (int64, error) {
	s = strings.TrimSpace(s)
	if s == "" {
		return 0, nil
	}
	lower := strings.ToLower(s)
	num, mul := lower, 1.0
	for _, u := range bandwidthUnits {
		if strings.HasSuffix(lower, u.suffix) {
			num, mul = strings.TrimSpace(lower[:len(lower)-len(u.suffix)]), u.mul
			break
		}
	}
	v, err := strconv.ParseFloat(num, 64)
	if err != nil || v <= 0 {
		return 0, fmt.Errorf(`maxBandwidth: cannot read %q as a rate (try "20mbps", "2MB/s", or bytes per second)`, s)
	}
	return int64(v * mul), nil
}

// LoadConfig reads and validates a JSON config file.
func LoadConfig(path string) (*Config, error) {
	data, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	var c Config
	if err := json.Unmarshal(data, &c); err != nil {
		return nil, fmt.Errorf("parse config: %w", err)
	}
	if c.Server == "" {
		return nil, fmt.Errorf("server is required")
	}
	if c.PSK == "" {
		return nil, fmt.Errorf("psk is required")
	}
	if c.MaxConn < 1 {
		c.MaxConn = 1
	}
	if c.MaxConn > 8 {
		c.MaxConn = 8
	}
	if c.PingIntervalMs <= 0 {
		c.PingIntervalMs = 20000
	}
	if c.PingIntervalMs < MinPingIntervalMs {
		c.PingIntervalMs = MinPingIntervalMs
	}
	// Parsed here only to fail fast on a bad value; New does the real conversion.
	bps, err := parseBandwidth(c.MaxBandwidth)
	if err != nil {
		return nil, err
	}
	if bps > 0 && bps < MinBandwidth {
		return nil, fmt.Errorf("maxBandwidth %q is only %d B/s; the minimum is %d B/s", c.MaxBandwidth, bps, MinBandwidth)
	}
	if len(c.Mappings) == 0 {
		return nil, fmt.Errorf("at least one mapping is required")
	}
	return &c, nil
}

// NormalizeAddress matches the hub's normalization: lower-cased, FML-suffix and
// trailing-dot stripped.
func NormalizeAddress(addr string) string {
	if i := strings.IndexByte(addr, 0); i >= 0 {
		addr = addr[:i]
	}
	addr = strings.ToLower(addr)
	addr = strings.TrimRight(addr, ".")
	return addr
}