package client

import (
	"encoding/json"
	"fmt"
	"os"
	"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

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

	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

	// 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
)

// 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
)

// 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
	Mappings          []Mapping `json:"mappings"`
}

// 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
	}
	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
}