package ircserver

import (
	"bufio"
	"context"
	"fmt"
	"log/slog"
	"net"
	"strconv"
	"strings"
	"sync"
	"time"

	"sneak.berlin/go/neoirc/internal/broker"
	"sneak.berlin/go/neoirc/internal/config"
	"sneak.berlin/go/neoirc/internal/db"
	"sneak.berlin/go/neoirc/internal/service"
	"sneak.berlin/go/neoirc/pkg/irc"
)

const (
	maxLineLen      = 512
	readTimeout     = 5 * time.Minute
	writeTimeout    = 30 * time.Second
	killWriteWindow = 2 * time.Second
	dnsTimeout      = 3 * time.Second
	pollInterval    = 100 * time.Millisecond
	pingInterval    = 90 * time.Second
	pongDeadline    = 30 * time.Second
	maxNickLen      = 32
	minPasswordLen  = 8
	maxHashcashBits = 40
)

// cmdHandler is the signature for registered IRC command
// handlers.
type cmdHandler func(ctx context.Context, msg *Message)

// Conn represents a single IRC client TCP connection.
type Conn struct {
	conn      net.Conn
	log       *slog.Logger
	database  *db.Database
	brk       *broker.Broker
	cfg       *config.Config
	svc       *service.Service
	serverSfx string
	commands  map[string]cmdHandler

	// writeMu serializes writes to conn.  A connection is
	// written to by its own read loop, by its relay
	// goroutine, and — when an operator KILLs it — by
	// another client's goroutine.
	writeMu sync.Mutex

	mu        sync.Mutex
	nick      string
	username  string
	realname  string
	hostname  string
	remoteIP  string
	sessionID int64
	clientID  int64

	registered bool
	gotNick    bool
	gotUser    bool
	passWord   string

	lastQueueID int64
	closed      bool
	killed      bool
}

func newConn(
	ctx context.Context,
	tcpConn net.Conn,
	log *slog.Logger,
	database *db.Database,
	brk *broker.Broker,
	cfg *config.Config,
	svc *service.Service,
) *Conn {
	host, _, _ := net.SplitHostPort(tcpConn.RemoteAddr().String())

	srvName := cfg.ServerName
	if srvName == "" {
		srvName = "neoirc"
	}

	conn := &Conn{ //nolint:exhaustruct // zero-value defaults
		conn:      tcpConn,
		log:       log,
		database:  database,
		brk:       brk,
		cfg:       cfg,
		svc:       svc,
		serverSfx: srvName,
		remoteIP:  host,
		hostname:  resolveHost(ctx, host),
	}

	conn.commands = conn.buildCommandMap()

	return conn
}

// Disconnect ends the connection for an operator KILL from
// either transport: the victim is sent KILL and ERROR, then
// its socket is closed, which ends serve() and with it the
// relay goroutine. It is called from the killer's goroutine
// and does not wait for the writes, so a victim that has
// stopped reading cannot stall the killer.
func (c *Conn) Disconnect(reason string) {
	c.mu.Lock()

	if c.closed {
		c.mu.Unlock()

		return
	}

	c.closed = true
	c.killed = true
	nick := c.nick
	host := c.hostname
	c.mu.Unlock()

	if nick == "" {
		nick = "*"
	}

	go func() {
		c.notifyKilledAndClose(nick, host, reason)
	}()
}

// notifyKilledAndClose sends a killed victim the KILL and
// ERROR lines, each bounded by killWriteWindow, and then
// closes its socket.
func (c *Conn) notifyKilledAndClose(
	nick, host, reason string,
) {
	defer c.conn.Close() //nolint:errcheck

	c.sendWithin(
		killWriteWindow,
		FormatMessage(
			c.serverSfx, irc.CmdKill, nick, reason,
		),
	)
	c.sendWithin(
		killWriteWindow,
		"ERROR :Closing Link: "+host+
			" ("+reason+")",
	)
}

// buildCommandMap returns a map from IRC command strings
// to handler functions.
func (c *Conn) buildCommandMap() map[string]cmdHandler {
	return map[string]cmdHandler{
		irc.CmdPing: func(_ context.Context, msg *Message) {
			c.handlePing(msg)
		},
		"PONG":         func(context.Context, *Message) {},
		irc.CmdNick:    c.handleNick,
		irc.CmdPrivmsg: c.handlePrivmsg,
		irc.CmdNotice:  c.handlePrivmsg,
		irc.CmdJoin:    c.handleJoin,
		irc.CmdPart:    c.handlePart,
		irc.CmdQuit: func(_ context.Context, msg *Message) {
			c.handleQuit(msg)
		},
		irc.CmdTopic:  c.handleTopic,
		irc.CmdMode:   c.handleMode,
		irc.CmdNames:  c.handleNames,
		irc.CmdList:   func(ctx context.Context, _ *Message) { c.handleList(ctx) },
		irc.CmdWhois:  c.handleWhois,
		irc.CmdWho:    c.handleWho,
		irc.CmdLusers: func(ctx context.Context, _ *Message) { c.handleLusers(ctx) },
		irc.CmdMotd:   func(context.Context, *Message) { c.deliverMOTD() },
		irc.CmdOper:   c.handleOper,
		irc.CmdAway:   c.handleAway,
		irc.CmdKick:   c.handleKick,
		irc.CmdPass:   c.handlePassPostReg,
		"INVITE":      c.handleInvite,
		"CAP": func(_ context.Context, msg *Message) {
			c.handleCAP(msg)
		},
		"USERHOST":     c.handleUserhost,
		irc.CmdVersion: func(context.Context, *Message) { c.handleVersion() },
		irc.CmdAdmin:   func(context.Context, *Message) { c.handleAdmin() },
		irc.CmdInfo:    func(context.Context, *Message) { c.handleInfo() },
		irc.CmdTime:    func(context.Context, *Message) { c.handleTime() },
		irc.CmdKill:    c.handleKillCmd,
		irc.CmdWallops: c.handleWallopsCmd,
	}
}

// resolveHost does a reverse DNS lookup, returning the IP
// on failure.
func resolveHost(ctx context.Context, addr string) string {
	ctx, cancel := context.WithTimeout(ctx, dnsTimeout)
	defer cancel()

	resolver := &net.Resolver{} //nolint:exhaustruct

	names, err := resolver.LookupAddr(ctx, addr)
	if err != nil || len(names) == 0 {
		return addr
	}

	return strings.TrimSuffix(names[0], ".")
}

// serve is the main loop for a single IRC client connection.
// Cancelling ctx when it returns stops the relay goroutine.
func (c *Conn) serve(ctx context.Context) {
	ctx, cancel := context.WithCancel(ctx)
	defer cancel()
	defer c.cleanup(ctx)

	scanner := bufio.NewScanner(c.conn)
	scanner.Buffer(make([]byte, maxLineLen), maxLineLen)

	for {
		_ = c.conn.SetReadDeadline(
			time.Now().Add(readTimeout),
		)

		if !scanner.Scan() {
			return
		}

		line := scanner.Text()
		if line == "" {
			continue
		}

		msg := ParseMessage(line)
		if msg == nil {
			continue
		}

		c.handleMessage(ctx, msg)

		if c.isClosed() {
			return
		}
	}
}

func (c *Conn) cleanup(ctx context.Context) {
	c.mu.Lock()
	wasRegistered := c.registered
	wasKilled := c.killed
	sessID := c.sessionID
	nick := c.nick
	c.closed = true
	c.mu.Unlock()

	if wasRegistered && sessID > 0 {
		c.svc.UnregisterWireConn(sessID, c)

		// A KILLed session has already been broadcast and
		// deleted by the killer; broadcasting again would
		// fan out a QUIT for a session row that is gone.
		if !wasKilled {
			c.svc.BroadcastQuit(
				ctx, sessID, nick, "Connection closed",
			)
		}
	}

	c.conn.Close() //nolint:errcheck,gosec
}

// isClosed reports whether the connection has been marked
// for teardown, either by QUIT or by an operator KILL.
func (c *Conn) isClosed() bool {
	c.mu.Lock()
	defer c.mu.Unlock()

	return c.closed
}

// currentNick returns the connection's registered nick.
// c.nick is written under c.mu during registration and
// NICK changes, so every read must take the mutex.
func (c *Conn) currentNick() string {
	c.mu.Lock()
	defer c.mu.Unlock()

	return c.nick
}

// send writes a formatted IRC line to the connection.
func (c *Conn) send(line string) {
	c.sendWithin(writeTimeout, line)
}

// sendWithin writes a formatted IRC line to the connection
// under the given write deadline.  Callers that must not be
// held hostage by an unresponsive peer pass a shorter window
// than writeTimeout.
func (c *Conn) sendWithin(
	timeout time.Duration,
	line string,
) {
	c.writeMu.Lock()
	defer c.writeMu.Unlock()

	_ = c.conn.SetWriteDeadline(
		time.Now().Add(timeout),
	)

	_, _ = fmt.Fprintf(c.conn, "%s\r\n", line)
}

// sendNumeric sends a numeric reply from the server.
func (c *Conn) sendNumeric(
	code irc.IRCMessageType,
	params ...string,
) {
	nick := c.nick
	if nick == "" {
		nick = "*"
	}

	allParams := make([]string, 0, 1+len(params))
	allParams = append(allParams, nick)
	allParams = append(allParams, params...)

	c.send(FormatMessage(
		c.serverSfx, code.Code(), allParams...,
	))
}

// sendFromServer sends a message from the server.
func (c *Conn) sendFromServer(
	command string, params ...string,
) {
	c.send(FormatMessage(c.serverSfx, command, params...))
}

// hostmask returns the client's full hostmask
// (nick!user@host).
func (c *Conn) hostmask() string {
	user := c.username
	if user == "" {
		user = c.nick
	}

	host := c.hostname
	if host == "" {
		host = c.remoteIP
	}

	return c.nick + "!" + user + "@" + host
}

// handleMessage dispatches a parsed IRC message using
// the command handler map.
func (c *Conn) handleMessage(
	ctx context.Context,
	msg *Message,
) {
	// Before registration, only NICK, USER, PASS, PING,
	// QUIT, and CAP are accepted.
	if !c.registered {
		c.handlePreRegistration(ctx, msg)

		return
	}

	handler, ok := c.commands[msg.Command]
	if !ok {
		c.sendNumeric(
			irc.ErrUnknownCommand,
			msg.Command, "Unknown command",
		)

		return
	}

	handler(ctx, msg)
}

// handlePreRegistration handles messages before the
// connection is registered (NICK+USER received).
func (c *Conn) handlePreRegistration(
	ctx context.Context,
	msg *Message,
) {
	switch msg.Command {
	case irc.CmdPass:
		if len(msg.Params) < 1 {
			c.sendNumeric(
				irc.ErrNeedMoreParams,
				"PASS", "Not enough parameters",
			)

			return
		}

		c.passWord = msg.Params[0]
	case irc.CmdNick:
		if len(msg.Params) < 1 {
			c.sendNumeric(
				irc.ErrNoNicknameGiven,
				"No nickname given",
			)

			return
		}

		c.nick = msg.Params[0]
		if len(c.nick) > maxNickLen {
			c.nick = c.nick[:maxNickLen]
		}

		c.gotNick = true
	case irc.CmdUser:
		if len(msg.Params) < 4 { //nolint:mnd
			c.sendNumeric(
				irc.ErrNeedMoreParams,
				"USER", "Not enough parameters",
			)

			return
		}

		c.username = msg.Params[0]
		c.realname = msg.Params[3]
		c.gotUser = true
	case irc.CmdPing:
		c.handlePing(msg)

		return
	case irc.CmdQuit:
		c.handleQuit(msg)

		return
	case "CAP":
		c.handleCAP(msg)

		return
	default:
		c.sendNumeric(
			irc.ErrNotRegistered,
			"You have not registered",
		)

		return
	}

	// Try to complete registration once we have both
	// NICK and USER.
	if c.gotNick && c.gotUser {
		c.completeRegistration(ctx)
	}
}

// completeRegistration creates a session and sends the
// welcome burst.
func (c *Conn) completeRegistration(ctx context.Context) {
	// Check if nick is valid.
	if c.nick == "" {
		c.sendNumeric(
			irc.ErrNoNicknameGiven, "No nickname given",
		)

		return
	}

	// Create session in DB.
	sessionID, clientID, _, err := c.database.CreateSession(
		ctx, c.nick, c.username, c.hostname, c.remoteIP,
	)
	if err != nil {
		if strings.Contains(err.Error(), "UNIQUE constraint") ||
			strings.Contains(err.Error(), "nick") {
			c.sendNumeric(
				irc.ErrNicknameInUse,
				c.nick, "Nickname is already in use",
			)

			return
		}

		c.log.Error(
			"failed to create session", "error", err,
		)
		c.send("ERROR :Internal server error")

		c.mu.Lock()
		c.closed = true
		c.mu.Unlock()

		return
	}

	c.mu.Lock()
	c.sessionID = sessionID
	c.clientID = clientID
	c.registered = true
	c.mu.Unlock()

	// Make this connection reachable by session ID so that
	// KILL from either transport can disconnect it.
	c.svc.RegisterWireConn(sessionID, c)

	// If PASS was provided before registration, set the
	// session password.
	if c.passWord != "" && len(c.passWord) >= minPasswordLen {
		c.setPassword(ctx, c.passWord)
	}

	// Send welcome burst.
	c.deliverWelcome()
	c.deliverLusers(ctx)
	c.deliverMOTD()

	// Start the message relay goroutine.
	go c.relayMessages(ctx)
}

// deliverWelcome sends 001-005 welcome numerics.
func (c *Conn) deliverWelcome() {
	c.sendNumeric(irc.RplWelcome, fmt.Sprintf(
		"Welcome to the %s Network, %s",
		c.serverSfx, c.hostmask(),
	))
	c.sendNumeric(irc.RplYourHost, fmt.Sprintf(
		"Your host is %s, running version neoirc",
		c.serverSfx,
	))
	c.sendNumeric(
		irc.RplCreated,
		"This server was created recently",
	)
	c.sendNumeric(
		irc.RplMyInfo,
		c.serverSfx, "neoirc", "", "mnst",
	)
	c.sendNumeric(
		irc.RplIsupport,
		"CHANTYPES=#",
		"NICKLEN=32",
		"PREFIX=(ov)@+",
		"CHANMODES=,,H,imnst",
		"NETWORK="+c.serverSfx,
		"are supported by this server",
	)
}

// deliverLusers sends 251/252/254/255 server statistics.
func (c *Conn) deliverLusers(ctx context.Context) {
	users, _ := c.database.GetUserCount(ctx)
	opers, _ := c.database.GetOperCount(ctx)
	channels, _ := c.database.GetChannelCount(ctx)

	c.sendNumeric(irc.RplLuserClient, fmt.Sprintf(
		"There are %d users and 0 invisible on 1 servers",
		users,
	))
	c.sendNumeric(
		irc.RplLuserOp,
		strconv.FormatInt(opers, 10),
		"operator(s) online",
	)
	c.sendNumeric(
		irc.RplLuserChannels,
		strconv.FormatInt(channels, 10),
		"channels formed",
	)
	c.sendNumeric(irc.RplLuserMe, fmt.Sprintf(
		"I have %d clients and 1 servers", users,
	))
}

// deliverMOTD sends 375/372/376 MOTD lines.
func (c *Conn) deliverMOTD() {
	motd := c.cfg.MOTD
	if motd == "" {
		c.sendNumeric(
			irc.ErrNoMotd, "MOTD File is missing",
		)

		return
	}

	c.sendNumeric(irc.RplMotdStart, fmt.Sprintf(
		"- %s Message of the Day -", c.serverSfx,
	))

	for line := range strings.SplitSeq(motd, "\n") {
		c.sendNumeric(irc.RplMotd, "- "+line)
	}

	c.sendNumeric(
		irc.RplEndOfMotd, "End of /MOTD command",
	)
}

// setPassword sets a bcrypt password on the session.
func (c *Conn) setPassword(ctx context.Context, pw string) {
	// Use the database's auth module to hash and store.
	err := c.database.SetPassword(ctx, c.sessionID, pw)
	if err != nil {
		c.log.Error(
			"failed to set password", "error", err,
		)
	}
}