//
// Copyright (c) 2014 - 2018, Přemysl Janouch <p@janouch.name>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
//

// hid is a straight-forward port of kike IRCd from C.
package main

/*

// ANSI terminal formatting, would be better if we had isatty() available
func tf(text string, ansi string) string {
	return "\x1b[0;" + ansi + "m" + text + "\x1b[0m"
}

func logErrorf(format string, args ...interface{}) {
	fmt.Fprintf(os.Stderr, tf("error: "+format+"\n", "1;31"), args...)
}

func logFatalf(format string, args ...interface{}) {
	fmt.Fprintf(os.Stderr, tf("fatal: "+format+"\n", "1;31"), args...)
	os.Exit(1)
}

func logFatal(object interface{}) {
	logFatalf("%s", object)
}

func getHome() (home string) {
	if u, _ := user.Current(); u != nil {
		home = u.HomeDir
	} else {
		home = os.Getenv("HOME")
	}
	return
}

// Only handling the simple case as that's what one mostly wants.
// TODO(p): Handle the generic case as well.
func expandTilde(path string) string {
	if strings.HasPrefix(path, "~/") {
		return getHome() + path[1:]
	}
	return path
}

func getXdgHomeDir(name, def string) string {
	env := os.Getenv(name)
	if env != "" && env[0] == '/' {
		return env
	}
	return filepath.Join(getHome(), def)
}

// Retrieve all XDG base directories for configuration files
func getXdgConfigDirs() (result []string) {
	home := getXdgHomeDir("XDG_CONFIG_HOME", ".config")
	if home != "" {
		result = append(result, home)
	}
	dirs := os.Getenv("XDG_CONFIG_DIRS")
	if dirs == "" {
		dirs = "/etc/xdg"
	}
	for _, path := range strings.Split(dirs, ":") {
		if path != "" {
			result = append(result, path)
		}
	}
	return
}

// Read a configuration file with the given basename w/o extension
func readConfigFile(name string, output interface{}) error {
	var suffix = filepath.Join(projectName, name+".json")
	for _, path := range getXdgConfigDirs() {
		full := filepath.Join(path, suffix)
		file, err := os.Open(full)
		if err != nil {
			if !os.IsNotExist(err) {
				return err
			}
			continue
		}
		defer file.Close()

		decoder := json.NewDecoder(file)
		err = decoder.Decode(output)
		if err != nil {
			return fmt.Errorf("%s: %s", full, err)
		}
		return nil
	}
	return errors.New("configuration file not found")
}

*/

import (
	"bufio"
	"crypto/sha256"
	"crypto/tls"
	"encoding/hex"
	"flag"
	"fmt"
	"io"
	"log"
	"net"
	"os"
	"os/signal"
	"path/filepath"
	"regexp"
	"strconv"
	"strings"
	"syscall"
	"time"
)

var debugMode = false

// --- Utilities ---------------------------------------------------------------

//
// Trivial SSL/TLS autodetection. The first block of data returned by Recvfrom
// must be at least three octets long for this to work reliably, but that should
// not pose a problem in practice. We might try waiting for them.
//
//  SSL2:      1xxx xxxx | xxxx xxxx |    <1>
//                (message length)  (client hello)
//  SSL3/TLS:    <22>    |    <3>    | xxxx xxxx
//            (handshake)|  (protocol version)
//
func detectTLS(sysconn syscall.RawConn) (isTLS bool) {
	sysconn.Read(func(fd uintptr) (done bool) {
		var buf [3]byte
		n, _, err := syscall.Recvfrom(int(fd), buf[:], syscall.MSG_PEEK)
		switch {
		case n == 3:
			isTLS = buf[0]&0x80 != 0 && buf[2] == 1
			fallthrough
		case n == 2:
			isTLS = buf[0] == 22 && buf[1] == 3
		case n == 1:
			isTLS = buf[0] == 22
		case err == syscall.EAGAIN:
			return false
		}
		return true
	})
	return isTLS
}

// --- Configuration -----------------------------------------------------------

// XXX: Do we really want to support default nil values?
var config = []struct {
	key         string // INI key
	def         []rune // default value, may be nil
	description string // documentation
}{
	// XXX: I'm not sure if Go will cooperate here.
	{"pid_file", nil, "Path or name of the PID file"},
	{"bind", []rune(":6667"), "Address of the IRC server"},
}

// --- Rate limiter ------------------------------------------------------------

type floodDetector struct {
	interval   uint    // interval for the limit
	limit      uint    // maximum number of events allowed
	timestamps []int64 // timestamps of last events
	pos        uint    // index of the oldest event
}

func newFloodDetector(interval, limit uint) *floodDetector {
	return &floodDetector{
		interval:   interval,
		limit:      limit,
		timestamps: make([]int64, limit+1),
		pos:        0,
	}
}

func (fd *floodDetector) check() bool {
	now := time.Now().Unix()
	fd.timestamps[fd.pos] = now

	fd.pos++
	if fd.pos > fd.limit {
		fd.pos = 0
	}

	var count uint
	begin := now - int64(fd.interval)
	for _, ts := range fd.timestamps {
		if ts >= begin {
			count++
		}
	}
	return count <= fd.limit
}

// --- IRC protocol ------------------------------------------------------------

//go:generate sh -c "./hid-gen-replies.sh > hid-replies.go < hid-replies"

func ircToLower(c byte) byte {
	switch c {
	case '[':
		return '{'
	case ']':
		return '}'
	case '\\':
		return '|'
	case '~':
		return '^'
	}
	if c >= 'A' && c <= 'Z' {
		return c + ('a' - 'A')
	}
	return c
}

// TODO: To support ALL CAPS initialization of maps, perhaps we should use
// ircToUpper instead.
// FIXME: This doesn't follow the meaning of strxfrm and perhaps should be
// renamed to ircNormalize.
func ircStrxfrm(ident string) string {
	var canon []byte
	for _, c := range []byte(ident) {
		canon = append(canon, ircToLower(c))
	}
	return string(canon)
}

func ircFnmatch(pattern string, s string) bool {
	pattern, s = ircStrxfrm(pattern), ircStrxfrm(s)
	// FIXME: This should not support [] ranges and handle / specially.
	// We could translate the pattern to a regular expression.
	matched, _ := filepath.Match(pattern, s)
	return matched
}

// TODO: We will need to add support for IRCv3 tags.
var reMsg = regexp.MustCompile(
	`^(?::([^! ]*)(?:!([^@]*)@([^ ]*))? +)?([^ ]+)(.*)?$`)
var reArgs = regexp.MustCompile(`:.*| [^: ][^ ]*`)

type message struct {
	nick    string   // optional nickname
	user    string   // optional username
	host    string   // optional hostname or IP address
	command string   // command name
	params  []string // arguments
}

// Everything as per RFC 2812
const (
	ircMaxNickname      = 9
	ircMaxHostname      = 63
	ircMaxChannelName   = 50
	ircMaxMessageLength = 510
)

// TODO: Port the IRC token validation part as needed.

const reClassSpecial = "\\[\\]\\\\`_^{|}"

var (
	// Extending ASCII to the whole range of Unicode letters.
	reNickname = regexp.MustCompile(
		`^[\pL` + reClassSpecial + `][\pL` + reClassSpecial + `0-9-]*$`)

	// Notably, this won't match invalid UTF-8 characters, although the
	// behaviour seems to be unstated in the documentation.
	reUsername = regexp.MustCompile(`^[^\0\r\n @]+$`)

	reChannelName = regexp.MustCompile(`^[^\0\7\r\n ,:]+$`)
)

func ircIsValidNickname(nickname string) bool {
	return len(nickname) <= ircMaxNickname && reNickname.MatchString(nickname)
}

func ircIsValidUsername(username string) bool {
	// XXX: We really should put a limit on this
	// despite the RFC not mentioning one.
	return reUsername.MatchString(username)
}

func ircIsValidChannelName(name string) bool {
	return len(name) <= ircMaxChannelName && reChannelName.MatchString(name)
}

// --- Clients (equals users) --------------------------------------------------

type connCloseWrite interface {
	net.Conn
	CloseWrite() error
}

const ircSupportedUserModes = "aiwros"

const (
	ircUserModeInvisible = 1 << iota
	ircUserModeRxWallops
	ircUserModeRestricted
	ircUserModeOperator
	ircUserModeRxServerNotices
)

const (
	ircCapMultiPrefix = 1 << iota
	ircCapInviteNotify
	ircCapEchoMessage
	ircCapUserhostInNames
	ircCapServerTime
)

type client struct {
	transport net.Conn       // underlying connection
	tls       *tls.Conn      // TLS, if detected
	conn      connCloseWrite // high-level connection
	inQ       []byte         // unprocessed input
	outQ      []byte         // unprocessed output
	reading   bool           // whether a reading goroutine is running
	writing   bool           // whether a writing goroutine is running
	closing   bool           // whether we're closing the connection
	killTimer *time.Timer    // hard kill timeout

	opened            int64 // when the connection was opened
	nSentMessages     uint  // number of sent messages total
	sentBytes         int   // number of sent bytes total
	nReceivedMessages uint  // number of received messages total
	receivedBytes     int   // number of received bytes total

	hostname string // hostname or IP shown to the network
	port     string // port of the peer as a string
	address  string // full network address

	pingTimer      *time.Timer // we should send a PING
	timeoutTimer   *time.Timer // connection seems to be dead
	registered     bool        // the user has registered
	capNegotiating bool        // negotiating capabilities
	capsEnabled    uint        // enabled capabilities
	capVersion     uint        // CAP protocol version

	tlsCertFingerprint string // client certificate fingerprint

	nickname string // IRC nickname (main identifier)
	username string // IRC username
	realname string // IRC realname (or e-mail)

	mode        uint            // user's mode
	awayMessage string          // away message
	lastActive  int64           // last PRIVMSG, to get idle time
	invites     map[string]bool // channel invitations by operators
	antiflood   floodDetector   // flood detector
}

// --- Channels ----------------------------------------------------------------

const ircSupportedChanModes = "ov" + "beI" + "imnqpst" + "kl"

const (
	ircChanModeInviteOnly = 1 << iota
	ircChanModeModerated
	ircChanModeNoOutsideMsgs
	ircChanModeQuiet
	ircChanModePrivate
	ircChanModeSecret
	ircChanModeProtectedTopic

	ircChanModeOperator
	ircChanModeVoice
)

type channel struct {
	name      string // channel name
	modes     uint   // channel modes
	key       string // channel key
	userLimit int    // user limit or -1
	created   int64  // creation time

	topic     string // channel topic
	topicWho  string // who set the topic
	topicTime int64  // when the topic was set

	userModes map[*client]uint // modes for all channel users

	banList       []string // ban list
	exceptionList []string // exceptions from bans
	inviteList    []string // exceptions from +I
}

func newChannel() *channel {
	return &channel{userLimit: -1}
}

// TODO: Port struct channel methods.

// --- IRC server context ------------------------------------------------------

type whowasInfo struct {
	nickname, username, realname, hostname string
}

func newWhowasInfo(c *client) *whowasInfo {
	return &whowasInfo{
		nickname: c.nickname,
		username: c.username,
		realname: c.realname,
		hostname: c.hostname,
	}
}

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

type ircCommand struct {
	name                 string
	requiresRegistration bool
	handler              func(*message, *client)

	nReceived     uint // number of commands received
	bytesReceived uint // number of bytes received total
}

type preparedEvent struct {
	client *client
	host   string // client's hostname or literal IP address
	isTLS  bool   // the client seems to use TLS
}

type readEvent struct {
	client *client
	data   []byte // new data from the client
	err    error  // read error
}

type writeEvent struct {
	client  *client
	written int   // amount of bytes written
	err     error // write error
}

// TODO: Port server_context. Maybe we want to keep it in a struct?
// XXX: Beware that maps with identifier keys need to be indexed correctly.
// We might want to enforce accessor functions for users and channels.
var (
	started int64 // when has the server been started

	users       map[string]*client  // maps nicknames to clients
	channels    map[string]*channel // maps channel names to data
	handlers    map[string]bool     // TODO message handlers
	capHandlers map[string]bool     // TODO CAP message handlers

	whowas map[string]*whowasInfo // WHOWAS registry

	serverName     string          // our server name
	pingInterval   uint            // ping interval in seconds
	maxConnections int             // max connections allowed or 0
	motd           []string        // MOTD (none if empty)
	operators      map[string]bool // TLS certificate fingerprints for IRCops
)

var (
	sigs     = make(chan os.Signal, 1)
	conns    = make(chan net.Conn)
	prepared = make(chan preparedEvent)
	reads    = make(chan readEvent)
	writes   = make(chan writeEvent)
	timeouts = make(chan *client)

	tlsConf  *tls.Config
	clients  = make(map[*client]bool)
	listener net.Listener
	// TODO: quitting, quitTimer as they are named in kike?
	inShutdown    bool
	shutdownTimer <-chan time.Time
)

// Forcefully tear down all connections.
func forceShutdown(reason string) {
	if !inShutdown {
		log.Fatalln("forceShutdown called without initiateShutdown")
	}

	log.Printf("forced shutdown (%s)\n", reason)
	for c := range clients {
		c.destroy("TODO")
	}
}

// Initiate a clean shutdown of the whole daemon.
func initiateShutdown() {
	log.Println("shutting down")
	if err := listener.Close(); err != nil {
		log.Println(err)
	}
	for c := range clients {
		c.closeLink("TODO")
	}

	shutdownTimer = time.After(5 * time.Second)
	inShutdown = true
}

// TODO: ircChannelCreate

func ircChannelDestroyIfEmpty(ch *channel) {
	// TODO
}

// TODO: ircSendToRoommates
// Broadcast to all /other/ clients (telnet-friendly, also in accordance to
// the plan of extending this to an IRCd).
func broadcast(line string, except *client) {
	for c := range clients {
		if c != except {
			c.send(line)
		}
	}
}

func ircSendToRoommates(c *client, message string) {
	// TODO
}

// --- Clients (continued) -----------------------------------------------------

// TODO: Perhaps we should append to *[]byte for performance.
func clientModeToString(m uint, mode *string) {
	if 0 != m&ircUserModeInvisible {
		*mode += "i"
	}
	if 0 != m&ircUserModeRxWallops {
		*mode += "w"
	}
	if 0 != m&ircUserModeRestricted {
		*mode += "r"
	}
	if 0 != m&ircUserModeOperator {
		*mode += "o"
	}
	if 0 != m&ircUserModeRxServerNotices {
		*mode += "s"
	}
}

func (c *client) getMode() string {
	mode := ""
	if c.awayMessage != "" {
		mode += "a"
	}
	clientModeToString(c.mode, &mode)
	return mode
}

func (c *client) send(line string) {
	if c.conn == nil || c.closing {
		return
	}

	// TODO: Rename inQ and outQ to recvQ and sendQ as they are usually named.
	oldOutQ := len(c.outQ)

	// So far there's only one message tag we use, so we can do it simple;
	// note that a 1024-character limit applies to messages with tags on.
	if 0 != c.capsEnabled&ircCapServerTime {
		c.outQ = time.Now().UTC().
			AppendFormat(c.outQ, "@time=2006-01-02T15:04:05.000Z ")
	}

	bytes := []byte(line)
	if len(bytes) > ircMaxMessageLength {
		bytes = bytes[:ircMaxMessageLength]
	}

	// TODO: Kill the connection above some "SendQ" threshold (careful!)
	c.outQ = append(c.outQ, bytes...)
	c.outQ = append(c.outQ, "\r\n"...)
	c.flushOutQ()

	// Technically we haven't sent it yet but that's a minor detail
	c.nSentMessages++
	c.sentBytes += len(c.outQ) - oldOutQ
}

func (c *client) sendf(format string, a ...interface{}) {
	c.send(fmt.Sprintf(format, a...))
}

func (c *client) addToWhowas() {
	// Only keeping one entry for each nickname.
	// TODO: Make sure this list doesn't get too long, for example by
	// putting them in a linked list ordered by time.
	whowas[ircStrxfrm(c.nickname)] = newWhowasInfo(c)
}

func (c *client) unregister(reason string) {
	if !c.registered {
		return
	}

	ircSendToRoommates(c, fmt.Sprintf(":%s!%s@%s QUIT :%s",
		c.nickname, c.username, c.hostname, reason))

	// The eventual QUIT message will take care of state at clients.
	for _, ch := range channels {
		delete(ch.userModes, c)
		ircChannelDestroyIfEmpty(ch)
	}

	c.addToWhowas()
	delete(users, ircStrxfrm(c.nickname))
	c.nickname = ""
	c.registered = false
}

// TODO: Rename to kill.
// Close the connection and forget about the client.
func (c *client) destroy(reason string) {
	if reason == "" {
		reason = "Client exited"
	}
	c.unregister(reason)

	// TODO: Log the address; seems like we always have c.address.
	log.Println("client destroyed")

	// Try to send a "close notify" alert if the TLS object is ready,
	// otherwise just tear down the transport.
	if c.conn != nil {
		_ = c.conn.Close()
	} else {
		_ = c.transport.Close()
	}

	// Clean up the goroutine, although a spurious event may still be sent.
	// TODO: Other timers if needed.
	if c.killTimer != nil {
		c.killTimer.Stop()
	}

	delete(clients, c)
}

// Tear down the client connection, trying to do so in a graceful manner.
func (c *client) closeLink(reason string) {
	// Let's just cut the connection, the client can try again later.
	// We also want to avoid accidentally writing to the socket before
	// address resolution has finished.
	if c.conn == nil {
		c.destroy(reason)
		return
	}
	if c.closing {
		return
	}

	nickname := c.nickname
	if nickname == "" {
		nickname = "*"
	}

	// We push an "ERROR" message to the write buffer and let the writer send
	// it, with some arbitrary timeout. The "closing" state makes sure
	// that a/ we ignore any successive messages, and b/ that the connection
	// is killed after the write buffer is transferred and emptied.
	// (Since we send this message, we don't need to call CloseWrite here.)
	c.sendf("ERROR :Closing link: %s[%s] (%s)",
		nickname, c.hostname /* TODO host IP? */, reason)
	c.closing = true

	c.unregister(reason)
	c.killTimer = time.AfterFunc(3*time.Second, func() {
		timeouts <- c
	})
}

func (c *client) inMaskList(masks []string) bool {
	client := fmt.Sprintf("%s!%s@%s", c.nickname, c.username, c.hostname)
	for _, mask := range masks {
		if ircFnmatch(mask, client) {
			return true
		}
	}
	return false
}

func (c *client) getTLSCertFingerprint() string {
	if c.tls == nil {
		return ""
	}

	peerCerts := c.tls.ConnectionState().PeerCertificates
	if len(peerCerts) == 0 {
		return ""
	}

	hash := sha256.Sum256(peerCerts[0].Raw)
	return hex.EncodeToString(hash[:])
}

// --- Timers ------------------------------------------------------------------

// TODO

// --- IRC command handling ----------------------------------------------------

// XXX: ap doesn't really need to be a slice.
func (c *client) makeReply(id int, ap []interface{}) string {
	nickname := c.nickname
	if nickname == "" {
		nickname = "*"
	}

	s := fmt.Sprintf(":%s %03d %s ", serverName, id, nickname)
	a := fmt.Sprintf(defaultReplies[id], ap...)
	return s + a
}

// XXX: This way we cannot typecheck the arguments, so we must be careful.
func (c *client) sendReply(id int, args ...interface{}) {
	c.send(c.makeReply(id, args))
}

/// Send a space-separated list of words across as many replies as needed.
func (c *client) sendReplyVector(id int, items []string, args ...interface{}) {
	common := c.makeReply(id, args)

	// We always send at least one message (there might be a client that
	// expects us to send this message at least once).
	for len(items) > 0 {
		// If not even a single item fits in the limit (which may happen,
		// in theory) it just gets cropped. We could also skip it.
		reply := append([]byte(common), items[0]...)

		// Append as many items as fits in a single message.
		for len(items) > 0 &&
			len(reply)+1+len(items[0]) <= ircMaxMessageLength {
			reply = append(reply, ' ')
			reply = append(reply, items[0]...)
			items = items[1:]
		}

		c.send(string(reply))
	}
}

func (c *client) sendMOTD() {
	if len(motd) == 0 {
		c.sendReply(ERR_NOMOTD)
		return
	}

	c.sendReply(RPL_MOTDSTART, serverName)
	for _, line := range motd {
		c.sendReply(RPL_MOTD, line)
	}
	c.sendReply(RPL_ENDOFMOTD)
}

func (c *client) sendLUSERS() {
	nUsers, nServices, nOpers, nUnknown := 0, 0, 0, 0
	for c := range clients {
		if c.registered {
			nUsers++
		} else {
			nUnknown++
		}
		if 0 != c.mode&ircUserModeOperator {
			nOpers++
		}
	}

	nChannels := 0
	for _, ch := range channels {
		if 0 != ch.modes&ircChanModeSecret {
			nChannels++
		}
	}

	c.sendReply(RPL_LUSERCLIENT, nUsers, nServices, 1 /* servers total */)
	if nOpers != 0 {
		c.sendReply(RPL_LUSEROP, nOpers)
	}
	if nUnknown != 0 {
		c.sendReply(RPL_LUSERUNKNOWN, nUnknown)
	}
	if nChannels != 0 {
		c.sendReply(RPL_LUSERCHANNELS, nChannels)
	}
	c.sendReply(RPL_LUSERME, nUsers+nServices+nUnknown, 0 /* peer servers */)
}

func isThisMe(target string) bool {
	// Target servers can also be matched by their users
	if ircFnmatch(target, serverName) {
		return true
	}
	_, ok := users[ircStrxfrm(target)]
	return ok
}

func (c *client) sendISUPPORT() {
	// Only # channels, +e supported, +I supported, unlimited arguments to MODE
	c.sendReply(RPL_ISUPPORT, "CHANTYPES=# EXCEPTS INVEX MODES"+
		" TARGMAX=WHOIS:,LIST:,NAMES:,PRIVMSG:1,NOTICE:1,KICK:"+
		" NICKLEN=%d CHANNELLEN=%d", ircMaxNickname, ircMaxChannelName)
}

func (c *client) tryFinishRegistration() {
	// TODO: Check if the realname is really required.
	if c.nickname == "" || c.username == "" || c.realname == "" {
		return
	}
	if c.registered || c.capNegotiating {
		return
	}

	c.registered = true
	c.sendReply(RPL_WELCOME, c.nickname, c.username, c.hostname)

	c.sendReply(RPL_YOURHOST, serverName, "TODO version")
	// The purpose of this message eludes me.
	c.sendReply(RPL_CREATED, time.Unix(started, 0).Format("Mon, 02 Jan 2006"))
	c.sendReply(RPL_MYINFO, serverName, "TODO version",
		ircSupportedUserModes, ircSupportedChanModes)

	c.sendISUPPORT()
	c.sendLUSERS()
	c.sendMOTD()

	if mode := c.getMode(); mode != "" {
		c.sendf(":%s MODE %s :+%s", c.nickname, c.nickname, mode)
	}

	c.tlsCertFingerprint = c.getTLSCertFingerprint()
	if c.tlsCertFingerprint != "" {
		c.sendf(":%s NOTICE %s :Your TLS client certificate fingerprint is %s",
			serverName, c.nickname, c.tlsCertFingerprint)
	}

	delete(whowas, ircStrxfrm(c.nickname))
}

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

// IRCv3 capability negotiation. See http://ircv3.org for details.

type ircCapArgs struct {
	subcommand string   // the subcommand being processed
	fullParams string   // whole parameter string
	params     []string // split parameters
	target     string   // target parameter for replies
}

var ircCapTable = []struct {
	flag uint   // flag
	name string // name of the capability
}{
	{ircCapMultiPrefix, "multi-prefix"},
	{ircCapInviteNotify, "invite-notify"},
	{ircCapEchoMessage, "echo-message"},
	{ircCapUserhostInNames, "userhost-in-names"},
	{ircCapServerTime, "server-time"},
}

func (c *client) handleCAPLS(a *ircCapArgs) {
	if len(a.params) == 1 {
		if ver, err := strconv.ParseUint(a.params[0], 10, 32); err != nil {
			c.sendReply(ERR_INVALIDCAPCMD, a.subcommand,
				"Ignoring invalid protocol version number")
		} else {
			c.capVersion = uint(ver)
		}
	}

	c.capNegotiating = true
	c.sendf(":%s CAP %s LS :multi-prefix invite-notify echo-message"+
		" userhost-in-names server-time", serverName, a.target)
}

func (c *client) handleCAPLIST(a *ircCapArgs) {
	caps := []string{}
	for _, cap := range ircCapTable {
		if 0 != c.capsEnabled&cap.flag {
			caps = append(caps, cap.name)
		}
	}

	c.sendf(":%s CAP %s LIST :%s", serverName, a.target,
		strings.Join(caps, " "))
}

func ircDecodeCapability(name string) uint {
	for _, cap := range ircCapTable {
		if cap.name == name {
			return cap.flag
		}
	}
	return 0
}

func (c *client) handleCAPREQ(a *ircCapArgs) {
	c.capNegotiating = true

	newCaps := c.capsEnabled
	ok := true
	for _, param := range a.params {
		removing := false
		name := param
		if name[:1] == "-" {
			removing = true
			name = name[1:]
		}

		if cap := ircDecodeCapability(name); cap == 0 {
			ok = false
		} else if removing {
			newCaps &= ^cap
		} else {
			newCaps |= cap
		}
	}

	if ok {
		c.capsEnabled = newCaps
		c.sendf(":%s CAP %s ACK :%s", serverName, a.target, a.fullParams)
	} else {
		c.sendf(":%s CAP %s NAK :%s", serverName, a.target, a.fullParams)
	}
}

func (c *client) handleCAPACK(a *ircCapArgs) {
	if len(a.params) > 0 {
		c.sendReply(ERR_INVALIDCAPCMD, a.subcommand,
			"No acknowledgable capabilities supported")
	}
}

func (c *client) handleCAPEND(a *ircCapArgs) {
	c.capNegotiating = false
	c.tryFinishRegistration()
}

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

// TODO: Beware of case sensitivity, probably need to index it by ircStrxfrm,
// which should arguably be named ircToLower and ircToUpper or something.
var ircCapHandlers = map[string]func(*client, *ircCapArgs){
	"LS":   (*client).handleCAPLS,
	"LIST": (*client).handleCAPLIST,
	"REQ":  (*client).handleCAPREQ,
	"ACK":  (*client).handleCAPACK,
	"END":  (*client).handleCAPEND,
}

// XXX: Maybe these also deserve to be methods for client?  They operato on
// global state, though.

func ircHandleCAP(msg *message, c *client) {
	if len(msg.params) < 1 {
		c.sendReply(ERR_NEEDMOREPARAMS, msg.command)
		return
	}

	// TODO: This really does seem to warrant a method.
	nickname := c.nickname
	if nickname == "" {
		nickname = "*"
	}

	args := &ircCapArgs{
		target:     nickname,
		subcommand: msg.params[0],
		fullParams: "",
		params:     []string{},
	}

	if len(msg.params) > 1 {
		args.fullParams = msg.params[1]
		// TODO: ignore_empty, likely create SplitSkipEmpty
		args.params = strings.Split(args.fullParams, " ")
	}

	// FIXME: We should ASCII ToUpper the subcommand.
	if fn, ok := ircCapHandlers[ircStrxfrm(args.subcommand)]; !ok {
		c.sendReply(ERR_INVALIDCAPCMD, args.subcommand,
			"Invalid CAP subcommand")
	} else {
		fn(c, args)
	}
}

func ircHandlePASS(msg *message, c *client) {
	if c.registered {
		c.sendReply(ERR_ALREADYREGISTERED)
	} else if len(msg.params) < 1 {
		c.sendReply(ERR_NEEDMOREPARAMS, msg.command)
	}

	// We have TLS client certificates for this purpose; ignoring.
}

func ircHandleNICK(msg *message, c *client) {
	if len(msg.params) < 1 {
		c.sendReply(ERR_NONICKNAMEGIVEN)
		return
	}

	nickname := msg.params[0]
	if !ircIsValidNickname(nickname) {
		c.sendReply(ERR_ERRONEOUSNICKNAME, nickname)
		return
	}

	nicknameNormalized := ircStrxfrm(nickname)
	if client, ok := users[nicknameNormalized]; ok && client != c {
		c.sendReply(ERR_NICKNAMEINUSE, nickname)
		return
	}

	if c.registered {
		c.addToWhowas()

		message := fmt.Sprintf(":%s!%s@%s NICK :%s",
			c.nickname, c.username, c.hostname, nickname)
		ircSendToRoommates(c, message)
		c.send(message)
	}

	// Release the old nickname and allocate a new one.
	if c.nickname != "" {
		delete(users, ircStrxfrm(c.nickname))
	}

	c.nickname = nickname
	users[nicknameNormalized] = c

	c.tryFinishRegistration()
}

func ircHandleUSER(msg *message, c *client) {
	if c.registered {
		c.sendReply(ERR_ALREADYREGISTERED)
		return
	}
	if len(msg.params) < 4 {
		c.sendReply(ERR_NEEDMOREPARAMS, msg.command)
		return
	}

	username, mode, realname := msg.params[0], msg.params[1], msg.params[3]

	// Unfortunately the protocol doesn't give us any means of rejecting it
	if !ircIsValidUsername(username) {
		username = "*"
	}

	c.username = username
	c.realname = realname

	c.mode = 0
	if m, err := strconv.ParseUint(mode, 10, 32); err != nil {
		if 0 != m&4 {
			c.mode |= ircUserModeRxWallops
		}
		if 0 != m&8 {
			c.mode |= ircUserModeInvisible
		}
	}

	c.tryFinishRegistration()
}

func ircHandleUSERHOST(msg *message, c *client) {
	if len(msg.params) < 1 {
		c.sendReply(ERR_NEEDMOREPARAMS, msg.command)
		return
	}

	// TODO
}

func ircHandleLUSERS(msg *message, c *client) {
	if len(msg.params) > 1 && !isThisMe(msg.params[1]) {
		c.sendReply(ERR_NOSUCHSERVER, msg.params[1])
		return
	}
	c.sendLUSERS()
}

func ircHandleMOTD(msg *message, c *client) {
	if len(msg.params) > 0 && !isThisMe(msg.params[0]) {
		c.sendReply(ERR_NOSUCHSERVER, msg.params[1])
		return
	}
	c.sendMOTD()
}

func ircHandlePING(msg *message, c *client) {
	// XXX: The RFC is pretty incomprehensible about the exact usage.
	if len(msg.params) > 1 && !isThisMe(msg.params[1]) {
		c.sendReply(ERR_NOSUCHSERVER, msg.params[1])
	} else if len(msg.params) < 1 {
		c.sendReply(ERR_NOORIGIN)
	} else {
		c.sendf(":%s PONG :%s", serverName, msg.params[0])
	}
}

func ircHandlePONG(msg *message, c *client) {
	// We are the only server, so we don't have to care too much.
	if len(msg.params) < 1 {
		c.sendReply(ERR_NOORIGIN)
		return
	}

	// Set a new timer to send another PING
	// TODO
}

func ircHandleQUIT(msg *message, c *client) {
	reason := c.nickname
	if len(msg.params) > 0 {
		reason = msg.params[0]
	}

	c.closeLink("Quit: " + reason)
}

func ircHandleTIME(msg *message, c *client) {
	if len(msg.params) > 0 && !isThisMe(msg.params[0]) {
		c.sendReply(ERR_NOSUCHSERVER, msg.params[0])
		return
	}

	c.sendReply(RPL_TIME, serverName,
		time.Now().Format("Mon Jan _2 2006 15:04:05"))
}

func ircHandleVERSION(msg *message, c *client) {
	if len(msg.params) > 0 && !isThisMe(msg.params[0]) {
		c.sendReply(ERR_NOSUCHSERVER, msg.params[0])
		return
	}

	postVersion := 0
	if debugMode {
		postVersion = 1
	}

	c.sendReply(RPL_VERSION, "TODO version", postVersion, serverName,
		"TODO program name"+" "+"TODO version")
	c.sendISUPPORT()
}

/*
func ircChannelMulticast(ch *channel, msg string, except *client) {
	for c, m := range ch.userModes {
		if c != except {
			c.send(msg)
		}
	}
}

func ircModifyMode(mask *uint, mode uint, add bool) bool {
	orig := *mask
	if add {
		*mask |= mode
	} else {
		*mask &= ^mode
	}
	return *mask != orig
}

func ircUpdateUserMode(c *client, newMode uint) {
	// TODO: Port, as well as all the other kike functions.
}
*/

func ircHandleMODE(msg *message, c *client) {
	if len(msg.params) < 1 {
		c.sendReply(ERR_NEEDMOREPARAMS, msg.command)
		return
	}

	// TODO
	target := msg.params[0]
	client := users[ircStrxfrm(target)]
	ch := users[ircStrxfrm(target)]

	if client != nil {
		// TODO: Think about strcmp.
		//if ircStrcmp(target, c.nickname) != 0 {
		//}
	} else if ch != nil {
		// TODO
	}
}

func ircHandleUserMessage(msg *message, c *client,
	command string, allowAwayReply bool) {
	if len(msg.params) < 1 {
		c.sendReply(ERR_NORECIPIENT, msg.command)
		return
	}
	if len(msg.params) < 2 || msg.params[1] == "" {
		c.sendReply(ERR_NOTEXTTOSEND)
		return
	}

	target, text := msg.params[0], msg.params[1]
	if client, ok := users[ircStrxfrm(target)]; ok {
		// TODO
		_ = client
		_ = text
	} else if ch, ok := channels[ircStrxfrm(target)]; ok {
		// TODO
		_ = ch
	} else {
		c.sendReply(ERR_NOSUCHNICK, target)
	}
}

// TODO: All the various real command handlers.

func ircHandleX(msg *message, c *client) {
}

// --- ? -----------------------------------------------------------------------

// Handle the results from initializing the client's connection.
func (c *client) onPrepared(host string, isTLS bool) {
	if isTLS {
		c.tls = tls.Server(c.transport, tlsConf)
		c.conn = c.tls
	} else {
		c.conn = c.transport.(connCloseWrite)
	}

	c.hostname = host
	c.address = net.JoinHostPort(host, c.port)

	// TODO: If we've tried to send any data before now, we need to flushOutQ.
	go read(c)
	c.reading = true
}

// Handle the results from trying to read from the client connection.
func (c *client) onRead(data []byte, readErr error) {
	if !c.reading {
		// Abusing the flag to emulate CloseRead and skip over data, see below.
		return
	}

	c.inQ = append(c.inQ, data...)
	for {
		// XXX: This accepts even simple LF newlines, even though they're not
		// really allowed by the protocol.
		advance, token, _ := bufio.ScanLines(c.inQ, false /* atEOF */)
		if advance == 0 {
			break
		}

		c.inQ = c.inQ[advance:]
		line := string(token)
		log.Printf("-> %s\n", line)

		m := reMsg.FindStringSubmatch(line)
		if m == nil {
			log.Println("error: invalid line")
			continue
		}

		msg := message{m[1], m[2], m[3], m[4], nil}
		for _, x := range reArgs.FindAllString(m[5], -1) {
			msg.params = append(msg.params, x[1:])
		}

		broadcast(line, c)
	}

	if readErr != nil {
		c.reading = false

		if readErr != io.EOF {
			log.Println(readErr)
			c.destroy("TODO")
		} else if c.closing {
			// Disregarding whether a clean shutdown has happened or not.
			log.Println("client finished shutdown")
			c.destroy("TODO")
		} else {
			log.Println("client EOF")
			c.closeLink("")
		}
	} else if len(c.inQ) > 8192 {
		log.Println("client inQ overrun")
		c.closeLink("inQ overrun")

		// tls.Conn doesn't have the CloseRead method (and it needs to be able
		// to read from the TCP connection even for writes, so there isn't much
		// sense in expecting the implementation to do anything useful),
		// otherwise we'd use it to block incoming packet data.
		c.reading = false
	}
}

// Spawn a goroutine to flush the outQ if possible and necessary.
func (c *client) flushOutQ() {
	if !c.writing && c.conn != nil {
		go write(c, c.outQ)
		c.writing = true
	}
}

// Handle the results from trying to write to the client connection.
func (c *client) onWrite(written int, writeErr error) {
	c.outQ = c.outQ[written:]
	c.writing = false

	if writeErr != nil {
		log.Println(writeErr)
		c.destroy("TODO")
	} else if len(c.outQ) > 0 {
		c.flushOutQ()
	} else if c.closing {
		if c.reading {
			c.conn.CloseWrite()
		} else {
			c.destroy("TODO")
		}
	}
}

// --- Worker goroutines -------------------------------------------------------

func accept(ln net.Listener) {
	for {
		if conn, err := ln.Accept(); err != nil {
			// TODO: Consider specific cases in error handling, some errors
			// are transitional while others are fatal.
			log.Println(err)
			break
		} else {
			conns <- conn
		}
	}
}

func prepare(client *client) {
	conn := client.transport
	host, _, err := net.SplitHostPort(conn.RemoteAddr().String())
	if err != nil {
		// In effect, we require TCP/UDP, as they have port numbers.
		log.Fatalln(err)
	}

	// The Cgo resolver doesn't pthread_cancel getnameinfo threads, so not
	// bothering with pointless contexts.
	ch := make(chan string, 1)
	go func() {
		defer close(ch)
		if names, err := net.LookupAddr(host); err != nil {
			log.Println(err)
		} else {
			ch <- names[0]
		}
	}()

	// While we can't cancel it, we still want to set a timeout on it.
	select {
	case <-time.After(5 * time.Second):
	case resolved, ok := <-ch:
		if ok {
			host = resolved
		}
	}

	// Note that in this demo application the autodetection prevents non-TLS
	// clients from receiving any messages until they send something.
	isTLS := false
	if sysconn, err := conn.(syscall.Conn).SyscallConn(); err != nil {
		// This is just for the TLS detection and doesn't need to be fatal.
		log.Println(err)
	} else {
		isTLS = detectTLS(sysconn)
	}

	// FIXME: When the client sends no data, we still initialize its conn.
	prepared <- preparedEvent{client, host, isTLS}
}

func read(client *client) {
	// A new buffer is allocated each time we receive some bytes, because of
	// thread-safety. Therefore the buffer shouldn't be too large, or we'd
	// need to copy it each time into a precisely sized new buffer.
	var err error
	for err == nil {
		var (
			buf [512]byte
			n   int
		)
		n, err = client.conn.Read(buf[:])
		reads <- readEvent{client, buf[:n], err}
	}
}

// Flush outQ, which is passed by parameter so that there are no data races.
func write(client *client, data []byte) {
	// We just write as much as we can, the main goroutine does the looping.
	n, err := client.conn.Write(data)
	writes <- writeEvent{client, n, err}
}

// --- Main --------------------------------------------------------------------

func processOneEvent() {
	select {
	case <-sigs:
		if inShutdown {
			forceShutdown("requested by user")
		} else {
			initiateShutdown()
		}

	case <-shutdownTimer:
		forceShutdown("timeout")

	case conn := <-conns:
		log.Println("accepted client connection")

		// In effect, we require TCP/UDP, as they have port numbers.
		address := conn.RemoteAddr().String()
		host, port, err := net.SplitHostPort(address)
		if err != nil {
			log.Fatalln(err)
		}

		c := &client{
			transport: conn,
			address:   address,
			hostname:  host,
			port:      port,
		}
		clients[c] = true
		go prepare(c)

	case ev := <-prepared:
		log.Println("client is ready:", ev.host)
		if _, ok := clients[ev.client]; ok {
			ev.client.onPrepared(ev.host, ev.isTLS)
		}

	case ev := <-reads:
		log.Println("received data from client")
		if _, ok := clients[ev.client]; ok {
			ev.client.onRead(ev.data, ev.err)
		}

	case ev := <-writes:
		log.Println("sent data to client")
		if _, ok := clients[ev.client]; ok {
			ev.client.onWrite(ev.written, ev.err)
		}

	case c := <-timeouts:
		if _, ok := clients[c]; ok {
			log.Println("client timeouted")
			c.destroy("TODO")
		}
	}
}

func main() {
	flag.BoolVar(&debugMode, "debug", false, "debug mode")
	version := flag.Bool("version", false, "show version and exit")
	flag.Parse()

	// TODO: Consider using the same version number for all subprojects.
	if *version {
		fmt.Printf("%s %s\n", "hid", "0")
		return
	}

	// TODO: Configuration--create an INI parser, probably;
	//   lift XDG_CONFIG_HOME from gitlab-notifier.
	if len(flag.Args()) != 3 {
		log.Fatalf("usage: %s KEY CERT ADDRESS\n", os.Args[0])
	}

	cert, err := tls.LoadX509KeyPair(flag.Arg(1), flag.Arg(0))
	if err != nil {
		log.Fatalln(err)
	}

	tlsConf = &tls.Config{
		Certificates:           []tls.Certificate{cert},
		ClientAuth:             tls.RequestClientCert,
		SessionTicketsDisabled: true,
	}
	listener, err = net.Listen("tcp", flag.Arg(2))
	if err != nil {
		log.Fatalln(err)
	}

	go accept(listener)
	signal.Notify(sigs, syscall.SIGINT, syscall.SIGTERM)

	for !inShutdown || len(clients) > 0 {
		processOneEvent()
	}
}