//
// Copyright (c) 2024, Přemysl Eric 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.
//

// xN is a simple IRC notifier.
package main

import (
	"bufio"
	"bytes"
	"crypto/tls"
	"errors"
	"flag"
	"fmt"
	"io"
	"log"
	"net"
	"net/url"
	"os"
	"os/user"
	"strconv"
	"strings"
)

const projectName = "xN"

var projectVersion = "?"

var debugMode = false

type parameters struct {
	conn      net.Conn // underlying network connection
	username  string   // nickname + username + realname
	password  string   // server password
	target    string   // where to send text
	isuser    bool     // the target is a user rather than a channel
	chankey   string   // channel key
	skipjoin  bool     // whether to send external messages to channels
	usenotice bool     // whether to use NOTICE rather than PRIVMSG
	message   []string // lines of the message to send
}

func (p *parameters) send(command string, args ...string) error {
	var buf bytes.Buffer
	buf.WriteString(command)
	for i, arg := range args {
		buf.WriteRune(' ')
		if i+1 == len(args) {
			buf.WriteRune(':')
		}
		buf.WriteString(arg)
	}
	buf.WriteString("\r\n")
	_, err := p.conn.Write(buf.Bytes())
	return err
}

const (
	rplWELCOME       = "001"
	errNICKNAMEINUSE = "433"
)

func notify(p *parameters) error {
	// The intro should comfortably fit in the TCP send buffer whole.
	if p.password != "" {
		p.send("PASS", p.password)
	}
	p.send("USER", p.username, "0", "*", p.username)
	p.send("NICK", p.username)

	scanner, nickCounter, issue := bufio.NewScanner(p.conn), 1, ""
	for scanner.Scan() {
		if debugMode {
			log.Println(scanner.Text())
		}

		m := ircParseMessage(scanner.Text())
		switch m.command {
		case "PING":
			p.send("PONG", m.params...)
		case rplWELCOME:
			if !p.isuser && !p.skipjoin {
				if p.chankey != "" {
					p.send("JOIN", p.target, p.chankey)
				} else {
					p.send("JOIN", p.target)
				}
			}
			for _, line := range p.message {
				if p.usenotice {
					p.send("NOTICE", p.target, line)
				} else {
					p.send("PRIVMSG", p.target, line)
				}
			}
			p.send("QUIT")
		case errNICKNAMEINUSE:
			p.send("NICK", fmt.Sprintf("%s%d", p.username, nickCounter))
			nickCounter++
		default:
			// Prevent hanging on unsuccessful registrations.
			numeric, _ := strconv.Atoi(m.command)
			if numeric >= 400 && numeric <= 599 {
				if len(m.params) > 1 {
					issue = strings.Join(m.params[1:], " ")
				} else {
					issue = strings.Join(m.params, " ")
				}
				p.send("QUIT")
			}
		}
	}
	if err := scanner.Err(); err != nil {
		return err
	}
	if issue != "" {
		return errors.New(issue)
	}
	return nil
}

func parse(rawURL string, text []byte) (
	p parameters, connect func() (net.Conn, error), err error) {
	u, err := url.Parse(rawURL)
	if err != nil {
		return p, nil, err
	} else if !u.IsAbs() || u.Opaque != "" {
		return p, nil, errors.New("need an absolute URL")
	} else if u.Path == "/" && u.Fragment != "" {
		// Try to handle the common but degenerate case.
		fragment := "%23" + u.Fragment
		u.Fragment, u.RawFragment = "", ""
		if u, err = url.Parse(u.String() + fragment); err != nil {
			return p, nil, err
		}
	}

	// Figure out registration details.
	p.username = projectName
	if u, _ := user.Current(); u != nil {
		p.username = u.Username
	}
	if u.User.Username() != "" {
		p.username = u.User.Username()
	}

	p.password, _ = u.User.Password()

	// Figure out the target, which for our intents must accept messages.
	path, _ := strings.CutPrefix(u.Path, "/")
	elements := strings.Split(path, ",")
	if path == "" || elements[0] == "" {
		return p, nil, errors.New("unspecified entity")
	}

	// The last entity type wins.
	p.target, p.isuser = elements[0], false
	for _, typ := range elements[1:] {
		switch typ {
		case "isuser":
			p.isuser = true
		case "ischannel":
			p.isuser = false
		case "isserver":
			// We do not support network names, and this is the default.
		default:
			return p, nil, errors.New("unsupported type: " + typ)
		}
	}
	if p.isuser {
		if i := strings.IndexAny(p.target, "!@"); i != -1 {
			p.target = p.target[:i]
		}
	} else if !strings.HasPrefix(p.target, "#") {
		// TODO(p): We should consult RPL_ISUPPORT rather than guess,
		// though other prefixes are rare.
		p.target = "#" + p.target
	}

	// Note that the draft RFC wants these to be case-insensitive.
	p.chankey = u.Query().Get("key")
	// Being able to skip channel join is our own requirement and invention,
	// as are notices (names taken from Travis CI configuration).
	p.skipjoin = u.Query().Has("skipjoin")
	p.usenotice = u.Query().Has("usenotice")

	// Ensure valid LF-separated UTF-8, and split it at lines.
	sanitized := strings.ReplaceAll(string([]rune(string(text))), "\r", "\n")
	for _, line := range strings.Split(sanitized, "\n") {
		if line != "" {
			p.message = append(p.message, line)
		}
	}

	hostname, port := u.Hostname(), u.Port()
	switch u.Scheme {
	case "irc":
		if port == "" {
			port = "6667"
		}
		connect = func() (net.Conn, error) {
			return net.Dial("tcp", net.JoinHostPort(hostname, port))
		}
	case "ircs":
		if port == "" {
			port = "6697"
		}
		connect = func() (net.Conn, error) {
			return tls.Dial("tcp", net.JoinHostPort(hostname, port), nil)
		}
	default:
		err = errors.New("unsupported scheme: " + u.Scheme)
	}
	return
}

// notifyByURL sends the given text to the IRC server specified by a URL.
// See draft-butcher-irc-url-04.txt for the URL scheme specification
// this function loosely follows.
func notifyByURL(rawURL string, text []byte) error {
	p, connect, err := parse(rawURL, text)
	if p.conn, err = connect(); err != nil {
		return err
	}
	defer p.conn.Close()
	return notify(&p)
}

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

	flag.Usage = func() {
		f := flag.CommandLine.Output()
		fmt.Fprintf(f, "Usage: %s [OPTION]... URL...\n", os.Args[0])
		flag.PrintDefaults()
	}
	flag.Parse()
	if flag.NArg() < 1 {
		flag.Usage()
		os.Exit(2)
	}

	if *version {
		fmt.Printf("%s %s\n", projectName, projectVersion)
		return
	}

	text, err := io.ReadAll(os.Stdin)
	if err != nil {
		log.Fatalln(err)
	}

	status := 0
	for _, rawURL := range flag.Args() {
		if err := notifyByURL(rawURL, text); err != nil {
			status = 1

			var ue *url.Error
			if errors.As(err, &ue) {
				log.Println(err)
			} else {
				log.Printf("notify %q: %s\n", rawURL, err)
			}
		}
	}
	os.Exit(status)
}