package main

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log"
	"os"
	"path/filepath"
	"strings"
	"sync"
	"time"
)

const (
	// telegramAPI is the Telegram Bot API service.
	telegramAPI = "https://api.telegram.org"

	// botConfigFileName is the bot config in the data directory. The daemon
	// runs the bot only when the file exists.
	botConfigFileName = "telegram.json"

	// offsetFileName is the update offset of the bot in the agent directory.
	offsetFileName = "telegram-offset.json"

	// telegramSource is the source of the user messages from Telegram.
	telegramSource = "telegram"

	// messageUnits is the most UTF-16 code units of one Telegram message.
	messageUnits = 4096

	// typingInterval is how often the bot shows the typing action while a
	// turn runs. Telegram shows the action for about five seconds.
	typingInterval = 4 * time.Second

	// textOnly is the answer to a message that is not text.
	textOnly = "Only text messages are supported."
)

// botConfig holds the settings of telegram.json: the bot token and the user ID
// of the owner. The token is a secret, so the file must have mode 0600.
type botConfig struct {
	Token string `json:"token"`
	Owner int64  `json:"owner"`
}

// telegramBot connects the private chat of the owner with the agent. It polls
// the Bot API for new messages and sends each text message of the owner to the
// agent. It ignores every other update. Its sender sends the replies, the
// errors, and the user messages of the other clients to the chat as plain text,
// and shows the typing action while a turn runs. Tool calls do not go to the
// chat.
//
// The offset file holds the ID of the next update. It moves past an update
// only after the message of the update is durable in the log, so a restart
// does not lose a message. A crash between the flush and the offset write
// makes Telegram send the update again, and the message repeats once.
//
// A failed Bot API call waits and tries again, from minWait up to maxWait, or
// as long as the Bot API asks. It never stops the agent.
type telegramBot struct {
	api        *botAPI
	owner      int64
	offsetPath string
	logger     *log.Logger
	minWait    time.Duration
	maxWait    time.Duration

	mutex sync.Mutex
	queue []Event
	ready chan struct{}
}

// offsetFile is the content of the offset file.
type offsetFile struct {
	Offset int64 `json:"offset"`
}

// loadBotConfig reads telegram.json in dir. It returns false when the file
// does not exist. An error never holds the token.
func loadBotConfig(dir string) (botConfig, bool, error) {
	path := filepath.Join(dir, botConfigFileName)
	info, err := os.Stat(path)
	if errors.Is(err, os.ErrNotExist) {
		return botConfig{}, false, nil
	}

	if err != nil {
		return botConfig{}, false, fmt.Errorf("read bot config: %w", err)
	}

	if info.Mode().Perm()&0o077 != 0 {
		return botConfig{}, false, fmt.Errorf("%s holds the bot token, so its mode must be 0600", botConfigFileName)
	}

	data, err := os.ReadFile(path)
	if err != nil {
		return botConfig{}, false, fmt.Errorf("read bot config: %w", err)
	}

	var config botConfig
	decoder := json.NewDecoder(bytes.NewReader(data))
	decoder.DisallowUnknownFields()
	err = decoder.Decode(&config)
	if err == nil && !errors.Is(decoder.Decode(&struct{}{}), io.EOF) {
		err = errors.New("data after the JSON object")
	}

	if err != nil {
		return botConfig{}, false, fmt.Errorf("%s is not valid JSON with the fields token and owner", botConfigFileName)
	}

	err = config.validate()
	if err != nil {
		return botConfig{}, false, fmt.Errorf("%s: %w", botConfigFileName, err)
	}

	return config, true, nil
}

// validate makes sure that the token has the form of a bot token and that the
// owner is a user ID. The token goes into a URL path, so it can only hold the
// characters of a bot token.
func (c botConfig) validate() error {
	id, secret, found := strings.Cut(c.Token, ":")
	if !found || id == "" || secret == "" || strings.Trim(id, "0123456789") != "" || strings.Trim(secret, "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-") != "" {
		return errors.New("token does not have the form of a bot token from BotFather")
	}

	if c.Owner <= 0 {
		return errors.New("owner must be the numeric Telegram user ID of the owner")
	}

	return nil
}

// newTelegramBot returns the bot of owner on api. The offset file is at
// offsetPath, and logger gets the failures. No log line holds the text of a
// message.
func newTelegramBot(api *botAPI, owner int64, offsetPath string, logger *log.Logger) *telegramBot {
	return &telegramBot{
		api:        api,
		owner:      owner,
		offsetPath: offsetPath,
		logger:     logger,
		minWait:    time.Second,
		maxWait:    time.Minute,
		ready:      make(chan struct{}, 1),
	}
}

// Notify queues event for the chat. It never blocks, so a slow network does not
// stop the agent.
func (b *telegramBot) Notify(event Event) {
	switch event.Kind {
	case "busy", "idle", "reply", "error":
	case "user":
		if event.Source == telegramSource {
			return
		}
	default:
		return
	}

	b.mutex.Lock()
	b.queue = append(b.queue, event)
	b.mutex.Unlock()

	select {
	case b.ready <- struct{}{}:
	default:
	}
}

// Run polls for the messages of the owner and sends the queued events to the
// chat until ctx ends. It returns an error only when a message or the offset
// cannot be written, because then the bot would lose or repeat messages.
func (b *telegramBot) Run(ctx context.Context, l *loop) error {
	ctx, cancel := context.WithCancel(ctx)
	defer cancel()

	var sender sync.WaitGroup
	sender.Add(1)
	go func() {
		defer sender.Done()

		b.send(ctx)
	}()

	err := b.poll(ctx, l)
	cancel()
	sender.Wait()

	return err
}

// send sends each queued event to the chat in order until ctx ends. While a
// turn runs, it shows the typing action every typingInterval and after each
// message, because a message ends the action.
func (b *telegramBot) send(ctx context.Context) {
	ticker := time.NewTicker(typingInterval)
	defer ticker.Stop()

	busy := false
	for ctx.Err() == nil {
		event, found := b.next()
		if !found {
			select {
			case <-b.ready:
			case <-ticker.C:
				if busy {
					b.typing(ctx)
				}
			case <-ctx.Done():
			}

			continue
		}

		switch event.Kind {
		case "busy":
			busy = true
			b.typing(ctx)
			ticker.Reset(typingInterval)
		case "idle":
			busy = false
		default:
			for _, part := range splitMessage(botText(event), messageUnits) {
				b.deliver(ctx, part)
			}

			if busy {
				b.typing(ctx)
			}
		}
	}
}

// next removes the first queued event and returns it.
func (b *telegramBot) next() (Event, bool) {
	b.mutex.Lock()
	defer b.mutex.Unlock()

	if len(b.queue) == 0 {
		return Event{}, false
	}

	event := b.queue[0]
	b.queue = b.queue[1:]

	return event, true
}

// typing shows the typing action once. A failure is not logged, because the
// next message or poll logs the state of the network.
func (b *telegramBot) typing(ctx context.Context) {
	_ = b.api.SendTyping(ctx, b.owner)
}

// deliver sends text to the chat. It tries again after each failure that a
// later try can fix, until ctx ends. It drops the text after any other failure.
func (b *telegramBot) deliver(ctx context.Context, text string) {
	wait := b.minWait
	for {
		err := b.api.SendMessage(ctx, b.owner, text)
		if err == nil || ctx.Err() != nil {
			return
		}

		retry, asked := retryable(err)
		if !retry {
			b.logger.Printf("message dropped: %v", err)
			return
		}

		wait = b.pause(ctx, err, wait, asked)
	}
}

// pause logs err and waits the longer of wait and asked, or until ctx ends. It
// returns the wait of the next failure: twice wait, up to maxWait.
func (b *telegramBot) pause(ctx context.Context, err error, wait, asked time.Duration) time.Duration {
	delay := max(wait, asked)
	b.logger.Printf("%v (next try in %s)", err, delay)

	timer := time.NewTimer(delay)
	defer timer.Stop()

	select {
	case <-timer.C:
	case <-ctx.Done():
	}

	return min(2*wait, b.maxWait)
}

// poll gets the updates from the stored offset on and handles each one, until
// ctx ends. Each update is handled to its end also when ctx ends during it, so
// a stop does not repeat its message.
func (b *telegramBot) poll(ctx context.Context, l *loop) error {
	offset, err := loadOffset(b.offsetPath)
	if err != nil {
		return err
	}

	wait := b.minWait
	for ctx.Err() == nil {
		updates, err := b.api.GetUpdates(ctx, offset)
		if ctx.Err() != nil {
			return nil
		}

		if err != nil {
			_, asked := retryable(err)
			wait = b.pause(ctx, err, wait, asked)

			continue
		}

		wait = b.minWait
		for _, update := range updates {
			if ctx.Err() != nil {
				return nil
			}

			err = b.handle(context.WithoutCancel(ctx), l, update)
			if err != nil {
				return err
			}

			offset = update.UpdateID + 1
			err = saveOffset(b.offsetPath, offset)
			if err != nil {
				return err
			}
		}
	}

	return nil
}

// handle runs one update. A text message of the owner in the private chat goes
// to the agent, and handle returns when it is durable in the log. The line
// /stop interrupts the turn. Any other message of the owner gets the answer
// that only text is supported. Every other update is ignored, and its content
// is not logged.
func (b *telegramBot) handle(ctx context.Context, l *loop, update botUpdate) error {
	message := update.Message
	if message == nil || message.Chat.Type != "private" || message.Chat.ID != b.owner || message.From == nil || message.From.ID != b.owner {
		b.logger.Print("update ignored: not a message of the owner in the private chat")
		return nil
	}

	switch {
	case message.Text == "":
		b.notice(textOnly)
	case strings.TrimSpace(message.Text) == stopCommand:
		if !l.Interrupt() {
			b.notice("No turn runs.")
		}
	default:
		_, err := l.Submit(telegramSource, message.Text)
		if err != nil {
			return err
		}

		err = l.Flush(ctx)
		if err != nil {
			return fmt.Errorf("flush the message from Telegram: %w", err)
		}
	}

	return nil
}

// notice queues a short answer of the bot for the chat.
func (b *telegramBot) notice(text string) {
	b.Notify(Event{
		Kind: "reply",
		Text: text,
	})
}

// loadOffset reads the update offset at path. A missing file gives 0, which
// asks for every update that Telegram keeps.
func loadOffset(path string) (int64, error) {
	data, err := os.ReadFile(path)
	if errors.Is(err, os.ErrNotExist) {
		return 0, nil
	}

	if err != nil {
		return 0, fmt.Errorf("read update offset: %w", err)
	}

	var file offsetFile
	err = json.Unmarshal(data, &file)
	if err != nil {
		return 0, fmt.Errorf("%s: %w", path, err)
	}

	return file.Offset, nil
}

// saveOffset replaces the update offset at path.
func saveOffset(path string, offset int64) error {
	data, err := json.Marshal(offsetFile{
		Offset: offset,
	})
	if err != nil {
		return fmt.Errorf("encode update offset: %w", err)
	}

	err = writeFileAtomic(path, append(data, '\n'))
	if err != nil {
		return fmt.Errorf("write update offset: %w", err)
	}

	return nil
}

// botText returns the text of event for the chat. A user message of another
// client has its source as a mark.
func botText(event Event) string {
	switch event.Kind {
	case "user":
		return "[" + event.Source + "] " + event.Text
	case "error":
		return "Error: " + event.Text
	default:
		return event.Text
	}
}

// splitMessage cuts text into parts of at most limit UTF-16 code units, the
// unit of the Telegram message limit. It cuts after the last line break or
// space of a part when one is in its second half, and else at the limit. It
// never cuts a character.
func splitMessage(text string, limit int) []string {
	var parts []string
	for {
		end := prefixEnd(text, limit)
		if end == len(text) {
			return append(parts, text)
		}

		cut := strings.LastIndexByte(text[:end], '\n') + 1
		if cut <= end/2 {
			cut = strings.LastIndexByte(text[:end], ' ') + 1
		}

		if cut <= end/2 {
			cut = end
		}

		parts = append(parts, text[:cut])
		text = text[cut:]
	}
}

// prefixEnd returns the byte length of the longest prefix of text with at most
// limit UTF-16 code units.
func prefixEnd(text string, limit int) int {
	units := 0
	for index, char := range text {
		size := 1
		if char > 0xFFFF {
			size = 2
		}

		if units+size > limit {
			return index
		}

		units += size
	}

	return len(text)
}