// Package bot turns a Telegram update into an answer about a CEP.
//
// It knows nothing about how the update arrived. [Bot.Handle] satisfies
// telegram.Handler, which the long-polling loop drives today and which a
// webhook receiver could drive tomorrow without touching anything here.
package bot

import (
	"context"
	"errors"
	"log/slog"
	"strings"

	cepis "cep.is/go"

	"cep.is/telegrambot/internal/telegram"
)

// Lookups is the subset of the cep.is client this bot needs. Depending on the
// interface rather than the concrete client keeps the seam narrow and visible.
//
// A failure may be a [*cepis.Error] carrying the municipality the CEP would
// belong to; nothing here has to know that, because [ErrorMessage] reads it.
type Lookups interface {
	Lookup(ctx context.Context, cep string) (*cepis.Address, error)
}

// Messenger is the subset of the Bot API this bot calls to answer.
type Messenger interface {
	SendMessage(ctx context.Context, req telegram.SendMessageRequest) error
	AnswerInlineQuery(ctx context.Context, req telegram.AnswerInlineQueryRequest) error
}

// Cache times for inline answers, in seconds. A found address is stable for a
// day — the DNE database changes twice a month — so Telegram may serve it from
// its own cache and spare both this bot and the API. Anything else is not
// cached, so a transient failure is not pinned to a query for hours.
const (
	inlineCacheFound = 86400
	inlineCacheOther = 0
)

// Bot answers CEP lookups.
type Bot struct {
	lookups   Lookups
	messenger Messenger
	limiter   *Limiter
	username  string
	logger    *slog.Logger
}

// Option configures a [Bot].
type Option func(*Bot)

// WithLimiter replaces the per-user token bucket.
func WithLimiter(limiter *Limiter) Option {
	return func(b *Bot) {
		if limiter != nil {
			b.limiter = limiter
		}
	}
}

// WithLogger sets the logger.
func WithLogger(logger *slog.Logger) Option {
	return func(b *Bot) {
		if logger != nil {
			b.logger = logger
		}
	}
}

// New builds a bot. The username is the bot's own, as reported by getMe, and is
// what makes "/cep@username" and the inline examples in the help text correct.
func New(lookups Lookups, messenger Messenger, username string, opts ...Option) *Bot {
	b := &Bot{
		lookups:   lookups,
		messenger: messenger,
		limiter:   NewLimiter(),
		username:  username,
		logger:    slog.Default(),
	}
	for _, opt := range opts {
		opt(b)
	}
	return b
}

// Handle dispatches one update. It implements telegram.Handler.
func (b *Bot) Handle(ctx context.Context, update telegram.Update) error {
	switch {
	case update.InlineQuery != nil:
		return b.handleInline(ctx, update.InlineQuery)
	case update.Message != nil:
		return b.handleMessage(ctx, update.Message)
	case update.ChannelPost != nil:
		return b.handleMessage(ctx, update.ChannelPost)
	}
	return nil
}

// handleMessage covers both messages and channel posts. They have the same
// shape, and the only difference that matters is that a channel post has no
// author, which the rate-limit key already accounts for.
func (b *Bot) handleMessage(ctx context.Context, message *telegram.Message) error {
	if message.From != nil && message.From.IsBot {
		return nil
	}

	command, isCommand := ParseCommand(message.Text)
	if isCommand {
		if !command.AddressedTo(b.username) {
			return nil
		}
		return b.handleCommand(ctx, message, command)
	}

	if !message.Chat.IsPrivate() {
		return nil
	}

	text := strings.TrimSpace(message.Text)
	if text == "" {
		return nil
	}
	if _, err := cepis.NormalizeCEP(text); err != nil {
		return b.reply(ctx, message, UnknownMessage)
	}
	return b.lookupAndReply(ctx, message, text)
}

func (b *Bot) handleCommand(ctx context.Context, message *telegram.Message, command Command) error {
	switch command.Name {
	case "start":
		return b.reply(ctx, message, StartMessage(b.username))
	case "help":
		return b.reply(ctx, message, HelpMessage(b.username))
	case "cep":
		if command.Argument == "" {
			return b.reply(ctx, message, UsageMessage(b.username))
		}
		return b.lookupAndReply(ctx, message, command.Argument)
	}

	if message.Chat.IsPrivate() {
		return b.reply(ctx, message, UnknownMessage)
	}
	return nil
}

func (b *Bot) lookupAndReply(ctx context.Context, message *telegram.Message, raw string) error {
	decision := b.limiter.Allow(rateKey(message))
	if !decision.Allowed {
		b.logger.Debug("rate limited", slog.Int64("key", rateKey(message)))
		if decision.Warn {
			return b.reply(ctx, message, RateLimitMessage(decision.RetryAfter))
		}
		return nil
	}

	address, err := b.lookups.Lookup(ctx, raw)
	if err != nil {
		b.logger.Info("lookup failed",
			slog.String("chat_type", message.Chat.Type),
			slog.String("error", err.Error()))
		return b.reply(ctx, message, ErrorMessage(err))
	}
	return b.reply(ctx, message, FormatAddress(address))
}

func (b *Bot) reply(ctx context.Context, message *telegram.Message, text string) error {
	req := telegram.SendMessageRequest{
		ChatID:             message.Chat.ID,
		Text:               text,
		ParseMode:          ParseMode,
		LinkPreviewOptions: &telegram.LinkPreviewOptions{IsDisabled: true},
	}
	if !message.Chat.IsPrivate() {
		req.ReplyParameters = &telegram.ReplyParameters{
			MessageID:                message.MessageID,
			AllowSendingWithoutReply: true,
		}
	}
	return b.messenger.SendMessage(ctx, req)
}

// handleInline answers an inline query.
//
// Telegram fires one query per keystroke, so a partial CEP must cost nothing: a
// query that is not eight digits is answered with an empty list, without
// touching the API and without spending a rate-limit token. Only a complete CEP
// becomes a lookup.
func (b *Bot) handleInline(ctx context.Context, query *telegram.InlineQuery) error {
	text := strings.TrimSpace(query.Query)
	if text == "" {
		return b.answerInline(ctx, query, nil, inlineCacheOther)
	}

	if _, err := cepis.NormalizeCEP(text); err != nil {
		if looksPartial(text) {
			return b.answerInline(ctx, query, nil, inlineCacheOther)
		}
		return b.answerInline(ctx, query, []telegram.InlineQueryResult{
			b.inlineNotice(query, "CEP inválido", "Use oito dígitos: 01001000", ErrorMessage(err)),
		}, inlineCacheOther)
	}

	decision := b.limiter.Allow(query.From.ID)
	if !decision.Allowed {
		return b.answerInline(ctx, query, nil, inlineCacheOther)
	}

	address, err := b.lookups.Lookup(ctx, text)
	if err != nil {
		b.logger.Info("inline lookup failed", slog.String("error", err.Error()))
		return b.answerInline(ctx, query, []telegram.InlineQueryResult{
			b.inlineNotice(query, inlineErrorTitle(err), inlineErrorDescription(err), ErrorMessage(err)),
		}, inlineCacheOther)
	}

	return b.answerInline(ctx, query, []telegram.InlineQueryResult{{
		Type:        "article",
		ID:          address.CEP,
		Title:       InlineTitle(address),
		Description: InlineDescription(address),
		InputMessageContent: telegram.InputMessageContent{
			MessageText:        FormatAddress(address),
			ParseMode:          ParseMode,
			LinkPreviewOptions: &telegram.LinkPreviewOptions{IsDisabled: true},
		},
	}}, inlineCacheFound)
}

func (b *Bot) answerInline(ctx context.Context, query *telegram.InlineQuery, results []telegram.InlineQueryResult, cacheTime int) error {
	if results == nil {
		results = []telegram.InlineQueryResult{}
	}
	return b.messenger.AnswerInlineQuery(ctx, telegram.AnswerInlineQueryRequest{
		InlineQueryID: query.ID,
		Results:       results,
		CacheTime:     cacheTime,
	})
}

func (b *Bot) inlineNotice(query *telegram.InlineQuery, title, description, text string) telegram.InlineQueryResult {
	return telegram.InlineQueryResult{
		Type:        "article",
		ID:          "notice-" + query.ID,
		Title:       title,
		Description: description,
		InputMessageContent: telegram.InputMessageContent{
			MessageText:        text,
			ParseMode:          ParseMode,
			LinkPreviewOptions: &telegram.LinkPreviewOptions{IsDisabled: true},
		},
	}
}

func inlineErrorTitle(err error) string {
	if isNotFound(err) {
		return "CEP não encontrado"
	}
	return "Não consegui consultar agora"
}

// inlineErrorDescription is the second line of the notice. Naming the
// municipality here is what lets someone see, in the results list, that the
// city they had in mind is right and the number is not — before sending
// anything to the chat.
//
// Titles and descriptions are plain text for Telegram, so nothing here is
// escaped: HTML applies to the message body alone.
func inlineErrorDescription(err error) string {
	if faixa := faixaOf(err); faixa != nil {
		return "Cairia em " + place(faixa) + " — toque para ver"
	}
	return "Toque para enviar o motivo"
}

// rateKey is the token bucket's key: the author when there is one, and the chat
// otherwise. A channel post has no author, and channel ids are negative, so the
// two spaces cannot collide.
func rateKey(message *telegram.Message) int64 {
	if message.From != nil {
		return message.From.ID
	}
	return message.Chat.ID
}

// looksPartial reports whether an inline query is plausibly a CEP still being
// typed, in which case answering with nothing is better than accusing the user
// of a mistake at every keystroke.
func looksPartial(text string) bool {
	if len(text) > 9 {
		return false
	}
	for _, r := range text {
		if (r < '0' || r > '9') && r != '-' && r != '.' {
			return false
		}
	}
	return true
}

func isNotFound(err error) bool { return errors.Is(err, cepis.ErrCEPNotFound) }