package main

// Telegram, sent to directly over HTTPS — never through the bus or the control node (novox/hq to-be 45
// §5): the one sender that does not pass through the control node. The same client as the
// operator-channel's holder (module messenger), kept here so the watcher depends on nothing it
// watches. The bot token is this module's own secret, accepted from the operator; the
// chat id is a setting. Neither is ever said: an error from the HTTP client carries the URL, and the
// URL carries the token, so every error is rebuilt here from its kind before it leaves this file.

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"net"
	"net/http"
	"net/url"
	"os"
	"regexp"
	"strings"
	"time"
)

// TelegramAPI is where the bot API answers; a test points it elsewhere.
var TelegramAPI = "https://api.telegram.org"

var (
	tokenShape  = regexp.MustCompile(`^\d{5,}:[A-Za-z0-9_-]{30,}$`)
	chatIDShape = regexp.MustCompile(`^(-?\d{1,20}|@[A-Za-z][A-Za-z0-9_]{4,})$`)
)

// TelegramConfig is where the token is and what the chat is; read each time it is used, so a secret
// accepted or a setting changed takes effect at the next message without a restart.
type TelegramConfig struct {
	TokenFile string
	ChatID    func() string
}

// Telegram sends to one chat.
type Telegram struct {
	Config TelegramConfig
	Client *http.Client
}

func NewTelegram(cfg TelegramConfig) *Telegram {
	return &Telegram{Config: cfg, Client: &http.Client{Timeout: 20 * time.Second}}
}

func (t *Telegram) Name() string { return "telegram" }

// Ready says whether the channel can send, in words.
func (t *Telegram) Ready() error {
	_, _, err := t.ready()
	return err
}

// ready says whether the channel can send, and when not, what the operator must give. The words name
// the setting and the secret, never a value.
func (t *Telegram) ready() (string, string, error) {
	token, err := t.token()
	if err != nil {
		return "", "", err
	}
	chat := strings.TrimSpace(t.Config.ChatID())
	if chat == "" {
		return "", "", errors.New("no chat to send to: the setting telegram-chat-id is not given " +
			"(`settings` for mesh-watcher with {\"telegram-chat-id\": \"<the chat's id>\"})")
	}
	if !chatIDShape.MatchString(chat) {
		return "", "", errors.New("the setting telegram-chat-id is not a chat id (a number, or @name of a channel)")
	}
	return token, chat, nil
}

func (t *Telegram) token() (string, error) {
	if t.Config.TokenFile == "" {
		return "", errors.New("no bot token: this module was started without a token file")
	}
	raw, err := os.ReadFile(t.Config.TokenFile)
	if errors.Is(err, os.ErrNotExist) {
		return "", errors.New("no bot token: the own secret telegram-token is not on this machine yet " +
			"(`secret accept <machine> mesh-watcher telegram-token`, then push the machine)")
	}
	if err != nil {
		return "", errors.New("the own secret telegram-token cannot be read: " + plainError(err))
	}
	token := strings.TrimSpace(string(raw))
	if token == "" {
		return "", errors.New("no bot token: the own secret telegram-token is empty")
	}
	if !tokenShape.MatchString(token) {
		// The mesh mints a random value for an own secret nobody accepted; that is not a bot token.
		return "", errors.New("the own secret telegram-token is not a bot token (digits, a colon, then the key " +
			"BotFather gave) — most likely the mesh made it because none was accepted: " +
			"`secret accept <machine> mesh-watcher telegram-token`, then push the machine")
	}
	return token, nil
}

// Send posts a message and answers its message id.
func (t *Telegram) Send(m Message) (string, error) {
	text := m.Text()
	token, chat, err := t.ready()
	if err != nil {
		return "", err
	}
	var out struct {
		MessageID int64 `json:"message_id"`
	}
	if err := t.call(token, "sendMessage", map[string]any{
		"chat_id": chat, "text": text, "disable_web_page_preview": true,
	}, &out); err != nil {
		return "", err
	}
	return fmt.Sprint(out.MessageID), nil
}

// Edit replaces the text of a message sent before: how a cleared condition is said (to-be 45 §5).
func (t *Telegram) Edit(id string, m Message) error {
	text := m.Text()
	token, chat, err := t.ready()
	if err != nil {
		return err
	}
	return t.call(token, "editMessageText", map[string]any{
		"chat_id": chat, "message_id": json.Number(id), "text": text, "disable_web_page_preview": true,
	}, nil)
}

// CanEdit: Telegram edits a message in place.
func (t *Telegram) CanEdit() bool { return true }

// Who asks the bot API who it is: the check that the token works, with no message sent.
func (t *Telegram) Who() (string, error) {
	token, _, err := t.ready()
	if err != nil {
		return "", err
	}
	var me struct {
		Username string `json:"username"`
	}
	if err := t.call(token, "getMe", map[string]any{}, &me); err != nil {
		return "", err
	}
	return me.Username, nil
}

func (t *Telegram) call(token, method string, body map[string]any, into any) error {
	raw, _ := json.Marshal(body)
	req, err := http.NewRequest(http.MethodPost, TelegramAPI+"/bot"+token+"/"+method, bytes.NewReader(raw))
	if err != nil {
		return errors.New("telegram " + method + ": the request could not be made")
	}
	req.Header.Set("Content-Type", "application/json")
	resp, err := t.Client.Do(req)
	if err != nil {
		return fmt.Errorf("telegram %s: %s", method, plainError(err))
	}
	defer resp.Body.Close()
	var answer struct {
		OK          bool            `json:"ok"`
		ErrorCode   int             `json:"error_code"`
		Description string          `json:"description"`
		Result      json.RawMessage `json:"result"`
	}
	if err := json.NewDecoder(resp.Body).Decode(&answer); err != nil {
		return fmt.Errorf("telegram %s: HTTP %d with an answer that is not the bot API's", method, resp.StatusCode)
	}
	if !answer.OK {
		d := strings.ReplaceAll(answer.Description, token, "<token>")
		return fmt.Errorf("telegram %s refused: %d %s", method, answer.ErrorCode, d)
	}
	if into != nil && len(answer.Result) > 0 {
		_ = json.Unmarshal(answer.Result, into)
	}
	return nil
}

// plainError is an error in words, without the URL a transport error carries.
func plainError(err error) string {
	var ue *url.Error
	if errors.As(err, &ue) {
		err = ue.Err
	}
	var ne net.Error
	switch {
	case errors.As(err, &ne) && ne.Timeout():
		return "no answer in time"
	case errors.Is(err, os.ErrPermission):
		return "permission denied"
	}
	var dns *net.DNSError
	if errors.As(err, &dns) {
		return "the bot API's name does not resolve"
	}
	var op *net.OpError
	if errors.As(err, &op) {
		return "cannot connect (" + op.Op + ")"
	}
	s := err.Error()
	if strings.Contains(s, "/bot") || strings.Contains(s, "://") {
		return "the request failed"
	}
	return s
}