package main

// The Bot API, as this channel uses it. The bot token is this module's own secret, read from its file at
// each use, owned by the account this module runs as and by no other (novox/hq ADR 0259 §8). It is never
// 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, and a description the API gives back has the
// token taken out.

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

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

var tokenShape = regexp.MustCompile(`^\d{5,}:[A-Za-z0-9_-]{30,}$`)

// Bot calls the Bot API with the token in its file.
type Bot struct {
	TokenFile string
	Client    *http.Client
}

// apiError is the API's refusal: its code, its words, and how long it asks to be left alone.
type apiError struct {
	Method     string
	Code       int
	Words      string
	RetryAfter time.Duration
}

func (e *apiError) Error() string {
	return fmt.Sprintf("telegram %s refused: %d %s", e.Method, e.Code, e.Words)
}

// permanent says whether trying again cannot help: a refusal of the request itself (400, 403) or of the
// token (401, 404), never a rate limit or the service failing (D6).
func permanent(err error) bool {
	var a *apiError
	return errors.As(err, &a) && (a.Code == 400 || a.Code == 401 || a.Code == 403 || a.Code == 404)
}

func retryAfter(err error) time.Duration {
	var a *apiError
	if errors.As(err, &a) && a.Code == 429 {
		if a.RetryAfter > 0 {
			return a.RetryAfter
		}
		return time.Second
	}
	return 0
}

// notModified is an edit to the text it already has: a success, never a reason to send again (D6).
func notModified(err error) bool {
	var a *apiError
	return errors.As(err, &a) && a.Code == 400 && strings.Contains(a.Words, "message is not modified")
}

func gone(err error) bool {
	var a *apiError
	return errors.As(err, &a) && a.Code == 400 && (strings.Contains(a.Words, "message to edit not found") ||
		strings.Contains(a.Words, "message to delete not found"))
}

// token reads the bot token, and says in words what is missing when it cannot: never the value.
func (b *Bot) token() (string, error) {
	if b.TokenFile == "" {
		return "", errors.New("no bot token: this module was started without a token file")
	}
	raw, err := os.ReadFile(b.TokenFile)
	if errors.Is(err, os.ErrNotExist) {
		return "", errors.New("no bot token: the own secret telegram-token is not on this machine yet; the operator " +
			"gives it to the vault (the module's README says how)")
	}
	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 makes a random value for an own secret nobody gave; 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 given; the operator gives it to the vault")
	}
	return token, nil
}

// call asks the API one method, decoding its result into `into`.
func (b *Bot) call(method string, body map[string]any, into any) error {
	token, err := b.token()
	if err != nil {
		return err
	}
	raw, err := json.Marshal(body)
	if err != nil {
		// Said, never discarded (D8).
		return fmt.Errorf("telegram %s: the request could not be written: %v", method, err)
	}
	req, err := http.NewRequest(http.MethodPost, API+"/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")
	client := b.Client
	if client == nil {
		client = &http.Client{Timeout: 20 * time.Second}
	}
	resp, err := 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"`
		Parameters  struct {
			RetryAfter int `json:"retry_after"`
		} `json:"parameters"`
	}
	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 {
		return &apiError{Method: method, Code: answer.ErrorCode, Words: strings.ReplaceAll(answer.Description, token, "<token>"),
			RetryAfter: time.Duration(answer.Parameters.RetryAfter) * time.Second}
	}
	if into != nil && len(answer.Result) > 0 {
		if err := json.Unmarshal(answer.Result, into); err != nil {
			return fmt.Errorf("telegram %s: its answer could not be read: %v", method, err)
		}
	}
	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
}

// User is a Telegram account as the API says it.
type User struct {
	ID        int64  `json:"id"`
	IsBot     bool   `json:"is_bot"`
	FirstName string `json:"first_name"`
	LastName  string `json:"last_name"`
	Username  string `json:"username"`
}

// Display is the name the service shows for an account.
func (u User) Display() string {
	name := strings.TrimSpace(u.FirstName + " " + u.LastName)
	if u.Username != "" {
		if name != "" {
			return name + " (@" + u.Username + ")"
		}
		return "@" + u.Username
	}
	return name
}

// Chat is where a message was written.
type Chat struct {
	ID   int64  `json:"id"`
	Type string `json:"type"`
}

// Msg is a message as the API says it.
type Msg struct {
	MessageID int64  `json:"message_id"`
	From      *User  `json:"from"`
	Chat      Chat   `json:"chat"`
	Text      string `json:"text"`
}

// Callback is a button tapped.
type Callback struct {
	ID      string `json:"id"`
	From    User   `json:"from"`
	Message *Msg   `json:"message"`
	Data    string `json:"data"`
}

// Update is one thing that happened to the bot.
type Update struct {
	UpdateID int64     `json:"update_id"`
	Message  *Msg      `json:"message"`
	Callback *Callback `json:"callback_query"`
}

// Updates long-polls for what happened since offset: outbound only, so no route into the mesh is needed, and
// a stolen token can read updates (a conflict says so) but never inject one (novox/hq ADR 0234).
func (b *Bot) Updates(offset int64, wait time.Duration) ([]Update, error) {
	var out []Update
	err := b.call("getUpdates", map[string]any{"offset": offset, "timeout": int(wait.Seconds()),
		"allowed_updates": []string{"message", "callback_query"}}, &out)
	return out, err
}

// Me is the bot's own account: the check that the token works, with nothing sent.
func (b *Bot) Me() (User, error) {
	var me User
	return me, b.call("getMe", map[string]any{}, &me)
}

// keyboard is buttons one to a row, each carrying only its ticket.
func keyboard(buttons [][2]string) map[string]any {
	rows := [][]map[string]string{}
	for _, b := range buttons {
		rows = append(rows, []map[string]string{{"text": b[0], "callback_data": b[1]}})
	}
	return map[string]any{"inline_keyboard": rows}
}

// Send posts a message, its text in ParseMode (rendered or escaped by the caller), and answers its id. silent:
// it arrives without sound (D4).
func (b *Bot) Send(chat int64, text string, buttons [][2]string, silent bool) (int64, error) {
	body := map[string]any{"chat_id": chat, "text": text, "parse_mode": ParseMode, "disable_notification": silent,
		"link_preview_options": map[string]any{"is_disabled": true}}
	if len(buttons) > 0 {
		body["reply_markup"] = keyboard(buttons)
	}
	var out Msg
	err := b.call("sendMessage", body, &out)
	return out.MessageID, err
}

// Edit replaces a message's text (in ParseMode, rendered or escaped by the caller) and buttons; none given, the
// buttons go.
func (b *Bot) Edit(chat, id int64, text string, buttons [][2]string) error {
	err := b.call("editMessageText", map[string]any{"chat_id": chat, "message_id": id, "text": text, "parse_mode": ParseMode,
		"reply_markup": keyboard(buttons), "link_preview_options": map[string]any{"is_disabled": true}}, nil)
	if notModified(err) {
		return nil
	}
	return err
}

// AnswerTap ends the phone's spinner for a tapped button, showing words over the chat when there are any.
func (b *Bot) AnswerTap(id, text string) error {
	body := map[string]any{"callback_query_id": id}
	if text != "" {
		body["text"] = text
	}
	return b.call("answerCallbackQuery", body, nil)
}

// Delete removes a message from the chat: a code once read.
func (b *Bot) Delete(chat, id int64) error {
	err := b.call("deleteMessage", map[string]any{"chat_id": chat, "message_id": id}, nil)
	if gone(err) {
		return nil
	}
	return err
}