go-telegram-bot/telegram_helpers.go

package main

import (
	"context"
	"fmt"
	"io"
	"net/http"

	tgbot "github.com/go-telegram/bot"
	"github.com/go-telegram/bot/models"
)

// largestPhotoSize returns the highest-resolution PhotoSize from the slice
// Telegram returns for a single photo. Telegram pre-renders each upload at
// several resolutions; we want the largest for vision quality. Falls back to
// the zero value when the slice is empty (caller should guard upstream).
func largestPhotoSize(photos []models.PhotoSize) models.PhotoSize {
	if len(photos) == 0 {
		return models.PhotoSize{}
	}
	largest := photos[0]
	largestArea := largest.Width * largest.Height
	for i := 1; i < len(photos); i++ {
		area := photos[i].Width * photos[i].Height
		if area > largestArea {
			largest = photos[i]
			largestArea = area
		}
	}
	return largest
}

// downloadTelegramFile resolves a Telegram file_id via the bot API, fetches the
// download URL, and reads the bytes into memory. The two-step dance (GetFile +
// fetch via FileDownloadLink) is required by Telegram's protocol — direct
// downloads aren't possible from file_id alone. Buffered into []byte because
// downstream callers (multipart uploads to ElevenLabs and Anthropic) re-read
// the body; streaming would require either tee-ing or a temp file.
func (b *Bot) downloadTelegramFile(ctx context.Context, fileID string) ([]byte, error) {
	fileInfo, err := b.tgBot.GetFile(ctx, &tgbot.GetFileParams{FileID: fileID})
	if err != nil {
		return nil, fmt.Errorf("telegram GetFile %s: %w", fileID, err)
	}
	downloadURL := b.tgBot.FileDownloadLink(fileInfo)
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, downloadURL, nil)
	if err != nil {
		return nil, fmt.Errorf("telegram download request %s: %w", fileID, err)
	}
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return nil, fmt.Errorf("telegram download %s: %w", fileID, err)
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("telegram download %s: status %d", fileID, resp.StatusCode)
	}
	data, err := io.ReadAll(resp.Body)
	if err != nil {
		return nil, fmt.Errorf("telegram download read %s: %w", fileID, err)
	}
	return data, nil
}