tweetdistributor/main.go

package main

import (
	"bytes"
	"fmt"
	"image"
	_ "image/gif"
	"image/jpeg"
	_ "image/png"
	"io"
	"net/http"
	"os"
	"path"
	"strings"
	"tweetdistributor/discord"
	"tweetdistributor/output"
	"unicode/utf8"

	"github.com/bwmarrin/discordgo"
	"golang.org/x/image/draw"
	_ "golang.org/x/image/webp"
)

const maxTweetLength = 140
const maxImagesPerPost = 4

// Bluesky rejects blobs over 2,000,000 bytes; Twitter allows up to 5MB.
const maxImageBytes = 2_000_000

// shrinkImage re-encodes (and if necessary downscales) an image until it
// fits within maxImageBytes. Images already small enough pass through
// untouched.
func shrinkImage(img output.Image) (output.Image, error) {
	if len(img.Data) <= maxImageBytes {
		return img, nil
	}

	src, _, err := image.Decode(bytes.NewReader(img.Data))
	if err != nil {
		return output.Image{}, fmt.Errorf("decoding %s: %w", img.Filename, err)
	}

	for scale := 1.0; scale > 0.05; scale *= 0.7 {
		width := int(float64(src.Bounds().Dx()) * scale)
		height := int(float64(src.Bounds().Dy()) * scale)
		if width < 1 || height < 1 {
			break
		}

		scaled := image.NewRGBA(image.Rect(0, 0, width, height))
		draw.CatmullRom.Scale(scaled, scaled.Bounds(), src, src.Bounds(), draw.Src, nil)

		var buf bytes.Buffer
		if err := jpeg.Encode(&buf, scaled, &jpeg.Options{Quality: 85}); err != nil {
			return output.Image{}, fmt.Errorf("encoding %s: %w", img.Filename, err)
		}

		if buf.Len() <= maxImageBytes {
			filename := strings.TrimSuffix(img.Filename, path.Ext(img.Filename)) + ".jpg"
			return output.Image{
				Data:        buf.Bytes(),
				ContentType: "image/jpeg",
				Filename:    filename,
			}, nil
		}
	}

	return output.Image{}, fmt.Errorf("%s could not be shrunk below %d bytes", img.Filename, maxImageBytes)
}

func downloadImages(attachments []*discordgo.MessageAttachment) ([]output.Image, error) {
	var images []output.Image
	for _, attachment := range attachments {
		if !strings.HasPrefix(attachment.ContentType, "image/") {
			continue
		}
		resp, err := http.Get(attachment.URL)
		if err != nil {
			return nil, fmt.Errorf("downloading %s: %w", attachment.Filename, err)
		}
		data, err := io.ReadAll(resp.Body)
		resp.Body.Close()
		if err != nil {
			return nil, fmt.Errorf("downloading %s: %w", attachment.Filename, err)
		}
		if resp.StatusCode != http.StatusOK {
			return nil, fmt.Errorf("downloading %s: status %s", attachment.Filename, resp.Status)
		}
		img, err := shrinkImage(output.Image{
			Data:        data,
			ContentType: attachment.ContentType,
			Filename:    attachment.Filename,
		})
		if err != nil {
			return nil, err
		}
		images = append(images, img)
	}
	return images, nil
}

func main() {
	d := discord.Discord(os.Getenv("DISCORD_TOKEN"), os.Getenv("DISCORD_CHANNEL"))

	tweetchannel := make(chan *discordgo.MessageCreate, 1)
	d.BeginRead(tweetchannel)

	d.Write("Tweetdistributor Started")

	var outputs []output.OutputInterface
	outputs = append(outputs, output.StdOutput())
	outputs = append(outputs, output.TwitterOutput(os.Getenv("TW_ACCESS_TOKEN"), os.Getenv("TW_ACCESS_SECRET")))
	outputs = append(outputs, output.BlueskyOutput(os.Getenv("BSKY_IDENTIFIER"), os.Getenv("BSKY_PASSWORD")))

	for tweet := range tweetchannel {
		if length := utf8.RuneCountInString(tweet.Content); length > maxTweetLength {
			errstr := fmt.Sprintf("Error: message is %d characters, exceeding the %d character limit; not posted", length, maxTweetLength)
			fmt.Fprintln(os.Stderr, errstr)
			d.Write(errstr)
			continue
		}

		images, err := downloadImages(tweet.Attachments)
		if err != nil {
			errstr := fmt.Sprintf("Error: %s; not posted", err)
			fmt.Fprintln(os.Stderr, errstr)
			d.Write(errstr)
			continue
		}
		if len(images) > maxImagesPerPost {
			errstr := fmt.Sprintf("Error: %d images attached, exceeding the limit of %d; not posted", len(images), maxImagesPerPost)
			fmt.Fprintln(os.Stderr, errstr)
			d.Write(errstr)
			continue
		}

		for _, output := range outputs {
			err := output.Write(tweet.Content, images)
			if err != nil {
				errstr := fmt.Sprintf("%s Error: %s", output.GetName(), err)
				fmt.Fprintln(os.Stderr, errstr)
				d.Write(errstr)
			}
		}
	}
}