ico: add module to deal with ico files

the ico file format is quite straightforward, it contains a few headers
followed by any number of full PNG/BMP images. for our usecase, we only
need to be able to embed a single favicon an ico file, this module
handles just the conversion of an image.Image to the ico format.

Signed-off-by: oppiliappan <[e-mail]>

appview/ogcard/card.go

···
 
 	// Handle SVG separately
 	if contentType == "image/svg+xml" || strings.HasSuffix(url, ".svg") {
-		return c.convertSVGToPNG(bodyBytes)
+		return convertSVGToPNG(bodyBytes)
 	}
 
 	// Support content types are in-sync with the allowed custom avatar file types
···
 }
 
 // convertSVGToPNG converts SVG data to a PNG image
-func (c *Card) convertSVGToPNG(svgData []byte) (image.Image, bool) {
+func convertSVGToPNG(svgData []byte) (image.Image, bool) {
 	// Parse the SVG
 	icon, err := oksvg.ReadIconStream(bytes.NewReader(svgData))
 	if err != nil {
···
 	draw.CatmullRom.Scale(scaledImg, scaledImg.Bounds(), img, srcBounds, draw.Src, nil)
 
 	// Draw the image with circular clipping
-	for cy := 0; cy < size; cy++ {
-		for cx := 0; cx < size; cx++ {
+	for cy := range size {
+		for cx := range size {
 			// Calculate distance from center
 			dx := float64(cx - center)
 			dy := float64(cy - center)

ico/ico.go

···
+package ico
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"image"
+	"image/png"
+)
+
+type IconDir struct {
+	Reserved uint16 // must be 0
+	Type     uint16 // 1 for ICO, 2 for CUR
+	Count    uint16 // number of images
+}
+
+type IconDirEntry struct {
+	Width        uint8 // 0 means 256
+	Height       uint8 // 0 means 256
+	ColorCount   uint8
+	Reserved     uint8  // must be 0
+	ColorPlanes  uint16 // 0 or 1
+	BitsPerPixel uint16
+	SizeInBytes  uint32
+	Offset       uint32
+}
+
+func ImageToIco(img image.Image) ([]byte, error) {
+	// encode image as png
+	var pngBuf bytes.Buffer
+	if err := png.Encode(&pngBuf, img); err != nil {
+		return nil, fmt.Errorf("failed to encode PNG: %w", err)
+	}
+	pngData := pngBuf.Bytes()
+
+	// get image dimensions
+	bounds := img.Bounds()
+	width := bounds.Dx()
+	height := bounds.Dy()
+
+	// prepare output buffer
+	var icoBuf bytes.Buffer
+
+	iconDir := IconDir{
+		Reserved: 0,
+		Type:     1, // ICO format
+		Count:    1, // One image
+	}
+
+	w := uint8(width)
+	h := uint8(height)
+
+	// width/height of 256 should be stored as 0
+	if width == 256 {
+		w = 0
+	}
+	if height == 256 {
+		h = 0
+	}
+
+	iconDirEntry := IconDirEntry{
+		Width:        w,
+		Height:       h,
+		ColorCount:   0, // 0 for PNG (32-bit)
+		Reserved:     0,
+		ColorPlanes:  1,
+		BitsPerPixel: 32, // PNG with alpha
+		SizeInBytes:  uint32(len(pngData)),
+		Offset:       6 + 16, // Size of ICONDIR + ICONDIRENTRY
+	}
+
+	// write IconDir
+	if err := binary.Write(&icoBuf, binary.LittleEndian, iconDir); err != nil {
+		return nil, fmt.Errorf("failed to write ICONDIR: %w", err)
+	}
+
+	// write IconDirEntry
+	if err := binary.Write(&icoBuf, binary.LittleEndian, iconDirEntry); err != nil {
+		return nil, fmt.Errorf("failed to write ICONDIRENTRY: %w", err)
+	}
+
+	// write PNG data directly
+	if _, err := icoBuf.Write(pngData); err != nil {
+		return nil, fmt.Errorf("failed to write PNG data: %w", err)
+	}
+
+	return icoBuf.Bytes(), nil
+}