1 files changed, 133 insertions, 0 deletions
diff --git a/pngmeta/altwriter.go b/pngmeta/altwriter.go
new file mode 100644
index 0000000..206b563
--- /dev/null
+++ b/pngmeta/altwriter.go
@@ -0,0 +1,133 @@
+package pngmeta
+
+import (
+	"bytes"
+	"gf-lt/models"
+	"encoding/base64"
+	"encoding/binary"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"hash/crc32"
+	"io"
+	"os"
+)
+
+const (
+	pngHeader     = "\x89PNG\r\n\x1a\n"
+	textChunkType = "tEXt"
+)
+
+// WriteToPng embeds the metadata into the specified PNG file and writes the result to outfile.
+func WriteToPng(metadata *models.CharCardSpec, sourcePath, outfile string) error {
+	pngData, err := os.ReadFile(sourcePath)
+	if err != nil {
+		return err
+	}
+	jsonData, err := json.Marshal(metadata)
+	if err != nil {
+		return err
+	}
+	base64Data := base64.StdEncoding.EncodeToString(jsonData)
+	embedData := PngEmbed{
+		Key:   "gf-lt", // Replace with appropriate key constant
+		Value: base64Data,
+	}
+	var outputBuffer bytes.Buffer
+	if _, err := outputBuffer.Write([]byte(pngHeader)); err != nil {
+		return err
+	}
+	chunks, iend, err := processChunks(pngData[8:])
+	if err != nil {
+		return err
+	}
+	for _, chunk := range chunks {
+		outputBuffer.Write(chunk)
+	}
+	newChunk, err := createTextChunk(embedData)
+	if err != nil {
+		return err
+	}
+	outputBuffer.Write(newChunk)
+	outputBuffer.Write(iend)
+	return os.WriteFile(outfile, outputBuffer.Bytes(), 0666)
+}
+
+// processChunks extracts non-tEXt chunks and locates the IEND chunk
+func processChunks(data []byte) ([][]byte, []byte, error) {
+	var (
+		chunks    [][]byte
+		iendChunk []byte
+		reader    = bytes.NewReader(data)
+	)
+	for {
+		var chunkLength uint32
+		if err := binary.Read(reader, binary.BigEndian, &chunkLength); err != nil {
+			if errors.Is(err, io.EOF) {
+				break
+			}
+			return nil, nil, fmt.Errorf("error reading chunk length: %w", err)
+		}
+		chunkType := make([]byte, 4)
+		if _, err := reader.Read(chunkType); err != nil {
+			return nil, nil, fmt.Errorf("error reading chunk type: %w", err)
+		}
+		chunkData := make([]byte, chunkLength)
+		if _, err := reader.Read(chunkData); err != nil {
+			return nil, nil, fmt.Errorf("error reading chunk data: %w", err)
+		}
+		crc := make([]byte, 4)
+		if _, err := reader.Read(crc); err != nil {
+			return nil, nil, fmt.Errorf("error reading CRC: %w", err)
+		}
+		fullChunk := bytes.NewBuffer(nil)
+		if err := binary.Write(fullChunk, binary.BigEndian, chunkLength); err != nil {
+			return nil, nil, fmt.Errorf("error writing chunk length: %w", err)
+		}
+		if _, err := fullChunk.Write(chunkType); err != nil {
+			return nil, nil, fmt.Errorf("error writing chunk type: %w", err)
+		}
+		if _, err := fullChunk.Write(chunkData); err != nil {
+			return nil, nil, fmt.Errorf("error writing chunk data: %w", err)
+		}
+		if _, err := fullChunk.Write(crc); err != nil {
+			return nil, nil, fmt.Errorf("error writing CRC: %w", err)
+		}
+		switch string(chunkType) {
+		case "IEND":
+			iendChunk = fullChunk.Bytes()
+			return chunks, iendChunk, nil
+		case textChunkType:
+			continue // Skip existing tEXt chunks
+		default:
+			chunks = append(chunks, fullChunk.Bytes())
+		}
+	}
+	return nil, nil, errors.New("IEND chunk not found")
+}
+
+// createTextChunk generates a valid tEXt chunk with proper CRC
+func createTextChunk(embed PngEmbed) ([]byte, error) {
+	content := bytes.NewBuffer(nil)
+	content.WriteString(embed.Key)
+	content.WriteByte(0) // Null separator
+	content.WriteString(embed.Value)
+	data := content.Bytes()
+	crc := crc32.NewIEEE()
+	crc.Write([]byte(textChunkType))
+	crc.Write(data)
+	chunk := bytes.NewBuffer(nil)
+	if err := binary.Write(chunk, binary.BigEndian, uint32(len(data))); err != nil {
+		return nil, fmt.Errorf("error writing chunk length: %w", err)
+	}
+	if _, err := chunk.Write([]byte(textChunkType)); err != nil {
+		return nil, fmt.Errorf("error writing chunk type: %w", err)
+	}
+	if _, err := chunk.Write(data); err != nil {
+		return nil, fmt.Errorf("error writing chunk data: %w", err)
+	}
+	if err := binary.Write(chunk, binary.BigEndian, crc.Sum32()); err != nil {
+		return nil, fmt.Errorf("error writing CRC: %w", err)
+	}
+	return chunk.Bytes(), nil
+}