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|
package pngmeta
import (
"bytes"
"elefant/models"
"encoding/base64"
"encoding/binary"
"encoding/json"
"errors"
"fmt"
"hash/crc32"
"io"
"os"
)
const (
pngHeader = "\x89PNG\r\n\x1a\n"
textChunkType = "tEXt"
)
// there should be no empty lines inside functions; ai!
// 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: "elefant", // 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
}
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