3 files changed, 126 insertions, 5 deletions

diff --git a/pngmeta/metareader.go b/pngmeta/metareader.go
index f8ea160..0ebedaa 100644
--- a/pngmeta/metareader.go
+++ b/pngmeta/metareader.go
@@ -8,15 +8,22 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"log/slog"
 	"os"
 	"path"
 	"strings"
 )
 
 const (
-	embType = "tEXt"
+	embType     = "tEXt"
+	cKey        = "chara"
+	IEND        = "IEND"
+	header      = "\x89PNG\r\n\x1a\n"
+	writeHeader = "\x89\x50\x4E\x47\x0D\x0A\x1A\x0A"
 )
 
+var tEXtChunkDataSpecification = "%s\x00%s"
+
 type PngEmbed struct {
 	Key   string
 	Value string
@@ -107,7 +114,7 @@ func readCardJson(fname string) (*models.CharCard, error) {
 	return &card, nil
 }
 
-func ReadDirCards(dirname, uname string) ([]*models.CharCard, error) {
+func ReadDirCards(dirname, uname string, log *slog.Logger) ([]*models.CharCard, error) {
 	files, err := os.ReadDir(dirname)
 	if err != nil {
 		return nil, err
@@ -121,7 +128,7 @@ func ReadDirCards(dirname, uname string) ([]*models.CharCard, error) {
 			fpath := path.Join(dirname, f.Name())
 			cc, err := ReadCard(fpath, uname)
 			if err != nil {
-				// logger.Warn("failed to load card", "error", err)
+				log.Warn("failed to load card", "error", err)
 				continue
 				// return nil, err // better to log and continue
 			}
diff --git a/pngmeta/partsreader.go b/pngmeta/partsreader.go
index b69e4c3..d345a16 100644
--- a/pngmeta/partsreader.go
+++ b/pngmeta/partsreader.go
@@ -14,8 +14,6 @@ var (
 	ErrBadLength     = errors.New("bad length")
 )
 
-const header = "\x89PNG\r\n\x1a\n"
-
 type PngChunk struct {
 	typ         string
 	length      int32
diff --git a/pngmeta/partswriter.go b/pngmeta/partswriter.go
new file mode 100644
index 0000000..7c36daf
--- /dev/null
+++ b/pngmeta/partswriter.go
@@ -0,0 +1,116 @@
+package pngmeta
+
+import (
+	"bytes"
+	"elefant/models"
+	"encoding/base64"
+	"encoding/binary"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"hash/crc32"
+	"io"
+	"os"
+)
+
+type Writer struct {
+	w io.Writer
+}
+
+func NewPNGWriter(w io.Writer) (*Writer, error) {
+	if _, err := io.WriteString(w, writeHeader); err != nil {
+		return nil, err
+	}
+	return &Writer{w}, nil
+}
+
+func (w *Writer) WriteChunk(length int32, typ string, r io.Reader) error {
+	if err := binary.Write(w.w, binary.BigEndian, length); err != nil {
+		return err
+	}
+	if _, err := w.w.Write([]byte(typ)); err != nil {
+		return err
+	}
+	checksummer := crc32.NewIEEE()
+	checksummer.Write([]byte(typ))
+	if _, err := io.CopyN(io.MultiWriter(w.w, checksummer), r, int64(length)); err != nil {
+		return err
+	}
+	if err := binary.Write(w.w, binary.BigEndian, checksummer.Sum32()); err != nil {
+		return err
+	}
+	return nil
+}
+
+func WriteToPng(c *models.CharCardSpec, fpath, outfile string) error {
+	data, err := os.ReadFile(fpath)
+	if err != nil {
+		return err
+	}
+	jsonData, err := json.Marshal(c)
+	if err != nil {
+		return err
+	}
+	// Base64 encode the JSON data
+	base64Data := base64.StdEncoding.EncodeToString(jsonData)
+	pe := PngEmbed{
+		Key:   cKey,
+		Value: base64Data,
+	}
+	w, err := WritetEXtToPngBytes(data, pe)
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(outfile, w.Bytes(), 0666)
+}
+
+func WritetEXtToPngBytes(inputBytes []byte, pe PngEmbed) (outputBytes bytes.Buffer, err error) {
+	if !(string(inputBytes[:8]) == header) {
+		return outputBytes, errors.New("wrong file format")
+	}
+	reader := bytes.NewReader(inputBytes)
+	pngr, err := NewPNGStepReader(reader)
+	if err != nil {
+		return outputBytes, fmt.Errorf("NewReader(): %s", err)
+	}
+	pngw, err := NewPNGWriter(&outputBytes)
+	if err != nil {
+		return outputBytes, fmt.Errorf("NewWriter(): %s", err)
+	}
+	for {
+		chunk, err := pngr.Next()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				break
+			}
+			return outputBytes, fmt.Errorf("NextChunk(): %s", err)
+		}
+		if chunk.Type() != embType {
+			// IENDChunkType will only appear on the final iteration of a valid PNG
+			if chunk.Type() == IEND {
+				// This is where we inject tEXtChunkType as the penultimate chunk with the new value
+				newtEXtChunk := []byte(fmt.Sprintf(tEXtChunkDataSpecification, pe.Key, pe.Value))
+				if err := pngw.WriteChunk(int32(len(newtEXtChunk)), embType, bytes.NewBuffer(newtEXtChunk)); err != nil {
+					return outputBytes, fmt.Errorf("WriteChunk(): %s", err)
+				}
+				// Now we end the buffer with IENDChunkType chunk
+				if err := pngw.WriteChunk(chunk.length, chunk.Type(), chunk); err != nil {
+					return outputBytes, fmt.Errorf("WriteChunk(): %s", err)
+				}
+			} else {
+				// writes back original chunk to buffer
+				if err := pngw.WriteChunk(chunk.length, chunk.Type(), chunk); err != nil {
+					return outputBytes, fmt.Errorf("WriteChunk(): %s", err)
+				}
+			}
+		} else {
+			if _, err := io.Copy(io.Discard, chunk); err != nil {
+				return outputBytes, fmt.Errorf("io.Copy(io.Discard, chunk): %s", err)
+			}
+		}
+		if err := chunk.Close(); err != nil {
+			return outputBytes, fmt.Errorf("chunk.Close(): %s", err)
+		}
+	}
+	return outputBytes, nil
+}