badge.go

package main

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"io"
	"log"
	"os"
)

// Block
// (4) length
// (4) type
// (n) data
// (4) CRC
func readNextBlock(f *os.File) (string, []byte, error) {
	var chunkLen uint32
	var chunkType [4]byte
	if err := binary.Read(f, binary.BigEndian, &chunkLen); err != nil {
		return "", nil, err
	}
	if err := binary.Read(f, binary.BigEndian, &chunkType); err != nil {
		return "", nil, err
	}

	data := make([]byte, chunkLen)
	if err := binary.Read(f, binary.BigEndian, &data); err != nil {
		return "", nil, err
	}

	f.Seek(4, os.SEEK_CUR) // skip CRC

	return string(chunkType[:]), data, nil
}

func readToNextNull(r io.ReadSeeker) ([]byte, error) {
	buf := []byte{}
	b := make([]byte, 1)
	for {
		_, err := r.Read(b)
		if err != nil {
			if err == io.EOF {
				return buf, nil
			}
			if err != nil {
				return nil, err
			}
		}
		if b[0] == 0x00 {
			return buf, nil
		}
		buf = append(buf, b[0])
	}
}

func main() {
	f, err := os.Open("./badge.png")
	if err != nil {
		log.Fatal(err)
	}
	defer f.Close()

	// check png header
	var header [8]byte
	if err := binary.Read(f, binary.BigEndian, &header); err != nil {
		log.Fatal(err)
	}
	pngHeader := []byte{0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a}
	if !bytes.Equal(header[:], pngHeader) {
		log.Fatal("Not a PNG file")
	}

	var itxt *bytes.Reader

	// check block & find itxt openbadges chunk
	for {
		t, data, err := readNextBlock(f)
		if err != nil {
			log.Fatal(err)
		}

		if t == "iTXt" {
			itxt = bytes.NewReader(data)
			kw, err := readToNextNull(itxt)
			if err != nil {
				log.Fatal(err)
			}
			if bytes.Compare(kw, []byte("openbadges")) == 0 {
				break
			}
		}
	}

	// skip compression
	itxt.Seek(1, os.SEEK_CUR)
	// skip compression format
	itxt.Seek(1, os.SEEK_CUR)
	// skip language tag
	readToNextNull(itxt)
	// skip translated keyword
	readToNextNull(itxt)

	text, err := readToNextNull(itxt)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("%s\n", string(text))
}