Solution to challenge from live stream (https://goo.gl/jRPkth)

png_fix.go

package main

/*

	Solution to challenge from live stream (https://goo.gl/jRPkth)

	Generate colormap
	perl -e ' print "\xff\x00\x00\x00\xff\x00\x00\x00\xff\xff\xff\x00"; print "\x00"x12;' > colormap.bin

	$: go run png_fix.go file.png colormap.bin
*/
import (
	"bytes"
	"encoding/binary"
	"fmt"
	"hash/crc32"
	"io"
	"io/ioutil"
	"os"
)

const (
	CHUNK_START_OFFSET = 8
)

type chunk struct {
	Length uint32
	Type   [4]byte
	Data   []byte
	CRC    uint32
}

func Decode(r io.ReadSeeker) []chunk {

	var chunks = make([]chunk, 0, 0)

	r.Seek(CHUNK_START_OFFSET, os.SEEK_SET)

	for {
		var ch chunk
		err := binary.Read(r, binary.BigEndian, &ch.Length)

		if err != nil {
			break
		}

		err = binary.Read(r, binary.BigEndian, &ch.Type)

		if err != nil {
			break
		}

		ch.Data = make([]byte, ch.Length)
		err = binary.Read(r, binary.BigEndian, ch.Data)

		if err != nil {
			break
		}

		err = binary.Read(r, binary.BigEndian, &ch.CRC)
		chunks = append(chunks, ch)
	}

	return chunks
}

func is_png(fd *os.File) bool {
	fd.Seek(1, os.SEEK_SET)

	buf := make([]byte, 3)

	if _, err := fd.Read(buf); err != nil {
		return false
	}

	if string(buf) == "PNG" {
		return true
	}

	return false
}

func main() {

	if len(os.Args) < 3 {
		fmt.Printf("file.png colormap.bin\n")
		os.Exit(-1)
	}

	path := os.Args[1]

	fd, err := os.Open(path)

	if err != nil {
		os.Exit(-2)
	}

	defer fd.Close()

	chunks := Decode(fd)

	fd_plte, err := os.Open(os.Args[2])

	if err != nil {
		panic(err.Error())
	}

	buff, err := ioutil.ReadAll(fd_plte)

	if err != nil {
		panic(err.Error())
	}

	fmt.Printf("Color map loaded\n")
	fmt.Printf("[%x]\n", buff)

	save_png(chunks, "mod.png", buff)

}

func save_png(chunks []chunk, fname string, plte []byte) {

	fd, err := os.OpenFile(fname, os.O_WRONLY|os.O_CREATE, 0660)

	if err != nil {
		panic(err.Error())
	}

	defer fd.Close()

	header := []byte{0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a}
	_, err = fd.Write(header)

	if err != nil {
		panic(err.Error())
	}

	fmt.Printf("CHUNK\tCRC32\n")
	fmt.Printf("--------------\n")

	for _, ch := range chunks {

		fmt.Printf("%s: 0x%x\n", ch.Type, ch.CRC)

		binary.Write(fd, binary.BigEndian, ch.Length)
		binary.Write(fd, binary.BigEndian, ch.Type)

		// I know right, it ain't perfect.
		if ch.Type[0] == 0x50 && plte != nil {
			binary.Write(fd, binary.BigEndian, plte)
			table := crc32.MakeTable(crc32.IEEE)

			var buffer bytes.Buffer

			binary.Write(&buffer, binary.BigEndian, ch.Type)
			buffer.Write(plte)

			crc := crc32.Checksum(buffer.Bytes(), table)
			binary.Write(fd, binary.BigEndian, crc)
			fmt.Printf("PLTE: 0x%x [UPDATE]\n", crc)

			continue
		}

		binary.Write(fd, binary.BigEndian, ch.Data)
		binary.Write(fd, binary.BigEndian, ch.CRC)
	}
}