Golang MapleStory ManualPatcher patchfile exporter

main.go

package main

import (
	"bytes"
	"encoding/binary"
	"encoding/hex"
	"errors"
	"flag"
	"fmt"
	"io"
	"io/ioutil"
	"os"
)

func main() {
	var input string
	flag.StringVar(&input, "input", "", "Input file")
	flag.Parse()
	if input == "" {
		fmt.Println("Try again")
		return
	}

	file, err := os.Open(input)
	if err != nil {
		panic(err)
	}

	newPos, err := file.Seek(0, os.SEEK_END)
	if err != nil {
		panic(err)
	}
	newPos -= 12

	b := make([]byte, 12)
	_, err = file.ReadAt(b, newPos)
	if err != nil && err != io.EOF {
		panic(err)
	}

	fmt.Println(hex.Dump(b))

	buf := bytes.NewReader(b)
	var patchfileSize, noticeSize uint32

	if err := binary.Read(buf, binary.LittleEndian, &patchfileSize); err != nil {
		panic(err)
	}
	if err := binary.Read(buf, binary.LittleEndian, &noticeSize); err != nil {
		panic(err)
	}

	if b[8] != 0xF3 || b[9] != 0xFB || b[10] != 0xF7 || b[11] != 0xF2 {
		panic(errors.New("Not a valid manualpatcher (incorrect magic value)"))
	}

	fmt.Println("Patch notice length: ", noticeSize)
	fmt.Println("Patch file size: ", patchfileSize)

	// Read notice
	b = make([]byte, noticeSize)

	newPos -= int64(noticeSize)
	_, err = file.ReadAt(b, newPos)
	if err != nil {
		panic(err)
	}

	fmt.Println("Notice:")
	fmt.Println("----------------------")
	fmt.Println(string(b))
	fmt.Println("----------------------")

	// Read patchfile

	b = make([]byte, patchfileSize)

	newPos -= int64(patchfileSize)
	_, err = file.ReadAt(b, newPos)
	if err != nil {
		panic(err)
	}

	ioutil.WriteFile(input+".patch", b, 0644)

	fmt.Println("Done.")

}