package main

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"log"
	"os"
)

const fname = "/home/rob/home-office/example.mp3"

const (
	maskSync        = 0b11111111111000000000000000000000
	maskVersion     = 0b00000000000110000000000000000000
	maskLayer       = 0b00000000000001100000000000000000
	maskProtection  = 0b00000000000000010000000000000000
	maskBitrate     = 0b00000000000000001111000000000000
	maskSamplerate  = 0b00000000000000000000110000000000
	maskPadding     = 0b00000000000000000000001000000000
	maskPrivate     = 0b00000000000000000000000100000000
	maskChannelMode = 0b00000000000000000000000011000000
	maskExtension   = 0b00000000000000000000000000110000
	maskCopyright   = 0b00000000000000000000000000001000
	maskOriginal    = 0b00000000000000000000000000000100
	maskEmphasis    = 0b00000000000000000000000000000011
)

type header uint32

func main() {
	f, err := os.Open(fname)
	if err != nil {
		log.Fatal(err)
	}

	bufreader := bufio.NewReader(f)

	buf := make([]byte, 4)
	var i int
	for {
		_, err := io.ReadFull(bufreader, buf)
		if err != nil {
			if err == io.EOF {
				break
			}
			log.Fatal(err)
		}

		header, err := newHeader(buf)

		if err == nil {
			fmt.Printf("Got header: %08X\n", header)
		}
		i++

		bufreader.Discard(3)
	}
}

// TODO use unsafe here? https://codereview.stackexchange.com/a/60161
func newHeader(buf []byte) (header, error) {
	if len(buf) < 4 {
		return 0, errors.New("insufficient bytes")
	}
	if buf[0] != 0xFF || buf[1]&0xE0 != 0xE0 {
		return 0, errors.New("no header")
	}

	// always little-endian for now
	n := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24

	return header(n), nil
}