package main

import (
	"encoding/binary"
	"errors"
	"io"
	"os"
)

func detectAudioFormat(path string) (string, error) {
	f, err := os.Open(path)
	if err != nil {
		return "", err
	}
	defer f.Close()
	b := make([]byte, 64*1024)
	n, err := f.Read(b)
	if err != nil && err != io.EOF {
		return "", err
	}
	b = b[:n]
	if len(b) >= 4 && string(b[:4]) == "fLaC" {
		return "flac", nil
	}
	if len(b) >= 12 && string(b[:4]) == "RIFF" && string(b[8:12]) == "WAVE" {
		return "wav", nil
	}
	if len(b) >= 12 && string(b[:4]) == "FORM" && (string(b[8:12]) == "AIFF" || string(b[8:12]) == "AIFC") {
		return "aiff", nil
	}
	if len(b) >= 4 && string(b[:4]) == "OggS" {
		return "ogg", nil
	}
	if len(b) >= 12 && string(b[4:8]) == "ftyp" {
		return "m4a", nil
	}
	start := 0
	if len(b) >= 10 && string(b[:3]) == "ID3" {
		size := int(b[6]&0x7f)<<21 | int(b[7]&0x7f)<<14 | int(b[8]&0x7f)<<7 | int(b[9]&0x7f)
		start = 10 + size
		if start > len(b) {
			start = 10
		}
	}
	for i := start; i+4 <= len(b) && i < start+4096; i++ {
		if b[i] == 0xff && b[i+1]&0xe0 == 0xe0 {
			h := binary.BigEndian.Uint32(b[i : i+4])
			layer := (h >> 17) & 3
			if layer == 1 {
				return "mp3", nil
			}
			if b[i+1]&0x06 == 0x06 {
				return "aac", nil
			}
		}
	}
	return "", errors.New("unknown audio format")
}