package transcode

import (
    "bytes"
    "io"
)

// sniffFormat detects container/codec from file header when the path has no extension.
func sniffFormat(r io.Reader) string {
    head := make([]byte, 32)
    n, _ := io.ReadFull(r, head)
    head = head[:n]
    if len(head) < 4 {
        return ""
    }
    if bytes.HasPrefix(head, []byte("RIFF")) && len(head) >= 12 && bytes.Equal(head[8:12], []byte("WAVE")) {
        return ".wav"
    }
    if bytes.HasPrefix(head, []byte("ID3")) {
        return ".mp3"
    }
    if len(head) >= 2 && head[0] == 0xFF && (head[1]&0xE0) == 0xE0 {
        return ".mp3"
    }
    if bytes.HasPrefix(head, []byte("fLaC")) {
        return ".flac"
    }
    if bytes.HasPrefix(head, []byte("OggS")) {
        return ".ogg"
    }
    if len(head) >= 8 && bytes.Equal(head[4:8], []byte("ftyp")) {
        return ".m4a"
    }
    if bytes.HasPrefix(head, []byte{0xFF, 0xF1}) || bytes.HasPrefix(head, []byte{0xFF, 0xF9}) {
        return ".aac"
    }
    if bytes.HasPrefix(head, []byte("FORM")) && len(head) >= 12 && bytes.Equal(head[8:12], []byte("AIFF")) {
        return ".aiff"
    }
    if bytes.HasPrefix(head, []byte{0x1A, 0x45, 0xDF, 0xA3}) {
        return ".webm"
    }
    return ""
}