7c18e34a |
1 | #include "mp3.h" |
2 | |
3 | int mp3_find_sync_word(const unsigned char *buf, int size) |
4 | { |
5 | const unsigned char *p, *pe; |
6 | |
7 | /* find byte-aligned syncword - need 12 (MPEG 1,2) or 11 (MPEG 2.5) matching bits */ |
8 | for (p = buf, pe = buf + size - 3; p <= pe; p++) |
9 | { |
10 | int pn; |
11 | if (p[0] != 0xff) |
12 | continue; |
13 | pn = p[1]; |
14 | if ((pn & 0xf8) != 0xf8 || // currently must be MPEG1 |
15 | (pn & 6) == 0) { // invalid layer |
16 | p++; continue; |
17 | } |
18 | pn = p[2]; |
19 | if ((pn & 0xf0) < 0x20 || (pn & 0xf0) == 0xf0 || // bitrates |
20 | (pn & 0x0c) != 0) { // not 44kHz |
21 | continue; |
22 | } |
23 | |
24 | return p - buf; |
25 | } |
26 | |
27 | return -1; |
28 | } |
29 | |