| 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 | |