skip junk in mp3_helix, refactor find_sync_word

[libpicofe.git] / common / mp3.c

diff --git a/common/mp3.c b/common/mp3.c
new file mode 100644 (file)
index 0000000..b00a5fb
--- /dev/null
+++ b/common/mp3.c
@@ -0,0 +1,29 @@
+#include "mp3.h"
+
+int mp3_find_sync_word(const unsigned char *buf, int size)
+{
+       const unsigned char *p, *pe;
+
+       /* find byte-aligned syncword - need 12 (MPEG 1,2) or 11 (MPEG 2.5) matching bits */
+       for (p = buf, pe = buf + size - 3; p <= pe; p++)
+       {
+               int pn;
+               if (p[0] != 0xff)
+                       continue;
+               pn = p[1];
+               if ((pn & 0xf8) != 0xf8 || // currently must be MPEG1
+                   (pn & 6) == 0) {       // invalid layer
+                       p++; continue;
+               }
+               pn = p[2];
+               if ((pn & 0xf0) < 0x20 || (pn & 0xf0) == 0xf0 || // bitrates
+                   (pn & 0x0c) != 0) { // not 44kHz
+                       continue;
+               }
+
+               return p - buf;
+       }
+
+       return -1;
+}
+