avcodec/ac3dec: split off code discarding garbage at the beginning of a packet

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/ac3_parser.c

@@ -53,6 +53,25 @@ static const uint8_t center_levels[4] = { 4, 5, 6, 5 };
  */
 static const uint8_t surround_levels[4] = { 4, 6, 7, 6 };
 
+int ff_ac3_find_syncword(const uint8_t *buf, int buf_size)
+{
+    int i;
+
+    for (i = 1; i < buf_size; i += 2) {
+        if (buf[i] == 0x77 || buf[i] == 0x0B) {
+            if ((buf[i] ^ buf[i-1]) == (0x77 ^ 0x0B)) {
+                i--;
+                break;
+            } else if ((buf[i] ^ buf[i+1]) == (0x77 ^ 0x0B)) {
+                break;
+            }
+        }
+    }
+    if (i >= buf_size)
+        return AVERROR_INVALIDDATA;
+
+    return i;
+}
 
 int ff_ac3_parse_header(GetBitContext *gbc, AC3HeaderInfo *hdr)
 {

libavcodec/ac3_parser_internal.h

@@ -79,4 +79,6 @@ int ff_ac3_parse_header(GetBitContext *gbc, AC3HeaderInfo *hdr);
 int avpriv_ac3_parse_header(AC3HeaderInfo **hdr, const uint8_t *buf,
                             size_t size);
 
+int ff_ac3_find_syncword(const uint8_t *buf, int buf_size);
+
 #endif /* AVCODEC_AC3_PARSER_INTERNAL_H */

libavcodec/ac3dec.c

@@ -1508,19 +1508,8 @@ static int ac3_decode_frame(AVCodecContext *avctx, AVFrame *frame,
     s->superframe_size = 0;
 
     buf_size = full_buf_size;
-    for (i = 1; i < buf_size; i += 2) {
-        if (buf[i] == 0x77 || buf[i] == 0x0B) {
-            if ((buf[i] ^ buf[i-1]) == (0x77 ^ 0x0B)) {
-                i--;
-                break;
-            } else if ((buf[i] ^ buf[i+1]) == (0x77 ^ 0x0B)) {
-                break;
-            }
-        }
-    }
-    if (i >= buf_size)
-        return AVERROR_INVALIDDATA;
-    if (i > 10)
+    i = ff_ac3_find_syncword(buf, buf_size);
+    if (i < 0 || i > 10)
         return i;
     buf += i;
     buf_size -= i;