avcodec/dca_parser: skip initial padding

Padding before the first sync word can be very large for DTS-in-WAV
streams. There is no reason to include this padding in parsed packet.
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/dca_parser.c

@@ -33,6 +33,7 @@ typedef struct DCAParseContext {
     uint32_t lastmarker;
     int size;
     int framesize;
+    unsigned int startpos;
     DCAExssParser exss;
     unsigned int sr_code;
 } DCAParseContext;
@@ -75,20 +76,27 @@ static int dca_find_frame_end(DCAParseContext *pc1, const uint8_t *buf,
 
     i = 0;
     if (!start_found) {
-        for (i = 0; i < buf_size; i++) {
+        for (; i < buf_size; i++) {
+            size++;
             state = (state << 8) | buf[i];
-            if (IS_MARKER(state)) {
-                if (!pc1->lastmarker ||
-                    pc1->lastmarker == CORE_MARKER(state) ||
-                    pc1->lastmarker == DCA_SYNCWORD_SUBSTREAM) {
-                    start_found = 1;
-                    if (IS_EXSS_MARKER(state))
-                        pc1->lastmarker = EXSS_MARKER(state);
-                    else
-                        pc1->lastmarker = CORE_MARKER(state);
-                    i++;
-                    break;
-                }
+
+            if (IS_MARKER(state) &&
+                (!pc1->lastmarker ||
+                  pc1->lastmarker == CORE_MARKER(state) ||
+                  pc1->lastmarker == DCA_SYNCWORD_SUBSTREAM)) {
+                if (!pc1->lastmarker)
+                    pc1->startpos = IS_EXSS_MARKER(state) ? size - 4 : size - 6;
+
+                if (IS_EXSS_MARKER(state))
+                    pc1->lastmarker = EXSS_MARKER(state);
+                else
+                    pc1->lastmarker = CORE_MARKER(state);
+
+                start_found = 1;
+                size        = 0;
+
+                i++;
+                break;
             }
         }
     }
@@ -284,6 +292,13 @@ static int dca_parse(AVCodecParserContext *s, AVCodecContext *avctx,
             *poutbuf_size = 0;
             return buf_size;
         }
+
+        /* skip initial padding */
+        if (buf_size  > pc1->startpos) {
+            buf      += pc1->startpos;
+            buf_size -= pc1->startpos;
+        }
+        pc1->startpos = 0;
     }
 
     /* read the duration and sample rate from the frame header */