avcodec/mpeg4_unpack_bframes_bsf: Use avpriv_find_start_code

instead of an ad-hoc function to search for start codes in order to
remove code duplication and to improve performance.

Improved performance of finding startcodes from 52606 decicycles to
9543 decicycles based upon 262144 runs for a 1 Mb/s MPEG4 video.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg4_unpack_bframes_bsf.c

@@ -21,50 +21,36 @@
 
 #include "avcodec.h"
 #include "bsf.h"
+#include "internal.h"
 #include "mpeg4video.h"
 
 typedef struct UnpackBFramesBSFContext {
     AVPacket *b_frame;
 } UnpackBFramesBSFContext;
 
-/* search next start code */
-static unsigned int find_startcode(const uint8_t *buf, int buf_size, int *pos)
-{
-    unsigned int startcode = 0xFF;
-
-    for (; *pos < buf_size;) {
-        startcode = ((startcode << 8) | buf[*pos]) & 0xFFFFFFFF;
-        *pos +=1;
-        if ((startcode & 0xFFFFFF00) != 0x100)
-            continue;  /* no startcode */
-        return startcode;
-    }
-
-    return 0;
-}
-
 /* determine the position of the packed marker in the userdata,
  * the number of VOPs and the position of the second VOP */
 static void scan_buffer(const uint8_t *buf, int buf_size,
                         int *pos_p, int *nb_vop, int *pos_vop2) {
-    unsigned int startcode;
-    int pos, i;
+    uint32_t startcode;
+    const uint8_t *end = buf + buf_size, *pos = buf;
 
-    for (pos = 0; pos < buf_size;) {
-        startcode = find_startcode(buf, buf_size, &pos);
+    while (pos < end) {
+        startcode = -1;
+        pos = avpriv_find_start_code(pos, end, &startcode);
 
         if (startcode == USER_DATA_STARTCODE && pos_p) {
             /* check if the (DivX) userdata string ends with 'p' (packed) */
-            for (i = 0; i < 255 && pos + i + 1 < buf_size; i++) {
-                if (buf[pos + i] == 'p' && buf[pos + i + 1] == '\0') {
-                    *pos_p = pos + i;
+            for (int i = 0; i < 255 && pos + i + 1 < end; i++) {
+                if (pos[i] == 'p' && pos[i + 1] == '\0') {
+                    *pos_p = pos + i - buf;
                     break;
                 }
             }
         } else if (startcode == VOP_STARTCODE && nb_vop) {
             *nb_vop += 1;
             if (*nb_vop == 2 && pos_vop2) {
-                *pos_vop2 = pos - 4; /* subtract 4 bytes startcode */
+                *pos_vop2 = pos - buf - 4; /* subtract 4 bytes startcode */
             }
         }
     }