The expected number of iterations may increase by one for an input
of alternating 0 and 1 bytes. Instead of incrementing by 2 everytime,
it now alternates between incrementing by 1 and by 3.
For the check p[-2] != 0:
This slightly reduces the number of iterations by starting with three
new bytes on the next iteration, instead of keeping byte p[-3] which
is invalid, since it is now known to be 01 when it must be 00.
No other observable change.
---
libavcodec/utils.c | 20 ++++++++++++++++----
1 file changed, 16 insertions(+), 4 deletions(-)
diff --git a/libavcodec/utils.c b/libavcodec/utils.c
index d6ab21b1a0..fc8cd87366 100644
--- a/libavcodec/utils.c
+++ b/libavcodec/utils.c
@@ -996,12 +996,24 @@ const uint8_t *avpriv_find_start_code(const uint8_t
*av_restrict p,
return p;
}
+ /* with memory address increasing left to right, we are looking for (in
hexadecimal):
+ * 00 00 01 XX
+ * p points at the address which should have the value of XX
+ */
while (p < end) {
- if (p[-1] > 1 ) p += 3;
- else if (p[-2] ) p += 2;
- else if (p[-3]|(p[-1]-1)) p++;
- else {
+ if (/* UU UU UU */ p[-1] < 1) { // equivalently p[-1] == 0
p++;
+ // could be in a start code, so check next byte
+ }
+ else if (/* UU UU UN */ p[-1] > 1 ||
+ /* UU UU 01 */ p[-2] != 0 ||
+ /* UU 00 01 */ p[-3] != 0) {
+ // start check over with 3 new bytes
+ p += 3;
+ }
+ else { /* 00 00 01 */
+ p++;
+ // p now points at the address following the start code value XX
break;
}
}
--
2.34.1
_______________________________________________
ffmpeg-devel mailing list
ffmpeg-devel@ffmpeg.org
https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
To unsubscribe, visit link above, or email
ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
