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.
No functional change, but now much clearer.
For the check p[-2] != 0:
Also reduce the number of iterations by correctly starting with three new bytes
on the next iteration,
instead of keeping byte p[-3] which is invalid, i.e. known to be 01 when it
must be 00.
---
libavcodec/utils.c | 25 ++++++++++++++++++++-----
1 file changed, 20 insertions(+), 5 deletions(-)
diff --git a/libavcodec/utils.c b/libavcodec/utils.c
index cb4437edc2..8f8cc820bd 100644
--- a/libavcodec/utils.c
+++ b/libavcodec/utils.c
@@ -957,12 +957,26 @@ 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++;
+ // UU UU UU
+ if (p[-1] > 1) p += 3; // start check over with 3 new bytes
+ else if (p[-1] == 0) p++; // could be in a start code, so check
next byte
+ // this should be one comparison against 1 since p is unsigned,
+ // i.e. p[-1] == 0 is equivalent to p[-1] < 1
+
+ // UU UU 01
+ else if (p[-2] != 0) p += 3; // we have UU YY 01, so increment by 3
+ // to start check over with 3 new bytes
+ // UU 00 01
+ else if (p[-3] != 0) p += 3; // we have YY 00 01, so increment by 3
+ // to start check over with 3 new bytes
+ // 00 00 01
else {
- p++;
+ p++; // p now points at the address following the start code value
XX
break;
}
}
@@ -972,7 +986,8 @@ const uint8_t *avpriv_find_start_code(const uint8_t
*av_restrict p,
// this will cause the last 4 bytes before end to be read,
// i.e. no out of bounds memory access occurs
- *state = AV_RB32(p - 4); // read the previous 4 bytes
+ *state = AV_RB32(p - 4);
+ // read the previous 4 bytes, i.e. bytes {p - 4, p - 3, p - 2, p - 1}
return p;
}
--
2.32.0
_______________________________________________
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".
