[FFmpeg-devel] [PATCH 1/2] avcodec/parser: simplify ff_mpeg4video_split()

[Michael Niedermayer]

On Sat, Feb 14, 2015 at 06:00:40PM +0800, zhaoxiu.zeng wrote:
> 在 2015/2/13 17:22, wm4 写道:
> > On Fri, 13 Feb 2015 13:50:23 +0800
> > Zhaoxiu Zeng <zhaoxiu.zeng at gmail.com> wrote:
> > 
> >> From 3cac16572aee4425377e4bc9e496ab5844200a51 Mon Sep 17 00:00:00 2001
> >> From: Zeng Zhaoxiu <zhaoxiu.zeng at gmail.com>
> >> Date: Fri, 13 Feb 2015 13:27:26 +0800
> >> Subject: [PATCH 1/2] avcodec/parser: simplify ff_mpeg4video_split()
> >>
> >> Signed-off-by: Zeng Zhaoxiu <zhaoxiu.zeng at gmail.com>
> >> ---
> >>  libavcodec/parser.c | 10 ++++++----
> >>  1 file changed, 6 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/libavcodec/parser.c b/libavcodec/parser.c
> >> index aa25481..83019e7 100644
> >> --- a/libavcodec/parser.c
> >> +++ b/libavcodec/parser.c
> >> @@ -28,6 +28,7 @@
> >>  #include "libavutil/mem.h"
> >>
> >>  #include "parser.h"
> >> +#include "internal.h"
> >>
> >>  static AVCodecParser *av_first_parser = NULL;
> >>
> >> @@ -308,13 +309,14 @@ void ff_parse_close(AVCodecParserContext *s)
> >>
> >>  int ff_mpeg4video_split(AVCodecContext *avctx, const uint8_t *buf,
> >> int buf_size)
> >>  {
> >> -    int i;
> >>      uint32_t state = -1;
> >> +    const uint8_t *ptr = buf, *end = buf + buf_size;
> >>
> >> -    for (i = 0; i < buf_size; i++) {
> >> -        state = state << 8 | buf[i];
> >> +    while (ptr < end) {
> >> +        ptr = avpriv_find_start_code(ptr, end, &state);
> >>          if (state == 0x1B3 || state == 0x1B6)
> >> -            return i - 3;
> >> +            return ptr - 4 - buf;
> >>      }
> >> +
> >>      return 0;
> >>  }
> > 
> > I don't know, it looks more complicated to me now. Following track of
> > an integer index is much easier than playing with pointer arithmetics.
> > Also, why this (apparently) can return a negative integer index just
> > fine, your pointer calculations can go out of the range [buf,
> > buf+buf_size], possibly leading to undefined behavior. The code
> 
> No problem. In this case, if state is 0x000001xx ptr must behind "buf + 4",
> else ptr equal to end.
> 
> > introduces a function call, without actually reducing the number of
> > lines.
> 
> avpriv_find_start_code is faster than the byte-by-byte version!
> Also, we would optimize avpriv_find_start_code in the future.

applied

thanks

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The misfortune of the wise is better than the prosperity of the fool.
-- Epicurus
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