[FFmpeg-cvslog] avcodec/cbs_mpeg2: Simplify splitting fragment

[Andreas Rheinhardt]

ffmpeg | branch: master | Andreas Rheinhardt <andreas.rheinhardt at outlook.com> | Fri Feb  4 14:17:33 2022 +0100| [115ac90ec99af35838f65fc5b8f1b27e2ca4ab37] | committer: Andreas Rheinhardt

avcodec/cbs_mpeg2: Simplify splitting fragment

avpriv_find_start_code() supports non-contiguous buffers
by maintaining a state that allows to find start codes
that span across multiple buffers; a consequence thereof
is that avpriv_find_start_code() is given a zero-sized
buffer, it does not modify this state, so that it appears
as if a start code was found if the state contained a start code.

This can e.g. happen with Sequence End units in MPEG-2 and
to counter this, cbs_mpeg2_split_fragment() reset the state
when it has already encountered the end of the fragment
in order to add the last unit (if it is only of the form 00 00 01 xy)
only once; it also used a flag to set whether this is the final unit.

Yet this can be improved by simply resetting state unconditionally
(thereby avoiding a branch); the flag can be removed by just checking
whether we have a valid start code (of the next unit to add)
at the end.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=115ac90ec99af35838f65fc5b8f1b27e2ca4ab37
---

 libavcodec/cbs_mpeg2.c | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/libavcodec/cbs_mpeg2.c b/libavcodec/cbs_mpeg2.c
index 2bf38c6001..11c43c2a47 100644
--- a/libavcodec/cbs_mpeg2.c
+++ b/libavcodec/cbs_mpeg2.c
@@ -149,7 +149,6 @@ static int cbs_mpeg2_split_fragment(CodedBitstreamContext *ctx,
     uint32_t start_code = -1;
     size_t unit_size;
     int err;
-    int final = 0;
 
     start = avpriv_find_start_code(frag->data, frag->data + frag->data_size,
                                    &start_code);
@@ -161,14 +160,11 @@ static int cbs_mpeg2_split_fragment(CodedBitstreamContext *ctx,
     do {
         unit_type = start_code & 0xff;
 
-        if (start == frag->data + frag->data_size) {
-            // The last four bytes form a start code which constitutes
-            // a unit of its own.  In this situation avpriv_find_start_code
-            // won't modify start_code at all so modify start_code so that
-            // the next unit will be treated as the last unit.
-            start_code = 0;
-        }
-
+        // Reset start_code to ensure that avpriv_find_start_code()
+        // really reads a new start code and does not reuse the old
+        // start code in any way (as e.g. happens when there is a
+        // Sequence End unit at the very end of a packet).
+        start_code = UINT32_MAX;
         end = avpriv_find_start_code(start--, frag->data + frag->data_size,
                                      &start_code);
 
@@ -183,7 +179,6 @@ static int cbs_mpeg2_split_fragment(CodedBitstreamContext *ctx,
         } else {
            // We didn't find a start code, so this is the final unit.
            unit_size = end - start;
-           final     = 1;
         }
 
         err = ff_cbs_append_unit_data(frag, unit_type, (uint8_t*)start,
@@ -192,7 +187,9 @@ static int cbs_mpeg2_split_fragment(CodedBitstreamContext *ctx,
             return err;
 
         start = end;
-    } while (!final);
+
+        // Do we have a further unit to add to the fragment?
+    } while ((start_code >> 8) == 0x000001);
 
     return 0;
 }