avcodec/wnv1: Use LE bitstream reader, avoid copying packet, fix memleak

The Winnov WNV1 format is designed for a little-endian bitstream reader;
yet our decoder reversed every byte bitwise (in a buffer only
allocated for this purpose) to use a big-endian bitstream reader. This
commit stops this.

Two things needed to be done to achieve this: The codes in the table used
to initialize a VLC reader needed to be reversed bitwise (when
initializing a VLC in LE mode, it is expected that the first bit to be
read is in the least significant bit; with BE codes the first bit to be
read is the most significant bit of the code) and the following
expression needed to be adapted:

ff_reverse[get_bits(&w->gb, 8 - w->shift)]

But this is easy: When only the bits read are reversed, they coincide
with what a little-endian bitstream reader reads that reads the
original, not-reversed data. But ff_reverse always reverses the full
eight bits and this also performs a shift by (8 - (8 - w->shift)) on top
of reversing the bits read. So the above line needs to be changed to

get_bits(&w->gb, 8 - w->shift) << w->shift

and this also shows why the variable shift is named the way it is.

Finally, this also fixes a hypothetical memleak: For gigantic packets,
initializing a GetBitContext can fail and in this case, the buffer
containing the reversed data would leak.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

0 files changed, 0 insertions, 0 deletions