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op_pcm_seek: Fix int64 overflow

Calculate discard_count directly with op_granpos_diff. this works
because _pcm_offset == (target_gp - pcm_start) and diff == (gp -
pcm_start).

Signed-off-by: Timothy B. Terriberry <[email protected]>

--- a/src/opusfile.c

+++ b/src/opusfile.c

@@ -2597,15 +2597,14 @@

     ogg_int64_t gp;

     gp=_of->prev_packet_gp;

     if(OP_LIKELY(gp!=-1)){

-      int nbuffered;

+      ogg_int64_t discard_count;

+      int         nbuffered;

       nbuffered=OP_MAX(_of->od_buffer_size-_of->od_buffer_pos,0);

       OP_ALWAYS_TRUE(!op_granpos_add(&gp,gp,-nbuffered));

       /*We do _not_ add cur_discard_count to gp.

         Otherwise the total amount to discard could grow without bound, and it

          would be better just to do a full seek.*/

-      if(OP_LIKELY(!op_granpos_diff(&diff,gp,pcm_start))){

-        ogg_int64_t discard_count;

-        discard_count=_pcm_offset-diff;

+      if(OP_LIKELY(!op_granpos_diff(&discard_count,target_gp,gp))){

         /*We use a threshold of 90 ms instead of 80, since 80 ms is the

            _minimum_ we would have discarded after a full seek.

           Assuming 20 ms frames (the default), we'd discard 90 ms on average.*/