Opus demuxing fixes.

This properly handles pre-skip, seeking pre-roll, and end-trim. It
uses the i_nb_samples field of block_t to signal to the decoder how
many samples should be returned after (possibly) skipping some at
the start of the block (for both pre-skip and pre-roll). In
addition, it abuses the i_length field of block_t to signal to the
decoder how many samples to trim from the end of the packet (for
end-trimming).

This patch does not compute correct timestamps for streams which
start at a non-zero offset (e.g., live streams joined in the
middle), nor does it correctly compute the length of a stream. But
neither of those things work for Vorbis, either. I'm leaving them
for a follow-up patch.
Signed-off-by: Rafaël Carré <funman@videolan.org>

modules/demux/ogg.h

@@ -55,6 +55,7 @@ typedef struct logical_stream_s
     int              i_packets_backup;
     void             *p_headers;
     int              i_headers;
+    ogg_int64_t      i_previous_granulepos;
 
     /* program clock reference (in units of 90kHz) derived from the previous
      * granulepos */
@@ -67,6 +68,8 @@ typedef struct logical_stream_s
     int i_granule_shift;
     /* Opus has a starting offset in the headers. */
     int i_pre_skip;
+    /* Vorbis and Opus can trim the end of a stream using granule positions. */
+    int i_end_trim;
 
     /* offset of first keyframe for theora; can be 0 or 1 depending on version number */
     int64_t i_keyframe_offset;

modules/demux/oggseek.c

@@ -776,14 +776,26 @@ int oggseek_find_frame ( demux_t *p_demux, logical_stream_t *p_stream, int64_t i
 
     i_tframe += p_stream->i_keyframe_offset;
 
+    i_cframe = i_tframe;
+    /* For Opus, seek back 80 ms before the target playback position. */
+    if ( p_stream->fmt.i_codec == VLC_CODEC_OPUS )
+    {
+        if ( i_tframe <= p_stream->i_pre_skip )
+            i_cframe = 0;
+        else if ( i_tframe < 80*48 )
+            i_cframe = 0;
+        else
+            i_cframe = i_tframe - 80*48;
+    }
+
     /* reduce the search domain */
-    fidx = get_bounds_for( p_stream, i_tframe, &i_pos_lower, &i_pos_upper );
+    fidx = get_bounds_for( p_stream, i_cframe, &i_pos_lower, &i_pos_upper );
 
     if ( fidx == NULL )
     {
-        /* no exact match found; search the domain for highest keyframe <= i_tframe */
+        /* no exact match found; search the domain for highest keyframe <= i_cframe */
 
-        i_granulepos = ogg_seek ( p_demux, p_stream, i_tframe, i_pos_lower, i_pos_upper,
+        i_granulepos = ogg_seek ( p_demux, p_stream, i_cframe, i_pos_lower, i_pos_upper,
                                   &i_pagepos, true );
         if ( i_granulepos == -1 )
         {