avformat/mp3dec: Make MP3 seek fast

When AVFMT_FLAG_FAST_SEEK is specified, make MP3 seek operation as
fast as possible.

When no "-usetoc" is specified, the default operation is using TOC
if available; otherwise, uses linear interpolation. This is useful
when seeking a large MP3 file with no TOC available. One example is
Podcast, many MP3 files are large, but no CBR/VBR tags. Most of
them are actually CBR. Even in VBR cases, this option sacrifices the
accuracy of playback time in exchange for responsiveness.

libavformat/mp3dec.c

@@ -342,7 +342,7 @@ static int mp3_read_header(AVFormatContext *s)
     int i;
 
     if (mp3->usetoc < 0)
-        mp3->usetoc = (s->flags & AVFMT_FLAG_FAST_SEEK) ? 0 : 2;
+        mp3->usetoc = (s->flags & AVFMT_FLAG_FAST_SEEK) ? 1 : 2;
 
     st = avformat_new_stream(s, NULL);
     if (!st)
@@ -489,19 +489,26 @@ static int mp3_seek(AVFormatContext *s, int stream_index, int64_t timestamp,
     AVStream *st = s->streams[0];
     int64_t ret  = av_index_search_timestamp(st, timestamp, flags);
     int64_t best_pos;
+    int fast_seek = (s->flags & AVFMT_FLAG_FAST_SEEK) ? 1 : 0;
+    int64_t filesize = mp3->header_filesize;
 
     if (mp3->usetoc == 2)
         return -1; // generic index code
 
-    if (   mp3->is_cbr
+    if (filesize <= 0) {
+        int64_t size = avio_size(s->pb);
+        if (size > 0 && size > s->internal->data_offset)
+            filesize = size - s->internal->data_offset;
+    }
+
+    if (   (mp3->is_cbr || fast_seek)
         && (mp3->usetoc == 0 || !mp3->xing_toc)
         && st->duration > 0
-        && mp3->header_filesize > s->internal->data_offset
-        && mp3->frames) {
+        && filesize > 0) {
         ie = &ie1;
         timestamp = av_clip64(timestamp, 0, st->duration);
         ie->timestamp = timestamp;
-        ie->pos       = av_rescale(timestamp, mp3->header_filesize, st->duration) + s->internal->data_offset;
+        ie->pos       = av_rescale(timestamp, filesize, st->duration) + s->internal->data_offset;
     } else if (mp3->xing_toc) {
         if (ret < 0)
             return ret;
@@ -515,7 +522,7 @@ static int mp3_seek(AVFormatContext *s, int stream_index, int64_t timestamp,
     if (best_pos < 0)
         return best_pos;
 
-    if (mp3->is_cbr && ie == &ie1) {
+    if (mp3->is_cbr && ie == &ie1 && mp3->frames) {
         int frame_duration = av_rescale(st->duration, 1, mp3->frames);
         ie1.timestamp = frame_duration * av_rescale(best_pos - s->internal->data_offset, mp3->frames, mp3->header_filesize);
     }