aviobuf: Do short seeks forward by reading and skipping data instead of a proper seek

This improves performance on e.g. seekable http.

Originally committed as revision 24280 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/aviobuf.c

@@ -28,6 +28,13 @@
 
 #define IO_BUFFER_SIZE 32768
 
+/**
+ * Do seeks within this distance ahead of the current buffer by skipping
+ * data instead of calling the protocol seek function, for seekable
+ * protocols.
+ */
+#define SHORT_SEEK_THRESHOLD 4096
+
 static void fill_buffer(ByteIOContext *s);
 #if LIBAVFORMAT_VERSION_MAJOR >= 53
 static int url_resetbuf(ByteIOContext *s, int flags);
@@ -153,7 +160,9 @@ int64_t url_fseek(ByteIOContext *s, int64_t offset, int whence)
         offset1 >= 0 && offset1 <= (s->buf_end - s->buffer)) {
         /* can do the seek inside the buffer */
         s->buf_ptr = s->buffer + offset1;
-    } else if(s->is_streamed && !s->write_flag && offset1 >= 0 &&
+    } else if ((s->is_streamed ||
+               offset1 <= s->buf_end + SHORT_SEEK_THRESHOLD - s->buffer) &&
+               !s->write_flag && offset1 >= 0 &&
               (whence != SEEK_END || force)) {
         while(s->pos < offset && !s->eof_reached)
             fill_buffer(s);