A possibly-better approach to seeking.

Before this commit, decoders try to guess whether they should
request a seek based on what they have in their buffers.  This
seems reasonable for video and audio, which will always (I think)
have some data lying around to give an indication of where their
parent decoders are in the timeline.

It doesn't work so well for subtitles, as the storage of subs is
cleared out based on time (+/- 5s of "now") so there is a good chance
that the storage will be empty.  This gives the subtitle decoder no
chance of knowing where its parent is, so it's very likely to seek.

This commit asks the parent decoder to seek if it wants to, and it
decides based on a knowledge of roughly where it is in the timeline.
Hence the sub-decoders just see if they have got the data that is being
requested, and if not they suggest to the parent that it might like
to seek.  They then start calling pass().  Hence the parent should only
seek if some calls to pass() are not going to elicit the required data
in a reasonable time.

1 files changed, 14 insertions, 8 deletions

diff --git a/src/lib/decoder.h b/src/lib/decoder.h
index 181fc6c2a..84661ee8c 100644
--- a/src/lib/decoder.h
+++ b/src/lib/decoder.h
@@ -52,6 +52,20 @@ public:
 		PASS_REASON_SUBTITLE
 	};
 
+	void maybe_seek (ContentTime time, bool accurate);
+
+	/** @return true if this decoder has already returned all its data and will give no more */
+	virtual bool pass (PassReason, bool accurate) = 0;
+
+	/** Ensure that any future get() calls return data that reflect
+	 *  changes in our content's settings.
+	 */
+	virtual void reset () {}
+
+protected:
+	boost::optional<ContentTime> _position;
+
+private:
 	/** Seek so that the next pass() will yield the next thing
 	 *  (video/sound frame, subtitle etc.) at or after the requested
 	 *  time.  Pass accurate = true to try harder to ensure that, at worst,
@@ -61,14 +75,6 @@ public:
 	 *  it may seek to just the right spot.
 	 */
 	virtual void seek (ContentTime time, bool accurate) = 0;
-
-	/** @return true if this decoder has already returned all its data and will give no more */
-	virtual bool pass (PassReason, bool accurate) = 0;
-
-	/** Ensure that any future get() calls return data that reflect
-	 *  changes in our content's settings.
-	 */
-	virtual void reset () {}
 };
 
 #endif