docs/random/wtay/timecache

1) intro
--------

For efficient and accurate seeking, a plugin typically needs
an index of possible seek positions (in time, bytes, samples, ...).

The purpose of GstTimecache is to provide an infrastructure for
plugins to maintain such an index.

By creating a generic interace to this functionality, it is also
possible for the application to make use of the generated index.
possible use cases are: saving/loading of indexes, obtain an 
overview of the indexed stream (where is sample N, where is
the keyframe...)


This document describes a proposed API for this functionality.


2) Requirements
---------------

Bare minimum:

- store mappings between format1 and format2 (bytes<->time, 
  samples<->tracks, ...)
- update the mappings (add/remove/change entries)
- query the index, locate entries.
- store arbitrary extra metadata in the timecache (media file,
  time, author, ...)
- set/get cache on elements.

Nice to have:

- cache groups with varying certainty that are merged.
- API to control the indexing process, what gets indexed, how much,
  ...)
- Get notification when a new entry is added, together with an
  option to turn off in-memory caching, this could be used to
  do direct-to-disk indexing (for large files) insteda of to-memory.

3) use cases
------------

a) indexing an mpeg systems stream

  - create an empty timecache object
  - create filesrc ! mpegparse
  - set timecache on mpegparse
  - run the pipeline to EOS
    - mpegparse will create a mapping from SCR to byte offset
      and add it to the cache.
  - read/save timecache

b) running an mpeg with a previously created cache

  - create timecache object from saved index
  - create filesrc ! mpegparse
  - set the timecache on mpegparse
  - run the pipeline, seek, ...
    - mpegparse uses the timecache to do accurate seek to 
      SCR timestamps.
  
c) indexing an mpeg systems stream, SCR, video and audio elementary
   streams.

  - create an empty timecache object
  - create filesrc ! mpegdemux
  - set timecache on mpegdemux
  - run the pipeline to EOS
    - mpegparse will create a mappings for: 
       * SCR to byte offset
       * PTS + streamid to byte offset
      and add it to the cache.
  - read/save timecache

d) complete indexing of mpeg systems streams and audio/video
   streams.

  - create 3 empty timecaches
  - create filesrc ! mpegdemux video_%02d! mpeg2dec 
                     mpegdemux0 audio_%02d! mad
  - set timecaches on mpegdemux, mpeg2dec, mad
  - run pipeline to EOS
    - mpegdemux creates timecache for SCR, PTS + streamid
    - mpeg2dec creates timecache for frames + I/P/B id
    - mad creates timecache for mpeg audio headers
  - read/save timecaches
  
  seeking in a fully indexed stream:
    - seek on timestamp in mpeg2dec
    - mpeg2dec locates previous I frame in cache, does
      a timeseek on sinkpad
    - mpegparse uses the cache to convert the timestamp
      to an offset, then does a seek on offset
    - filesrc seeks to offset

4) Entry types
--------------

 - mapping between formats:

    <padid> <flags> <format> <position>, <format> <position>, ...

 - metadata, streaminfo, other objects:

    <padid> <format> <position>, <keyword> gpointer

    
5) implementation
-----------------

We define an abstract base class GstTimeCache with following
abstract methods:


6) cache format example
-----------------------

We define a binary format for the cache for space/speed reasons.
It is entirely possible to create alternative implementations.