Don't let the buffer grow until it is full, flush it whenever there is
data available.

Parse ID3 tags, even when they are in the middle of the stream.  Very
few streams provide embedded ID3 tags.  Most of them send only
Shoutcast "icy" tags, which limits the practical usefulness of this
patch.

When a command is received, decode_next_frame_header() and
decodeNextFrame() return DECODE_BREAK.  This is already checked by
both callers, which means that we can eliminate lots of
decoder_get_command() checks.

When a tag is updated, the old tag was freed before the new one was
created.  Reverse the order to be sure that other threads always see a
valid pointer.

This still leaves a possible race condition, but it will be addressed
later.

When the tag of the current song changes (e.g. a new tag was sent in
the stream), update the playlist, so clients pick up the new tag.

The stream_decode() and file_decode() methods returned a boolean,
indicating whether they were able to decode the song.  This is
redundant, since we already know that: if decoder_initialized() has
been called (and dc.state==DECODE), the plugin succeeded.  Change both
methods to return void.

os_compat.h is deprecated and should be replaced with well-defined
standard headers.

Eliminated deprecated libraries.

The currently replay_gain_apply() implementation duplicates code from
pcm_volume(), except that it uses a floating point scale.  Eliminate
all duplicated code from and make it utilize the pcm_volume() library
function.  This introduces replay gain support for 24 bit audio.

Allow pcm_volume() to increase volume.

It may be desirable to change the range of integer volume levels
(e.g. to 1024, which may utilize shifts instead of expensive integer
divisions).  Introduce the constant PCM_VOLUME_1 which describes the
integer value for "100% volume".  This is currently 1000.

Having an array instead of individual variables allows the use of the
replay_gain_mode enum as an array index.

The function simplifies wavpack_replaygain(), because it already
contains the float parser, and it works with a fixed buffer instead of
doing expensive heap allocations.

This allows us to remove the "static char[]" hack.

Some code simplification.  Avoid pointers to pointers.

Prefer C enums over CPP macros.

The author's name is already mentioned in the copyright declaration.

Renamed functions and variables.

No CamelCase file names.

The assertion on dc.state in decoder_read() was too strict: when a
decoder tried to call decoder_read() from tag_dup(), the decoder state
was NONE.  Allow this special case.

Instead of having a seprate try_decode() method, let the
stream_decode() and file_decode() methods decide whether they are able
to decode the song.

When a plugin is unable to decode a song, try the other plugins.

Added lots of assertions to the wrapper functions.

Added a lot of assertions checking the state of the decoder plugin.

The flac plugin wasn't initialized properly when an OGG file was being
decoded.  For some reason, flac_process_metadata() was explicitly not
called for OGG files.  Since that seems to fix the issue, make it
always call flac_process_metadata().

Since decoder_list.c does not include the libflac headers, it cannot
know whether to add the oggflac plugin to the decoder list.  Solve
this by always enabling the oggflac sub-plugin, even with older
libflac versions.  When the libflac API cannot support oggflac,
disable the plugin at runtime by returning "false" from its init()
method.

The "oggflac" plugin was enabled only if HAVE_FLAC_COMMON was
defined.  HAVE_FLAC_COMMON however is only an automake variable, and
is never available in decoder_list.c.  Make decoder_list.c depend on
HAVE_FLAC||HAVE_OGGFLAC instead.

Updated documentation on the stream_decode() and file_decode() return
values.

Wavpack's try_decode() implementation does nothing useful, it only
duplicates code already in stream_decode() / file_decode(), and slows
down MPD.

Disable flac's "oggflac" sub-plugin when libflac does not support
ogg-flac files.

The player did not care about the exact error value, it only checked
whether an error has occured.  This could fit well into
decoder_control.state - introduce a new state "DECODE_STATE_ERROR".

decoder.plugin was a write-only attribute.

This patch makes 24-bit samples available for mpd. I tested with the
WavPack Test Suite found on wavpack.com:

 http://www.rarewares.org/wavpack/test_suite.zip

Every test file worked fine.

At this moment the wavpack lib doesn't use the return value of the
push_back function, which has an equivalent meaning of the return
value of ungetc().  This is a lucky situation, because so far it
simply returned with 1 as a hard coded value.  From now on the
function will return EOF on error.  (This function makes exactly one
byte pushable back.)

A new function has been added to do a cast and a little check in the
wavpack-mpd input stream wrapper.

I think this makes the code more easily modifiable and prevents some
annoying mistakes.

There are some functions in the wavpack-mpd input streams wrapper
which had too commonly used names (especially can_seek). I prefixed
these with "wavpack_input_".