In Linux, Makefiles can be used to cross-compile with the MingW headers, but may not be updated to generate native code at the moment. It should be fixable with some effort. Autotools should build it as vgmstream-cli instead (see the Audacious section).
**With MSVC**: To build in Visual Studio, run *./init-build.bat*, open *./vgmstream_full.sln* and compile. To build from the command line, run *./build.bat*.
The build script will automatically handle obtaining dependencies and making the project changes listed in the foobar2000 section (you may need to get some PowerShell .NET packages).
You can also call MSBuild directly in the command line (see the foobar2000 section for dependencies and examples)
FDK-AAC/QAAC can be safely disabled by removing *VGM_USE_MP4V2* and *VGM_USE_FDKAAC* in the compiler/linker options and the project dependencies, as FFmpeg is used instead to support their codecs.
You can also manually use the command line to compile with MSBuild, if you don't want to touch the .vcxproj files, register VS after trial, get PowerShell dependencies for the build script, or only have VC++/MSBuild tools.
Windows CMD example for foobar2000:
```
prompt $P$G$_$S
set PATH=C:\Program Files (x86)\Git\usr\bin;%PATH%
set PATH=C:\Program Files (x86)\MSBuild\14.0\Bin;%PATH%
Requires the dev version of Audacious (and dependencies), automake/autoconf, and gcc/make (C++11). It must be compiled and installed into Audacious, where it should appear in the plugin list as "vgmstream".
The plugin needs Audacious 3.5 or higher. New Audacious releases can break plugin compatibility so it may not work with the latest version unless adapted first.
C should be restricted to features VS2010 understands. This mainly means declaring variables at the start of a { .. } block (declare+initialize is fine, as long as it doesn't reference variables declared in the same block) and avoiding C99 features like variable-length arrays (but certain others like // comments are fine).
There are no hard coding rules but for consistency should follow general C conventions and the style used in most files: 4 spaces instead of tabs, underscore_and_lowercase_names, brackets starting in the same line (`if (..) { CRLF ... }`), etc. Some of the code may be a bit inefficient or duplicated at places, but it isn't much of a problem if gives clarity. vgmstream's performance is fast enough (as it mainly deals with playing songs in real time) so that favors clarity over optimization. Similarly some code may segfault or even cause infinite loops on bad data, but it's fixed as encountered rather than worrying to much about improbable cases.
vgmstream works by parsing a music stream header (*meta/*), preparing/controlling data and sample buffers (*layout/*) and decoding the compressed data into listenable PCM samples (*coding/*).
- player closes the VGMSTREAM once the stream is finished
### Components
#### STREAMFILEs
Structs with I/O callbacks that vgmstream uses in place of stdio/FILEs. All I/O must be done through STREAMFILEs as it lets plugins set up their own. This includes reading data or opening other STREAMFILEs (ex. when a header has companion files that need to be parsed, or during setup).
Players should open a base STREAMFILE and pass it to init_vgmstream. Once it's done this STREAMFILE must be closed, as internally vgmstream opens its own copy (using the base one's callbacks).
For optimization purposes vgmstream may open a copy of the FILE per channel, as each has its own I/O buffer, and channel data can be too separate to fit a single buffer.
Custom STREAMFILEs wrapping base STREAMFILEs may be used for complex I/O cases (ex. if data needs decryption, parts needs to be skipped on the fly, or a file is composed of multiple sub-files), as some codecs are not easy to feed chunked data.
The VGMSTREAM (caps) is the main struct created during init when a file is successfully recognized and parsed. It holds the file's configuration (channels, sample rate, decoder, layout, samples, loop points, etc) and decoder state (STREAMFILEs, offsets per channel, current sample, etc), and is used to interact with the API.
- *src/meta/(format-name).c*: create new init_vgmstream_(format-name) parser that tests the extension and header id, reads all needed info from the stream header and sets up the VGMSTREAM
Ultimately the meta must alloc the VGMSTREAM, set config and initial state. vgmstream needs the total of number samples to work, so at times must convert from data sizes to samples (doing calculations or using helpers).
It also needs to open and assign to the VGMSTREAM one or several STREAMFILEs (usually reopening the base one, but could be any other file) to do I/O during decode, as well as setting the starting offsets of each channel and other values; this gives metas full flexibility at the cost of some repetition. The STREAMFILE passed to the meta will be discarded and its pointer must not be reused.
The .c file is usually named after the format's main extension or header id, optionally with affixes. Each file should parse one format and/or its variations (regardless of accepted extensions or decoders used) for consistency, but deviations may be found in the codebase. Sometimes a format is already parsed but not accepted due to bugs though.
Different formats may use the same extension but this isn't a problem as long as the header id or some other validation tells them apart, and should be implemented in separate .c files. If the format is headerless and the extension isn't unique enough it probably needs a generic GENH/TXTH header instead of direct support.
If the format supports subsongs it should read the stream index (subsong number) in the passed STREAMFILE, and use it to parse a section of the file. Then it must report the number of subsongs in the VGMSTREAM, to signal this feature is enabled. The index is 1-based (first subsong is 1, 0 is default/first). This makes possible to directly use bank-like formats like .FSB, and while vgmstream could technically support any container (like generic bigfiles or even .zip) it should be restricted to files that actually are audio banks.
- receive external buffer to fill with PCM samples
- detect when looping must be done
- find max number of samples to do next decoder call (usually one frame, less if loop starts/ends)
- call decoder
- do post-process if necessary (move offsets, check stuff, etc)
- repeat until buffer is filled
Available layouts, depending on how codec data is laid out:
- none/flat: straight data. Decoder should handle channel offsets and other details normally.
- interleave: one data block per channel, mixed in configurable sizes. Once one channel block is fully decoded this layout skips the other channels, so the decoder only handles one at a time.
- blocked: data is divided into blocks, often with a header. Layout detects when a block is done and asks a helper function to fix offsets (skipping the header and pointing to data per channel), depending on the block format.
- others: uncommon cases may need its own custom layout (ex.- multistream/subfiles)
The layout used mainly depends on the decoder. MP3 data (that may have 1 or 2 channels per frame) uses flat layout, while DSP ADPCM (that only decodes one channel at a time) is interleaved. In case of mono files either could be used as there won't be any actual difference.
Layouts expect the VGMSTREAM to be properly initialized during the meta processing (channel offsets must point to each channel start offset).
#### decoders
Decoders take a sample buffer, convert data to PCM samples and fill one or multiple channels at a time, depending on the decoder itself. Usually its data is divided into frames with a number of samples, and should only need to do one frame at a time (when size is fixed/informed; vgmstream gives flexibility to the decoder), but must take into account that the sample buffer may be smaller than the frame samples, and that may start some samples into the frame.
Every call the decoder will need to find out the current frame offset (usually per channel). This is usually done with a base channel offset (from the VGMSTREAM) plus deriving the frame number (thus sub-offset, but only if frames are fixed) through the current sample, or manually updating the channel offsets every frame. This second method is not suitable to use with the interleave layout as it advances the offsets assuming they didn't change (this is a limitation/bug at the moment). Similarly, the blocked layout cannot contain interleaved data, and must use alt decoders with internal interleave (also a current limitation). Thus, some decoders and layouts don't mix.
If the decoder needs to keep state between calls it may use the VGMSTREAM for common values (like ADPCM history), or alloc a custom data struct. In that case the decoder should provide init/free functions so the meta or vgmstream may use. This is the case with decoders implemented using external libraries (*ext_libs*), as seen in *#ifdef VGM_USE_X ... #endif* sections.
- *src/coding/(decoder-name).c*: create `decode_x` function that decodes stream data into the passed sample buffer. If the codec requires custom internals it may need `init/reset/seek/free_x`, or other helper functions.
- *src/vgmstream.h*: define new coding type in the list. If the codec requires custom internals, define new `x_codec_data` struct.
- *src/vgmstream.c: reset_vgmstream*: call `reset_x` if needed
- *src/vgmstream.c: close_vgmstream*: call `free_x` if needed
- *src/vgmstream.c: get_vgmstream_samples_per_frame*: define so vgmstream only asks for N samples per decode_x call. May return 0 if variable/unknown/etc (decoder must handle setting arbitrary number of samples)
- *src/vgmstream.c: get_vgmstream_frame_size*: define so vgmstream can do certain internal calculations. May return 0 if variable/unknown/etc, but blocked/interleave layouts will need to be used in a certain way.
- *src/vgmstream.c: decode_vgmstream*: call `decode_x`, possibly once per channel if the decoder works with a channel at a time.
- *src/vgmstream.c: vgmstream_do_loop*: call `seek_x` if needed
- *src/vgmstream.c: reset_vgmstream*: call `reset_x` if needed
- *src/formats.c*: add coding type description
- *src/libvgmstream.vcproj/vcxproj/filters*: add to compile new (decoder-name).c parser in VS
- *src/vgmstream.c*: add parser init to the init list
- if the codec depends on a external library don't forget to mark parts with: *#ifdef VGM_USE_X ... #endif*
