# LibXtract
[](https://github.com/jamiebullock/LibXtract/actions/workflows/ci.yml)
LibXtract is a simple, portable, lightweight library of audio feature extraction functions. The purpose of the library is to provide a relatively exhaustive set of feature extraction primitives that are designed to be 'cascaded' to create extraction hierarchies.
For example, 'variance', 'average deviation', 'skewness' and 'kurtosis', all require the 'mean' of the input vector to be precomputed. However, rather than compute the 'mean' 'inside' each function, it is expected that the 'mean' will be passed in as an argument. This means that if the user wishes to use all of these features, the mean is calculated only once, and then passed to any functions that require it.
This philosophy of 'cascading' features is followed throughout the library, for example with features that operate on the magnitude spectrum of a signal vector (e.g. 'irregularity'), the magnitude spectrum is not calculated 'inside' the respective function, instead, a pointer to the first element in an array containing the magnitude spectrum is passed in as an argument.
Hopefully this not only makes the library more efficient when computing large numbers of features, but also makes it more flexible because extraction functions can be combined arbitrarily (one can take the irregularity of the Mel Frequency Cepstral Coefficients for example).
A complete list of features can be found by viewing the header files, or reading the doxygen documentation.
## Building
### Prerequisites
- A C99 compiler (gcc, clang, MinGW)
- make
On macOS, the library uses Apple's Accelerate framework for FFT. On Linux and Windows, it uses the bundled Ooura FFT.
On Windows, an MSYS2/MinGW environment is required to provide `make` and a POSIX-compatible shell. Install [MSYS2](https://www.msys2.org), then from the MinGW 64-bit shell run `pacman -S mingw-w64-x86_64-gcc make`.
### Build and test
```bash
make # build library and examples
make check # build and run tests
make mutation # mutation-test the suite with Mull (needs clang-18 and mull-18; CI runs it)
```
### Install
```bash
make install # install to /usr/local
make install PREFIX=/somewhere/else # install elsewhere
```
## Python bindings
To build the Python bindings, [SWIG](http://www.swig.org) and Python are required:
```bash
make swig
```
## Documentation
LibXtract headers are documented using [doxygen](https://www.doxygen.nl) comments. If you have doxygen installed:
```bash
make doc
```
The generated HTML documentation can then be viewed by opening `doc/html/index.html`.
The documentation for the current release is published at
. It is rebuilt from `main` by the
`Docs` workflow on every push.
## Code standard
LibXtract is written in **C99**. Declarations are placed at the top of the
block (C89 style) as a project convention; both the standard and the
convention are enforced by the build flags (`-std=c99 -pedantic
-Wdeclaration-after-statement -Wstrict-prototypes`). Thread-local storage
uses a portability macro that resolves to C11 `_Thread_local`,
`__declspec(thread)` (MSVC) or `__thread` (GCC/Clang). Bundled third-party
code (`ooura`, `c-ringbuf`, `dywapitchtrack`) keeps its upstream style.
Initialisation convention: initialise a variable at its declaration when
the initial value is known and meaningful (accumulators to zero, copies of
parameters); otherwise declare it uninitialised and assign on every path
before use. Never defensively zero a variable that is unconditionally
assigned later — dead stores hide missing-assignment bugs from the
compiler. This is checked by `-Wconditional-uninitialized` (clang builds)
and by `make analyze`, which runs the clang static analyzer over the
first-party sources and fails on any finding; CI runs it on Linux and
macOS.
Formatting follows the Allman brace style (the opening brace on its own
line) with four-space indentation, no tabs, and the pointer asterisk bound
to the variable (`const double *data`). It is the standard clang-format
`Microsoft` preset, captured in `.clang-format` (the only deviations are
disabling include sorting and line reflow). Run `make format-check` (or
`clang-format --dry-run `) to check, and `make format` (or
`clang-format -i `) to apply it.
## API stability
LibXtract follows [Semantic Versioning](https://semver.org). The public API
is the set of headers installed under `include/xtract/` — `libxtract.h` and
the headers it includes. Symbols declared in `src/`, and the bundled
third-party code (`ooura`, `c-ringbuf`, `dywapitchtrack`), are internal and
not part of the public API.
Within the 1.x series the library guarantees **source compatibility**: code
that compiles against one 1.x release continues to compile against every
later 1.x release. Concretely, across 1.x:
- existing public functions are never removed and their signatures never
change;
- existing `XTRACT_*` enum and macro values are never renumbered or removed;
- changes are additive — new functions, new enum values appended to the end,
and new features may be added in a minor release.
The following are explicitly **not** guaranteed within 1.x:
- **Binary (ABI) compatibility.** There is no library-versioning (soname)
mechanism yet, and struct layouts may change, so a new minor release may
require recompiling and relinking dependent code rather than dropping in a
replacement shared library. A formal ABI policy will be revisited once the
context/handle API stabilises the layouts (planned for 2.0).
- **Numerical output.** A feature function may return different values in a
later release when a correctness fix changes a computation; buggy output is
not frozen for compatibility. Such changes are called out in the release
notes.
Breaking changes — removing or renaming a public function, changing a
signature, or renumbering an enum — are batched into the next major release
(2.0) so that dependent code has a single migration. Where practical, a
function to be removed in 2.0 is deprecated in a preceding 1.x release first.
## Licence
zlib. See `LICENSE`. The bundled third-party code under `src/` keeps its own
licences, listed in `DEPENDENCIES.md`; one of them (dywapitchtrack, MIT)
requires its copyright notice to accompany binary distributions.