easyfft

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A Rust library crate providing an easy FFT API for arrays and slices

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# easyfft
A Rust library crate providing an [FFT] API for arrays and slices. This crate wraps the
[rustfft] and [realfft] crates that does the heavy lifting behind the scenes.

### What does easyfft actually do?
* Manages the [FftPlanner] and scratch buffers for you in thread local storage.
* Exposes extension traits on arrays and slices for terse and ergonomic
  expression of the underlying logic.
* Implementation on arrays has the signal sizes verified at compile time using
  the type checker.
* Size calculations for real-valued signals are done for you so you can't mess
  it up.
* Wrapper scructs ([RealDft] and [DynRealDft]), that forces you to call the
  correct version of `real_ifft`.

### Example
The `nightly` dependent features are commented out.
```rust
// NOTE: Only required for real arrays
// #![allow(incomplete_features)]
// #![feature(generic_const_exprs)]

use easyfft::prelude::*;
use easyfft::num_complex::Complex;

// Complex arrays
let complex_array = [Complex::new(1.0, 0.0); 100];
let complex_array_dft = complex_array.fft();
let _complex_array_dft_idft = complex_array_dft.ifft();

// Real to complex arrays
let real_array = [1.0; 100];
let _real_array_dft = real_array.fft();

// // Real arrays
// let real_array = [1.0; 100];
// let real_array_dft = real_array.real_fft();
// let _real_array_dft_idft = real_array_dft.real_ifft();

// Complex slices
let complex_slice: &[_] = &[Complex::new(1.0, 0.0); 100];
let complex_slice_dft = complex_slice.fft();
let _complex_slice_dft_idft = complex_slice_dft.ifft();

// Real to complex slices
let real_slice: &[_] = &[1.0; 100];
let _real_slice_dft = real_slice.fft();

// Real slices
let real_slice: &[_] = &[1.0; 100];
let real_slice_dft = real_slice.real_fft();
let _real_slice_dft_idft = real_slice_dft.real_ifft();

// In-place mutation on complex -> complex transforms
let mut complex_slice = [Complex::new(1.0, 0.0); 100];
complex_slice.fft_mut();
complex_slice.ifft_mut();
let mut complex_array = [Complex::new(1.0, 0.0); 100];
complex_array.fft_mut();
complex_array.ifft_mut();
```

### Current limitations
* The `const-realfft` feature requires the `nightly` compiler because it depends on
  the [generic_const_exprs] feature.
* There are no methods for in-place mutation for complex -> real or real ->
  complex transforms.
* easyfft stores the [FftPlanner] and scratch buffer in thread local storage.
  This means IT'S NOT APPROPRIATE FOR APPLICATIONS THAT SPIN UP A NEW THREAD
  FOR EACH FFT OPERATION. It will work, it'll just reduce performance.

### ⚠️ SCALING ⚠️
Like [rustfft] and [realfft], `easyfft` does NOT rescale output! This is
intentional and done for performance reasons. Many applications do an `fft`,
manipulate the data, and do an `ifft` before actually analysing the data. The
two normalisation steps (`1/len().sqrt()`) can be combined into a single step
`1/len()`, halving the number of expensive division instructions.

### The `fallible` feature
The `DynRealDft` struct has some associated operations which can panic. This is
because the rust language does not have the ability to encode properties of the
length of slices in the type system. This might become possible in the future
if the rust team manages to extend const generics to fully fledged
[dependent types]. For now, we're limited to using arrays where we can ensure
these properties. If safety is your primary concern I recommend you take a step
back and consider if you REALLY need to work with slices instead of arrays.
Many applications can get away with knowing the size of their signal at compile
time. You can opt out of these panic-able operations by removing the `fallible`
feature flag, which is enabled by default.

### Why a prelude-glob import?
I generally don't like glob imports because it makes code harder to reason
about. Explicit imports means you can easily search for the items and see which
packages they come from. I believe there is an exception: Traits. Traits don't
allow you to do textual matching to figure out which trait is being used and
which package it comes from. Since it's already a bit implicit and convoluted,
you may aswell get the benefit of importing all of them in a single short line:
```rust
use easyfft::prelude::*;
```
For this reason, easyfft only exposes Traits via the prelude module.

[FFT]: https://en.wikipedia.org/wiki/Fast_Fourier_transform
[rustfft]: https://docs.rs/rustfft/latest/rustfft/
[realfft]: https://docs.rs/realfft/latest/realfft/
[arrays]: https://doc.rust-lang.org/std/primitive.array.html
[generic_const_exprs]: https://github.com/rust-lang/rust/issues/76560
[Result]: https://doc.rust-lang.org/std/result/enum.Result.html
[Error]: https://doc.rust-lang.org/std/result/enum.Result.html#variant.Err
[realfft module]: https://docs.rs/easyfft/latest/easyfft/realfft/index.html
[dependent types]: https://en.wikipedia.org/wiki/Dependent_type
[FftPlanner]: https://docs.rs/rustfft/latest/rustfft/struct.FftPlanner.html
[RealDft]: https://docs.rs/easyfft/latest/easyfft/const_size/realfft/struct.RealDft.html
[DynRealDft]: https://docs.rs/easyfft/latest/easyfft/dyn_size/realfft/struct.DynRealDft.html

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