Only check whether fft is power of two when not using rustfft-backend

[danny900714]

Resolves #18

[gemini-code-assist]

Summary of Changes

Hello @danny900714, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the flexibility of the StftConfig by making the fft_size validation conditional. Specifically, the requirement for fft_size to be a power of two is now bypassed when the rustfft-backend feature is enabled, accommodating backends that can handle arbitrary FFT sizes. This change improves compatibility and usability for different backend implementations.

Highlights

• Conditional FFT Size Validation: The StftConfig constructor now only enforces that the fft_size must be a power of two when the rustfft-backend feature is not enabled. This change allows for non-power-of-two FFT sizes when utilizing the rustfft-backend.
• Test Suite Adaptation: Existing unit tests that specifically validate the power-of-two constraint for fft_size have been updated to be conditionally compiled. These tests will now only run when the rustfft-backend feature is not active, aligning with the new conditional validation logic in the core library.

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[gemini-code-assist]

Code Review

This pull request correctly updates the FFT size validation to be conditional based on the rustfft-backend feature. The power-of-two check for fft_size is now correctly skipped when rustfft-backend is enabled, as that backend supports non-power-of-two sizes. The associated tests have also been updated with the appropriate cfg attribute to run only when this check is active. The changes are logical and well-implemented. I have one suggestion to improve the readability of the conditional logic in src/lib.rs.

[gemini-code-assist]

While logically correct, this condition can be made more readable. By applying De Morgan's law, you can express the logic without negating a grouped expression, which is often easier to reason about. The rewritten form !cfg!(feature = "rustfft-backend") && !fft_size.is_power_of_two() makes it immediately clear that the power-of-two constraint applies only when not using the rustfft-backend.

Suggested change

	if fft_size == 0 || !(cfg!(feature = "rustfft-backend") || fft_size.is_power_of_two()) {
	if fft_size == 0 || !cfg!(feature = "rustfft-backend") && !fft_size.is_power_of_two() {

[wizenink]

Added a few tests for checking invalid fft size on other backends. Other than that, this seems perfect. Thank you very much for this contribution. This should be released in the next version, along with your other issue :)