ffts VS Simple-FFT

You might as well assume (c) whenever you use a library provided free of charge. Be prepared to either fix the bugs yourself or make sure there are alternatives you can use as a replacement. Even if the author is responsive now, there's no guarantee you'll get help once your code is in production a few months later. The library might even be archived at that point.

Lets say you've used FFTS, the Fastest Fourier Transform in the South [0]. Performance is spectacular, but now it's no longer supported and you find a bug. The code is impressive: Thousands of lines of code that dynamically generate SIMD instructions. But it's impossible to understand. You have chosen a library you cannot repair yourself. This is fine when there's alternatives, and there are for FFTs, and they are easy to swap out. If you can't easily swap out one library with another, you better make sure that you can repair it yourself.

[0] https://github.com/anthonix/ffts

I quite like ffts. Compared to fftw, it has a similar interface, a more permissive license, and as far as I can tell, it has fairly comparable performance.

I've been using this guy's FFT library for as long as I've been doing random personal projects in C++, it's not the fastest but it's very simple to use and works great for my purposes: https://github.com/d1vanov/Simple-FFT

FFTW is a pretty popular and very fast FFT library, if you're after absolute speed. I generally just use Simple-FFT, you just need to wrap the container of your choice (your own, a std::vector or std::array or std::span) in a type that is subscriptable with parenthesis (operator()) and off you go.