This is a fork of https://github.com/dujianyi/ardFFT
ArduinoFastFFT is a readable FFT code with simple comments. Everyone with C knowledge should be able to read it and modify it.
ArduinoFastFFT increases performance of ardFFT by precalculating a 2D array of needed sines and cosines.
calculateFFT(Xr, Xi, re, im, N, 0, 1)
or
calculateFFT2(Xr, Xi, 0, re, im, N, 0, 1)
where Xr and Xi store the real and imagine parts of the frequency, and re and im are the time signal. Usually im are set to be zero. N is the number of samples.
More information can be found in the comments.