FFTEASY is a series of C functions for doing Fast Fourier Transforms (FFTs) of real or complex data in any number of dimensions. The routines are designed to be short, readable, and easy to use. If you are comfortable programming in C and are somewhat familiar with FFTs it shouldn't take you more than an hour to read these instructions, link FFTEASY into your C program, and start doing Fourier transforms to your heart's content.
FFTEASY is not the fastest or highest precision FFT routine out there. If you want to do millions of transforms of large three-dimensional grids you probably want to find a better optimized set of routines. FFTEASY also requires that the number of data points in each dimension be a power of two. The advantages of FFTEASY are that it is simple to understand, simple to use, free, and capable of handling arbitrary dimensions of real or complex data. (In my experience most FFT routines handle either multi-dimensional complex data or one-dimensional real data, but for some reason there seem to be very few out there for multi-dimensional real data, which is what I actually wrote the routines for in the first place.)
FFTEASY consists of the four C functions fftc1, fftcn, fftr1, and fftrn. FFTEASY is free. I am not in any way, shape, or form expecting to make money off of these routines. I wrote them because I needed them for some work I was doing and I'm putting them out on the Internet in case other people might find them useful. Feel free to download them, incorporate them into your code, modify them, translate the comment lines into Swahili, or whatever else you want. What I do want is the following: