There is FFT Source code on the Internet in C. The Hard part is knowing how to use it. Fortunately it is well known and there is a lot of write ups and youtube videos on the subject.
If you are trying to detect a zero crossing in a AC line you have to have external hardware to keep the AC blowing out the pin on your microcontroller.
Probably the easiest is a 3 or 3.3V zener diode and a high value resistor. (Do not do it with powerline AC because of no isolation, you will kill yourself. Or at least get shocked and maybe blow up your computer.)
What you will see is a square wave. And a dead time. The Square wave will be less time than the dead time because the square wave will appear high when the voltage is +something like on the rising side 2.7V until it is less than something like 0.8V on the falling side. from those two references and the on time and the off time (or frequency) you can calculate the zero crossing points.
Another way to do it is with a comparator at the zero crossing point. This is best if the signal is all above 0. (Has a bias voltage).
