FIR filter coding in arduino

I am creating a digital harmonic percentage differential relay which is used to protect transformers from internal faults.

My problem is that i need an idea to calculate the 2nd harmonic to differentiate if the output current is rush or inrush current. I did do some researches and i found that the main algorithm used is FIR filters but i didn' t find anything about it on arduino so is there any thing or codes for FIR filter on arduino ?

Thanks ^^

Let me Google that for you…

Looks to be what you want…

Yeah i know that library , and i know how to use google -" uhhh weird"- but that FIR filter code only works on frequency generated only by sound and such ... , i just want to be able to enter current samples to an equation to calculate the 2nd harmonic.

Why should it only work on sound? It's just a FIR. Calculate the right coefficients and off you go...