The Ping Pong Modulo Function

Earlier today I had to spend a whole 20 minutes coming up with this function. It’s like a modulo function, but instead of looping back to zero every cycle, it transitions back down to zero. It cycles every 2(n-1) elements. There’s probably a proper name for it, but I can’t figure it out.

In case anyone else would rather google it than spend time figuring it out, here it is:

def ping_pong_mod(i, n):
    cycle = 2 * (n - 1)
    i = i % cycle
    if i >= n:
        return cycle - i
    return i

I hope this is helpful to a fellow lazy person in the future.