854 Pisano Periods 2 -- v2.sf

#!/usr/bin/ruby

# Pisano Periods 2
# https://projecteuler.net/problem=854

# See also:
#   https://oeis.org/A005013

# Runtime: 4.014s

func P(n, m=1_234_567_891) {

    var v = 2

    var (a,b) = (0, 1)
    var (c,d) = (2, 1)

    for k in (2..n `by` 2) {
        v = mulmod(v, (k.is_congruent(0, 4) ? b : d), m)
        (a,b) = (b, addmod(a,b,m))
        (c,d) = (d, addmod(c,d,m))
    }

    return v
}

assert_eq(P(10), 264)

say P(1e6)