# Rosalind: PMCH — Perfect Matchings and RNA Secondary Structures
# https://rosalind.info/problems/pmch/
#
# Given: An RNA string s with the same number of A as U and of G as C.
# Return: The total number of perfect matchings of basepair edges.

let s = "AGCUAGUCAU"

fn count_base(seq, base) {
    range(0, len(seq)) |> count_if(|i| substr(seq, i, 1) == base)
}

fn factorial(n) {
    range(1, n + 1) |> reduce(|acc, i| acc * i, 1)
}

# Every A pairs with some U and every G with some C, independently, so the
# count is |A|! * |G|!.
let adenine = count_base(s, "A")
let guanine = count_base(s, "G")
let result = factorial(adenine) * factorial(guanine)

println("Result:   " + str(result))
println("Expected: 12")

fn test_pmch_perfect_matchings() {
    assert result == 12, "PMCH: got " + str(result)
}