open Base

type nucleotide = A | C | G | T

let toint = function
    | A -> 0
    | C -> 1
    | G -> 2
    | T -> 3

let hamming_distance a b = match (a, b) with
    | [], [] -> Ok 0
    | [], _ -> Error "left strand must not be empty"
    | _, [] -> Error "right strand must not be empty"
    | l1, l2 -> match List.zip l1 l2 with
        | Ok res -> Ok (List.filter ~f:(fun (x, y) -> (toint x) <> (toint y)) res |> List.length)
        | Unequal_lengths -> Error "left and right strands must be of equal length"