module _39

let rightTriPerimeter =

    let pythagreonTriple m n =
        (m * m - n * n, 2 * m * n, m * m + n * n)

    let sumTriple (a,b,c) =
        a + b + c

    let parity a b =
        (a + b) % 2 = 1

    let multiples lim n  =
        seq { n .. n .. lim }

    common.coprimeArray 33
    |> Array.mapi (fun i cp ->
        cp 
        |> Seq.filter (parity i)
        |> Seq.map ((pythagreonTriple i)  >> sumTriple)
        |> Seq.collect (multiples 1000)
        )
    |> Seq.collect (fun l -> l)
    |> Seq.groupBy (fun n -> n)
    |> Seq.maxBy (fun g -> Seq.length (snd g))