module _14

let collatzConjecture n =
    match n with
        | 0L -> None
        | 1L -> Some(1L,0L)
        | n -> Some(n,
                    if n % 2L = 0L then n/2L
                    else if n % 2L = 1L then 3L*n + 1L
                    else 1L)

let getLongestChain =
    [1L..1000000L]
        |> Seq.map (fun i -> (i |> Seq.unfold collatzConjecture |> Seq.length, i))
        |> Seq.max