euler14

euler14.fsx

//n → n/2 (n is even)
//n → 3n + 1 (n is odd)
let collatzSeq = Seq.unfold (fun n -> match n with
                                        | 0L -> None
                                        | 1L -> Some(1L, 0L)
                                        | n  -> Some(n, if n % 2L = 0L then n/2L else 3L*n+1L) )

let r = {1L..999999L} |> Seq.map(fun i -> (i, Seq.length(collatzSeq(i)))) |> Seq.maxBy(fun (i,s) -> s)

printfn "%A" r