Problem14.fs

#light

open System
open System.Collections.Generic
open Microsoft.FSharp.Math
 
let N =
    Seq.unfold (fun x -> Some(x, x + 1I)) 1I
    
let isEven = function
             | x when x % 2I = 0I -> true
             | x -> false
    
let f = function
        | x when isEven x -> x / 2I
        | x -> (3I * x) + 1I
        
let rec makeSequence = function
                       | x when x = 1I -> [x]
                       | x -> let next = f x
                              x :: makeSequence next
                              
let sequenceLengths m n =
    [m .. n]
    |> Seq.map (fun x -> if x % 10000I = 0I then do printfn "at %A" x
                         (x, makeSequence x |> Seq.length))                              

let longestSequence m n =
    sequenceLengths m n |> Seq.max_by (snd)