gistfile1.fsx

open System.Collections.Generic

let memoize f = 
    let cache = Dictionary<_, _>()

    fun x ->
        match cache.TryGetValue x with
        | true, res -> res
        | _ -> 
            let res = f x
            cache.[x] <- res
            res

let next n = if n%2L = 0L then n/2L else 3L*n+1L

let rec collatzLengthCached =
    let loop start =
        let mutable current = start

        if current > 1L 
        then 1 + collatzLengthCached (next current)
        else 1

    memoize loop