euler 92

euler_92.fsx

open System.Collections.Generic

/// Applies memoization to the supplied function f
let memoize (f : 'a -> 'b) =
    let cache = new Dictionary<'a, 'b>()

    let memoizedFunc (input : 'a) =
        // check if there is a cached result for this input
        match cache.TryGetValue(input) with
        | true, x   -> x
        | false, _  ->
            // evaluate and add result to cache
            let result = f input
            cache.Add(input, result)
            result

    // return the memoized version of f
    memoizedFunc

let squareDigit = 
    let inner (c : char) = pown (int(c.ToString())) 2
    memoize inner

let sosd = 
    let inner (n : int) = n.ToString().ToCharArray() |> Array.sumBy squareDigit
    memoize inner

let rec f n = 
    match sosd n with
    | 89 -> true
    | 1  -> false
    | n  -> f n
 
let t = [1..9999999] |> List.filter f |> List.length