Project Euler 5

Euler05.fsx

let getPrimeFactors n =
    let allFactors = seq { yield 2; yield! Seq.initInfinite (fun i -> 2 * i + 3) } |> Seq.cache
    let addFactor factors d =
        match Map.tryFind d factors with
        | None -> Map.add d 1 factors
        | Some count -> Map.remove d factors |> Map.add d (count + 1)
    let rec gPF n factors =
        allFactors
        |> Seq.takeWhile (fun d -> d <= (n |> float |> sqrt |> int))
        |> Seq.tryFind (fun d -> n % d = 0)
        |> function
           | None -> addFactor factors n
           | Some d -> addFactor factors d |> gPF (n / d)
    gPF n Map.empty;;

Seq.init 20 ((+) 1)
|> Seq.map getPrimeFactors
|> Seq.concat
|> Seq.map (|KeyValue|)
|> Seq.groupBy fst
|> Seq.map (fun (f, counts) -> pown f (counts |> Seq.map snd |> Seq.max))
|> Seq.reduce (*)