[<RequireQualifiedAccess>]
module Array =
    [<RequireQualifiedAccess>]
    module Parallel =
        let filter predicate array =
            array |> Array.Parallel.choose (fun x -> if predicate x then Some x else None)

module NumericLiteralG =
    let inline FromZero () = LanguagePrimitives.GenericZero
    let inline FromOne () = LanguagePrimitives.GenericOne

let inline isNumPalindrome number =
    let ten =
        let two = 1G + 1G
        let three = two + 1G
        let five = three + two
        five + five
    let hundred = ten * ten

    let rec findHighDiv div =
        let div' = div * ten
        match number / div' with
          | x when x = 0G    -> div
          | _                -> findHighDiv div'
    let rec impl n = function
      | x when x = 0G    -> true
      | div              -> match n / div, n % ten with
                              | a, b when a <> b -> false
                              | _                -> impl (n % div / ten) (div / hundred)
    impl number (findHighDiv 1G)

do
    Array.Parallel.init 900 ((+) 100 >> fun n -> Array.init (1000 - n) ((+) n >> (*) n))
    |> Array.concat
    |> Array.Parallel.filter isNumPalindrome
    |> Array.max
    |> stdout.WriteLine