Project Euler 4

Euler04.fsx

let generate a b = seq {
        yield (a, b)
        let rec down (a, b) diff secondPass = seq {
                let next = a - diff, b + diff - 1
                yield next
                yield! up next diff (not secondPass)
            }
        and up (a,b) diff secondPass = seq {
                yield! Seq.init diff (fun i -> a + 1 + i, b - 1 - i)
                yield! down (a + diff, b - diff) (if secondPass then diff + 1 else diff) secondPass
            }

        yield! down (a, b) 0 false
    };;

generate 999 999
|> Seq.map (fun (a,b) -> a * b)
|> Seq.find (fun v -> let s = string v in new string(s.ToCharArray() |> Array.rev) = s)