birthday_odds_1

birthday_odds_1

open System


type Item = O = 0 | X = 1 | A = 2 

let inline add char = function
| struct (Item.O,Item.O) -> struct (char, Item.O)
| x, Item.O -> x, char
| _, y -> y, char
    
let rec calc numA numX = function
  | _ when numX = 0 -> 0
  | _ when numA = 0 -> 1
  | struct (Item.A,Item.A) as state  ->
       calc numA (numX - 1) (add Item.X state)
  | state ->
     calc (numA - 1) numX (add Item.A state) 
     +  calc numA (numX - 1) (add Item.X state)
 
open System.Diagnostics

let sw = Stopwatch.StartNew()

calc  10 30 (Item.O,Item.O) |> printf "%i"

printfn "%A" sw.Elapsed