gistfile1.fs

let max = 1000001L
let memo = Array.create (max|>int) 0L

// Computes Collatz sequence length, given a
// positive integer, x.   
let rec collatzSeqLength (x:int64):int64 = 
  let rec seqLength' (n:int64)  contd = 
    match n with 
    | 1L        -> contd 1L // initalizing sequence length with 1 
    | _         -> 
      if n < max && memo.[n|>int] <> 0L then 
        contd memo.[n|>int]
      else 
        seqLength' (nextCollatz n) (fun x -> 
                                      let x' = x+1L //incrementing length and storing it in memo.
                                      if n<max then 
                                        memo.[n|>int] <- x'
                                      else
                                        ()
                                      contd x'
                                  )
  x|>(fun i -> seqLength' i id)