euler14.fs

let max = 1000001L
let memo = Array.create (max|>int) 0L
  
let rec collatzSeqLength (x:int64):int64 = 
  
  let nextCollatz (n:int64) = 
    if n%2L = 0L then n/2L else 3L*n+1L
  
  let rec seqLength' (n:int64)  contd = 
    match n with 
    | 1L        -> contd n
    | _         -> 
      if n < max && memo.[n|>int] <> 0L then 
        contd memo.[n|>int]
      else 
        seqLength' (nextCollatz n) (fun x -> 
                                      let x' = x+1L
                                      //printf "%d " n
                                      if n<max then 
                                        memo.[n|>int] <- x'
                                      else
                                        ()
                                      contd x'
                                  )

  x|>(fun i -> seqLength' i id)

let maxCollazSeqLength' (x:int64,y:int64) = 
  let x',y' = System.Math.Min(x,y), System.Math.Max(x,y)
  let mutable maxL = 0L
  let mutable num = 0

  for i=(x'|>int) to (y'|>int) do 
    let r = collatzSeqLength (i|>int64) 
    if r>maxL then
      maxL <- r 
      num  <- i
    else 
      ()
  maxL,num

run.fsx

maxCollazSeqLength' (1L, max-1L);;

// result : val it : int64 * int = (525L, 837799)
// 837799 has the maxLength := 525