let brent f x0 =
  let rec affiche_suite x = function
    | 0 -> ()
    | n -> Printf.printf "%d " x; affiche_suite (f x) (n-1)
  in
  let rec brent' xh h =
    let rec parcours_k xk k=
      if xk = xh then (Printf.printf "xh = xk pour h=%d, k=%d, xh = xk = %d\n" h k xk; (k, xk))
      else if k >= 2*h+1 then  (Printf.printf "k : [%d, %d] termine.\n" (h+1) (2*h+1); (k, xk))
      else  parcours_k (f xk) (k+1)
    in
    let k, xk = parcours_k (f xh) (h+1) in
    if xk = xh then (h, k)
    else brent' xk k
  in
  let h, k = brent' x0 0 in
  affiche_suite x0 (k+1); (h, k, k-h)
;;

let n = 1589;;
let f x = (x*x+1) mod n;;
brent f 0;;