Mine

fun collatz (n: Int64.int,length) =
  if (n = 1) then
    length + 1
  else if (n mod 2) = 0 then
    collatz ((n div 2),length + 1)
  else
    collatz ((3 * n + 1),length + 1);

fun max (a,b) =
  if (a > b) then
    a
  else
    b;

fun loop (i,j,max_cycle) =
  if (i = j) then
    max_cycle
  else
    loop (i+1,j,max(collatz(i,0),max_cycle));

fun tnp1 i j =
  loop (i,j,0);