let cur_max = ref 0;;

let next n =
    match n with
    | n when (n mod 2 = 0) -> n/2
    | n -> 3*n + 1
in

let rec calc num steps =
    if num = 1 then steps else calc (next num) (steps + 1)
in

let rec check_million start =
    let steps = calc start 1 in
    Printf.printf "Checking %d\n" start;
    if start mod 10000 = 0 then (Printf.printf "Starting at %d\n" start);
    if steps > !cur_max then (
        Printf.printf "New maximum value: %d for number %d\n" steps start;
        flush stdout;
        cur_max := steps;
        ()
    );
    if start <= 1000000 then check_million (start + 1);
in
check_million 1;;