fun hail (x = 1) = [1]
       | (x rem 2 = 0) = x :: hail (x div 2)
       | x = x :: hail (x * 3 + 1)

fun hailstorm
		([], i, largest, largest_at) = (largest_at, largest)
	| 	(x :: xs, i, largest, largest_at) =
		let
			val k = len (hail x)
		in
			if k > largest then
				hailstorm (xs, i + 1, k, i)
			else
				hailstorm (xs, i + 1, largest, largest_at)
			end
	| 	(x :: xs) = hailstorm (x :: xs, 1, 0, 0)

 ;

val h27 = hail 27;
print "hailstone sequence for the number 27 has ";
print ` len (h27);
print " elements starting with ";
print ` sub (h27, 0, 4);
print " and ending with ";
print ` sub (h27, len(h27)-4, len h27);
println ".";

val biggest = hailstorm ` iota (100000 - 1);

print "The number less than 100,000 which has the longest ";
print "hailstone sequence is at element ";
print ` ref (biggest, 0);
print " and is of length ";
println ` ref (biggest, 1);