import util.

go =>
   println("H27:"),
   H27 = hailstoneseq(27),
   H27Len = H27.len,
   println(len=H27.len),
   println(take(H27,4)++['...']++drop(H27,H27Len-4)),
   nl,

   println("Longest sequence < 100_000:"),
   longest_seq(99_999),

   nl.

% The Hailstone value of a number
hailstone(N) = N // 2, N mod 2 == 0 => true.
hailstone(N) = 3*N+1, N mod 2 == 1 => true.

% Sequence for a number
hailstoneseq(N) = Seq =>
   Seq := [N],
   while (N > 1)
      N := hailstone(N),
      Seq := Seq ++ [N]
   end.

%
% Use a map to cache the lengths.
% Here we don't care about the actual sequence.
%
longest_seq(Limit) =>
   Lens = new_map(), % caching the lengths
   MaxLen = 0,
   MaxN = 1,

   foreach(N in 1..Limit-1)
      M = N,
      CLen = 1,
      while (M > 1)
         if Lens.has_key(M) then
            CLen := CLen + Lens.get(M) - 1,
            M := 1
         else
            M := hailstone(M), % call the
            CLen := CLen + 1
         end
      end,
      Lens.put(N, CLen),
      if CLen > MaxLen then
         MaxLen := CLen,
         MaxN := N
      end
   end,
   println([maxLen=MaxLen, maxN=MaxN]),
   nl.