A-M baby

Task/Hailstone-sequence/Erlang/hailstone-sequence.erl

@@ -0,0 +1,24 @@
+-module(hailstone).
+-import(io).
+-export([main/0]).
+
+hailstone(1) -> [1];
+hailstone(N) when N band 1 == 1 -> [N|hailstone(N * 3 + 1)];
+hailstone(N) when N band 1 == 0 -> [N|hailstone(N div 2)].
+
+max_length(Start, Stop) ->
+    F = fun (N) -> {length(hailstone(N)), N} end,
+    Lengths = lists:map(F, lists:seq(Start, Stop)),
+    lists:max(Lengths).
+
+main() ->
+    io:format("hailstone(4): ~w~n", [hailstone(4)]),
+    Seq27 = hailstone(27),
+    io:format("hailstone(27) length: ~B~n", [length(Seq27)]),
+    io:format("hailstone(27) first 4: ~w~n",
+              [lists:sublist(Seq27, 4)]),
+    io:format("hailstone(27) last 4: ~w~n",
+              [lists:nthtail(length(Seq27) - 4, Seq27)]),
+    io:format("finding maximum hailstone(N) length for 1 <= N <= 100000..."),
+    {Length, N} = max_length(1, 100000),
+    io:format(" done.~nhailstone(~B) length: ~B~n", [N, Length]).