Sync

Task/Hailstone-sequence/Elixir/hailstone-sequence.elixir

@@ -0,0 +1,22 @@
+defmodule Hailstone do
+  def step(1), do: 0
+  def step(n) when Integer.even?(n), do: div(n,2)
+  def step(n) when Integer.odd?(n), do: n*3 + 1
+  def sequence(n) do
+    Enum.to_list(Stream.take_while(Stream.iterate(n, &step/1), &(&1 > 0)))
+  end
+
+  def run do
+    seq27 = Hailstone.sequence(27)
+    len27 = length(seq27)
+    repr = String.replace(inspect(seq27, limit: 4), "]",
+                          String.replace(inspect(Enum.drop(seq27,len27-4)), "[", ", "))
+    IO.puts("Hailstone(27) has #{len27} elements: #{repr}")
+
+    {start, len}  = Enum.max_by( Enum.map(1..100_000, fn(n) -> {n, length(Hailstone.sequence(n))} end),
+                                 fn({_,len}) -> len end )
+    IO.puts("Longest sequence starting under 100000 begins with #{start} and has #{len} elements.")
+  end
+end
+
+Hailstone.run