2016 Update

Task/Hofstadter-Q-sequence/Elixir/hofstadter-q-sequence.elixir

@@ -0,0 +1,24 @@
+defmodule Hofstadter do
+  defp flip(v2, v1) when v1 > v2, do: 1
+  defp flip(_v2, _v1), do: 0
+
+  defp list_terms(max, n, acc), do: Enum.map_join(n..max, ", ", &acc[&1])
+
+  defp hofstadter(n, n, acc, flips) do
+    IO.puts "The first ten terms are: #{list_terms(10, 1, acc)}"
+    IO.puts "The 1000'th term is #{acc[1000]}"
+    IO.puts "Number of flips: #{flips}"
+  end
+  defp hofstadter(max, n, acc, flips) do
+    qn1 = acc[n-1]
+    qn = acc[n - qn1] + acc[n - acc[n-2]]
+    hofstadter(max, n+1, Map.put(acc, n, qn), flips + flip(qn, qn1))
+  end
+
+  def main(max \\ 100_000) do
+    acc = %{1 => 1, 2 => 1}
+    hofstadter(max+1, 3, acc, 0)
+  end
+end
+
+Hofstadter.main