Add 2015 Day3

2015/day3.exs

@@ -0,0 +1,67 @@
+defmodule Day3 do
+  def part1(input), do: move(input, MapSet.new([{0, 0}]), {0, 0})
+
+  def move(<<?<, rest::binary>>, houses, {x, y}), do: deliver(rest, houses, {x - 1, y})
+  def move(<<?>, rest::binary>>, houses, {x, y}), do: deliver(rest, houses, {x + 1, y})
+  def move(<<?^, rest::binary>>, houses, {x, y}), do: deliver(rest, houses, {x, y - 1})
+  def move(<<?v, rest::binary>>, houses, {x, y}), do: deliver(rest, houses, {x, y + 1})
+  def move(<<>>, houses, _addr), do: MapSet.size(houses)
+
+  def deliver(moves, houses, addr), do: move(moves, MapSet.put(houses, addr), addr)
+
+  def part2(input), do: move2(input, MapSet.new([{0, 0}]), {0, 0}, {0, 0})
+
+  def move2(<<ms, mb, rest::binary>>, houses, santa, bot) do
+    santa = next(ms, santa)
+    bot = next(mb, bot)
+    move2(rest, MapSet.union(houses, MapSet.new([santa, bot])), santa, bot)
+  end
+
+  def move2(<<>>, houses, _santa, _bot), do: MapSet.size(houses)
+
+  def next(?<, {x, y}), do: {x - 1, y}
+  def next(?>, {x, y}), do: {x + 1, y}
+  def next(?^, {x, y}), do: {x, y - 1}
+  def next(?v, {x, y}), do: {x, y + 1}
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+    else
+      _ -> :error
+    end
+  end
+
+  #######################
+  # HERE BE BOILERPLATE #
+  #######################
+
+  def run do
+    case input() do
+      :error -> print_usage()
+      input -> run_parts_with_timer(input)
+    end
+  end
+
+  defp run_parts_with_timer(input) do
+    run_with_timer(1, fn -> part1(input) end)
+    run_with_timer(2, fn -> part2(input) end)
+  end
+
+  defp run_with_timer(part, fun) do
+    {time, result} = :timer.tc(fun)
+    IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
+    IO.puts("#{result}\n")
+  end
+
+  defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
+  defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
+  defp format_time(μsec), do: "#{μsec / 1_000_000}s"
+
+  defp print_usage do
+    IO.puts("Usage: elixir day3.exs input_filename")
+  end
+end
+
+Day3.run()