Add 2015 day1

2015/day1.exs

@@ -0,0 +1,52 @@
+defmodule Day1 do
+  def part1(<<>>), do: 0
+  def part1(<<?(, rest::binary>>), do: 1 + part1(rest)
+  def part1(<<?), rest::binary>>), do: -1 + part1(rest)
+
+  def part2(input), do: find_basement(input, _floor = 0, _pos = 1)
+
+  def find_basement(<<?), _::binary>>, 0, pos), do: pos
+  def find_basement(<<?), rest::binary>>, floor, pos), do: find_basement(rest, floor - 1, pos + 1)
+  def find_basement(<<?(, rest::binary>>, floor, pos), do: find_basement(rest, floor + 1, pos + 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 day1.exs input_filename")
+  end
+end
+
+Day1.run()