2024 day 3

2024/day3.exs

@@ -0,0 +1,60 @@
+#!/usr/bin/env elixir
+defmodule Day3 do
+  def part1(input) do
+    Regex.scan(~r/mul\((\d+),(\d+)\)/, input, capture: :all_but_first)
+    |> Enum.map(fn [a, b] -> String.to_integer(a) * String.to_integer(b) end)
+    |> Enum.sum()
+  end
+
+  def part2(input) do
+    Regex.scan(~r/mul\((\d+),(\d+)\)|(do\(\))|(don't\(\))/, input, capture: :all_but_first)
+    |> Enum.reduce({0, :process}, fn
+      ["", "", "do()"], {sum, _} -> {sum, :process}
+      ["", "", "", "don't()"], {sum, _} -> {sum, :skip}
+      _, {sum, :skip} -> {sum, :skip}
+      [a, b], {sum, :process} -> {sum + String.to_integer(a) * String.to_integer(b), :process}
+    end)
+    |> elem(0)
+  end
+
+  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("#{inspect(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()