2024 Day 2

2024/day2.exs

@@ -0,0 +1,72 @@
+#!/usr/bin/env elixir
+defmodule Day2 do
+  def part1(reports) do
+    Enum.count(reports, &check_report/1)
+  end
+
+  def check_report([a, b | _]) when abs(b - a) not in [1, 2, 3], do: false
+  def check_report([a, b | rest]), do: check_report([b | rest], b - a)
+  def check_report([_], _dir), do: true
+
+  def check_report([a, b | rest], dir) do
+    new_dir = b - a
+
+    if ((dir < 0 and new_dir < 0) or (dir > 0 and new_dir > 0)) and abs(new_dir) <= 3 do
+      check_report([b | rest], new_dir)
+    else
+      false
+    end
+  end
+
+  def part2(reports) do
+    Enum.count(reports, fn report ->
+      Enum.find_value(0..(length(report) - 1), false, fn dampen ->
+        {_, dampened} = List.pop_at(report, dampen)
+        check_report(dampened)
+      end)
+    end)
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split("\n", trim: true)
+      |> Enum.map(fn line -> line |> String.split() |> Enum.map(&String.to_integer/1) end)
+    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 day2.exs input_filename")
+  end
+end
+
+Day2.run()