AdventOfCode/2024/day8.exs

#!/usr/bin/env elixir
defmodule Day8 do
  def part1({x, y, frequencies}) do
    Enum.reduce(frequencies, MapSet.new(), fn antennas, antinodes ->
      antennas
      |> find_pairs()
      |> Enum.flat_map(fn pair -> find_antinodes(pair, x, y) end)
      |> Enum.into(antinodes)
    end)
    |> MapSet.size()
  end

  def find_pairs([_]), do: []
  def find_pairs([a | rest]), do: for(b <- rest, do: {a, b}) ++ find_pairs(rest)

  def find_antinodes({{x_a, y_a}, {x_b, y_b}}, max_x, max_y) do
    dx = x_a - x_b
    dy = y_a - y_b
    antinode_1 = {x_a + dx, y_a + dy}
    antinode_2 = {x_b - dx, y_b - dy}

    Enum.filter([antinode_1, antinode_2], fn
      {x, y} -> 0 <= x and x < max_x and 0 <= y and y < max_y
    end)
  end

  def part2({x, y, frequencies}) do
    Enum.reduce(frequencies, MapSet.new(), fn antennas, antinodes ->
      antennas
      |> find_pairs()
      |> Enum.flat_map(fn pair -> find_resonant_harmonics(pair, x, y) end)
      |> Enum.into(antinodes)
    end)
    |> MapSet.size()
  end

  def find_resonant_harmonics({{x_a, y_a}, {x_b, y_b}}, max_x, max_y) do
    dx = x_a - x_b
    dy = y_a - y_b

    [{x_a, y_a}, {x_b, y_b}] ++
      resonate(x_a, y_a, dx, dy, max_x, max_y) ++
      resonate(x_b, y_b, dx * -1, dy * -1, max_x, max_y)
  end

  def resonate(x, y, dx, dy, max_x, max_y, multiplier \\ 1) do
    next_x = x + dx * multiplier
    next_y = y + dy * multiplier

    if next_x < 0 or next_x >= max_x or next_y < 0 or next_y >= max_y do
      []
    else
      [{next_x, next_y} | resonate(x, y, dx, dy, max_x, max_y, multiplier + 1)]
    end
  end

  def input do
    with [input_filename] <- System.argv(),
         {:ok, input} <- File.read(input_filename) do
      {_, x, y, antennas} =
        for <<char::binary-1 <- input>>, reduce: {0, 0, 0, []} do
          {x, max_x, y, antennas} ->
            case char do
              "." -> {x + 1, max_x, y, antennas}
              "\n" -> {0, x, y + 1, antennas}
              freq -> {x + 1, max_x, y, [{freq, {x, y}} | antennas]}
            end
        end

      {x, y, antennas |> Enum.group_by(&elem(&1, 0), &elem(&1, 1)) |> Map.values()}
    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 day8.exs input_filename")
  end
end

Day8.run()