AdventOfCode/2024/day14.exs

#!/usr/bin/env elixir
defmodule Day14 do
  def part1({robots, size_x, size_y}) do
    mid_x = div(size_x, 2)
    mid_y = div(size_y, 2)

    robots
    |> Enum.reduce({0, 0, 0, 0}, fn [px, py, vx, vy], {a, b, c, d} ->
      x = Integer.mod(px + vx * 100, size_x)
      y = Integer.mod(py + vy * 100, size_y)

      case {x, y} do
        {x, y} when x < mid_x and y < mid_y -> {a + 1, b, c, d}
        {x, y} when x < mid_x and y > mid_y -> {a, b + 1, c, d}
        {x, y} when x > mid_x and y < mid_y -> {a, b, c + 1, d}
        {x, y} when x > mid_x and y > mid_y -> {a, b, c, d + 1}
        {x, y} when x == mid_x or y == mid_y -> {a, b, c, d}
      end
    end)
    |> Tuple.product()
  end

  def part2({robots, size_x, size_y}) do
    # The tree looks like this
    # XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
    # X                             X
    # X                             X
    # X                             X
    # X                             X
    # X              X              X
    # X             XXX             X
    # X            XXXXX            X
    # X           XXXXXXX           X
    # X          XXXXXXXXX          X
    # X            XXXXX            X
    # X           XXXXXXX           X
    # X          XXXXXXXXX          X
    # X         XXXXXXXXXXX         X
    # X        XXXXXXXXXXXXX        X
    # X          XXXXXXXXX          X
    # X         XXXXXXXXXXX         X
    # X        XXXXXXXXXXXXX        X
    # X       XXXXXXXXXXXXXXX       X
    # X      XXXXXXXXXXXXXXXXX      X
    # X        XXXXXXXXXXXXX        X
    # X       XXXXXXXXXXXXXXX       X
    # X      XXXXXXXXXXXXXXXXX      X
    # X     XXXXXXXXXXXXXXXXXXX     X
    # X    XXXXXXXXXXXXXXXXXXXXX    X
    # X             XXX             X
    # X             XXX             X
    # X             XXX             X
    # X                             X
    # X                             X
    # X                             X
    # X                             X
    # XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
    # Could improve this by searching for that pattern explicitly. In this case I just searched
    # manually: noticed things looked suspicious after 27 seconds
    # also noticed that repeated every 101 seconds
    # Checked each 101 second jump until found the above tree
    find_tree(robots, size_x, size_y)
  end

  def find_tree(robots, size_x, size_y, count \\ 27, acc \\ 27) do
    robots = tick(robots, size_x, size_y, count)
    robomap = MapSet.new(robots, fn [x, y, _, _] -> {x, y} end)

    for y <- 0..(size_y - 1) do
      for x <- 0..(size_x - 1) do
        if MapSet.member?(robomap, {x, y}) do
          IO.write("X")
        else
          IO.write(" ")
        end
      end

      IO.puts("|")
    end

    line = IO.gets("#{acc} tree [y/N]?")

    if line |> String.trim() |> String.downcase() == "y" do
      acc
    else
      find_tree(robots, size_x, size_y, 101, acc + 101)
    end
  end

  def tick(robots, size_x, size_y, times) do
    Enum.map(robots, fn [px, py, vx, vy] ->
      [Integer.mod(px + vx * times, size_x), Integer.mod(py + vy * times, size_y), vx, vy]
    end)
  end

  def input do
    with [input_filename, size_x, size_y] <- System.argv(),
         {:ok, input} <- File.read(input_filename),
         {size_x, ""} <- Integer.parse(size_x),
         {size_y, ""} <- Integer.parse(size_y) do
      robots =
        input
        |> String.split(["p=", ",", " ", "v=", "\n"], trim: true)
        |> Enum.map(&String.to_integer/1)
        |> Enum.chunk_every(4)

      {robots, size_x, size_y}
    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 day14.exs input_filename size_x size_y")
  end
end

Day14.run()