Advent of Code 2023 Day 16

Mix.install([
  {:kino_aoc, "~> 0.1.5"}
])

Prep

{:ok, puzzle_input} =
  KinoAOC.download_puzzle("2023", "16", System.fetch_env!("LB_AOC_SESSION"))

grid =
  for {row, i} <- puzzle_input |> String.split() |> Stream.with_index(),
      {val, j} <- row |> String.to_charlist() |> Stream.with_index(),
      into: %{},
      do: {{i, j}, val}

defmodule AoC2023.Day16 do
  def part1(grid) do
    shoot(grid, {0, 0}, {0, 1}, MapSet.new())
    |> count_energized()
  end

  def part2(grid) do
    {max_i, max_j} = grid |> Map.keys() |> Enum.max()

    count =
      0..max_i
      |> Enum.map(&{&1, 0})
      |> Task.async_stream(&count_energized(shoot(grid, &1, {0, 1}, MapSet.new())),
        ordered: false
      )
      |> Enum.max()

    count =
      0..max_i
      |> Enum.map(&{&1, max_j})
      |> Task.async_stream(&count_energized(shoot(grid, &1, {0, -1}, MapSet.new())),
        ordered: false
      )
      |> Enum.reduce(count, &max/2)

    count =
      0..max_j
      |> Enum.map(&{0, &1})
      |> Task.async_stream(&count_energized(shoot(grid, &1, {1, 0}, MapSet.new())),
        ordered: false
      )
      |> Enum.reduce(count, &max/2)

    count =
      0..max_j
      |> Enum.map(&{max_i, &1})
      |> Task.async_stream(&count_energized(shoot(grid, &1, {-1, 0}, MapSet.new())),
        ordered: false
      )
      |> Enum.reduce(count, &max/2)

    elem(count, 1)
  end

  defp count_energized(set) do
    set
    |> Enum.map(&elem(&1, 0))
    |> Enum.uniq()
    |> length()
  end

  defp shoot(grid, {i, j} = position, {di, dj} = direction, acc) do
    if {position, direction} in acc do
      acc
    else
      case grid[position] do
        nil ->
          acc

        ?. ->
          shoot(grid, {i + di, j + dj}, direction, MapSet.put(acc, {position, direction}))

        ?\\ ->
          shoot(grid, {i + dj, j + di}, {dj, di}, MapSet.put(acc, {position, direction}))

        ?/ ->
          shoot(grid, {i - dj, j - di}, {-dj, -di}, MapSet.put(acc, {position, direction}))

        ?- when di == 0 ->
          shoot(grid, {i + di, j + dj}, direction, MapSet.put(acc, {position, direction}))

        ?| when dj == 0 ->
          shoot(grid, {i + di, j + dj}, direction, MapSet.put(acc, {position, direction}))

        ?- ->
          acc = shoot(grid, {i, j - 1}, {0, -1}, MapSet.put(acc, {position, direction}))
          shoot(grid, {i, j + 1}, {0, 1}, MapSet.put(acc, {position, direction}))

        ?| ->
          acc = shoot(grid, {i - 1, j}, {-1, 0}, MapSet.put(acc, {position, direction}))
          shoot(grid, {i + 1, j}, {1, 0}, MapSet.put(acc, {position, direction}))
      end
    end
  end
end

AoC2023.Day16.part1(grid)

AoC2023.Day16.part2(grid)