Advent of Code 2023 - Day 18 (incomplete)

Mix.install([
  {:kino_aoc, "~> 0.1"}
])

Introduction

— Day 18: Lavaduct Lagoon —

Thanks to your efforts, the machine parts factory is one of the first factories up and running since the lavafall came back. However, to catch up with the large backlog of parts requests, the factory will also need a large supply of lava for a while; the Elves have already started creating a large lagoon nearby for this purpose.

However, they aren’t sure the lagoon will be big enough; they’ve asked you to take a look at the dig plan (your puzzle input). For example:

R 6 (#70c710)
D 5 (#0dc571)
L 2 (#5713f0)
D 2 (#d2c081)
R 2 (#59c680)
D 2 (#411b91)
L 5 (#8ceee2)
U 2 (#caa173)
L 1 (#1b58a2)
U 2 (#caa171)
R 2 (#7807d2)
U 3 (#a77fa3)
L 2 (#015232)
U 2 (#7a21e3)

The digger starts in a 1 meter cube hole in the ground. They then dig the specified number of meters up (U), down (D), left (L), or right (R), clearing full 1 meter cubes as they go. The directions are given as seen from above, so if “up” were north, then “right” would be east, and so on. Each trench is also listed with the color that the edge of the trench should be painted as an RGB hexadecimal color code.

When viewed from above, the above example dig plan would result in the following loop of trench (#) having been dug out from otherwise ground-level terrain (.):

#######
#.....#
###...#
..#...#
..#...#
###.###
#...#..
##..###
.#....#
.######

At this point, the trench could contain 38 cubic meters of lava. However, this is just the edge of the lagoon; the next step is to dig out the interior so that it is one meter deep as well:

#######
#######
#######
..#####
..#####
#######
#####..
#######
.######
.######

Now, the lagoon can contain a much more respectable 62 cubic meters of lava. While the interior is dug out, the edges are also painted according to the color codes in the dig plan.

The Elves are concerned the lagoon won’t be large enough; if they follow their dig plan, how many cubic meters of lava could it hold?

— Part Two —

The Elves were right to be concerned; the planned lagoon would be much too small.

After a few minutes, someone realizes what happened; someone swapped the color and instruction parameters when producing the dig plan. They don’t have time to fix the bug; one of them asks if you can extract the correct instructions from the hexadecimal codes.

Each hexadecimal code is six hexadecimal digits long. The first five hexadecimal digits encode the distance in meters as a five-digit hexadecimal number. The last hexadecimal digit encodes the direction to dig: 0 means R, 1 means D, 2 means L, and 3 means U.

So, in the above example, the hexadecimal codes can be converted into the true instructions:

• #70c710 = R 461937
• #0dc571 = D 56407
• #5713f0 = R 356671
• #d2c081 = D 863240
• #59c680 = R 367720
• #411b91 = D 266681
• #8ceee2 = L 577262
• #caa173 = U 829975
• #1b58a2 = L 112010
• #caa171 = D 829975
• #7807d2 = L 491645
• #a77fa3 = U 686074
• #015232 = L 5411
• #7a21e3 = U 500254

Digging out this loop and its interior produces a lagoon that can hold an impressive 952408144115 cubic meters of lava.

Convert the hexadecimal color codes into the correct instructions; if the Elves follow this new dig plan, how many cubic meters of lava could the lagoon hold?

Puzzle

{:ok, puzzle_input} =
  KinoAOC.download_puzzle("2023", "18", System.fetch_env!("LB_AOC_SESSION"))

IO.puts(puzzle_input)

Parser

Code - Parser

defmodule Parser do
  def parse(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.map(fn line ->
      [direction, value, color] = String.split(line, [" ", "(#", ")"], trim: true)

      {direction, String.to_integer(value), color}
    end)
  end
end

Tests - Parser

ExUnit.start(autorun: false)

defmodule ParserTest do
  use ExUnit.Case, async: true
  import Parser

  @input """
  R 6 (#70c710)
  D 5 (#0dc571)
  L 2 (#5713f0)
  D 2 (#d2c081)
  R 2 (#59c680)
  D 2 (#411b91)
  L 5 (#8ceee2)
  U 2 (#caa173)
  L 1 (#1b58a2)
  U 2 (#caa171)
  R 2 (#7807d2)
  U 3 (#a77fa3)
  L 2 (#015232)
  U 2 (#7a21e3)
  """
  @expected [
    {"R", 6, "70c710"},
    {"D", 5, "0dc571"},
    {"L", 2, "5713f0"},
    {"D", 2, "d2c081"},
    {"R", 2, "59c680"},
    {"D", 2, "411b91"},
    {"L", 5, "8ceee2"},
    {"U", 2, "caa173"},
    {"L", 1, "1b58a2"},
    {"U", 2, "caa171"},
    {"R", 2, "7807d2"},
    {"U", 3, "a77fa3"},
    {"L", 2, "015232"},
    {"U", 2, "7a21e3"}
  ]

  test "parse test" do
    assert parse(@input) == @expected
  end
end

ExUnit.run()

Part One

Code - Part 1

defmodule PartOne do
  def solve(input) do
    IO.puts("--- Part One ---")
    IO.puts("Result: #{run(input)}")
  end

  def run(input) do
    points =
      input
      |> Parser.parse()
      |> Enum.reduce([{0, 0}], fn {direction, value, _}, [{x, y} | _] = acc ->
        case direction do
          "U" ->
            (y - 1)..(y - value)
            |> Enum.reduce(acc, fn v, acc_up ->
              [{x, v} | acc_up]
            end)

          "D" ->
            (y + 1)..(y + value)
            |> Enum.reduce(acc, fn v, acc_down ->
              [{x, v} | acc_down]
            end)

          "L" ->
            (x - 1)..(x - value)
            |> Enum.reduce(acc, fn v, acc_left ->
              [{v, y} | acc_left]
            end)

          "R" ->
            (x + 1)..(x + value)
            |> Enum.reduce(acc, fn v, acc_right ->
              [{v, y} | acc_right]
            end)

          _ ->
            acc
        end
      end)
      |> Enum.uniq()
      |> MapSet.new()

    {x, y} =
      points
      |> Enum.to_list()
      |> Enum.sort(:asc)
      |> List.first()

    trenches_pid =
      points
      |> dig_interior({x + 1, y + 1})

    trenches = Agent.get(trenches_pid, fn trenches -> trenches end)

    Agent.stop(trenches_pid)

    MapSet.union(points, trenches)
    |> Enum.count()
  end

  defp dig_interior(points, start) do
    {:ok, trenches_pid} = Agent.start_link(fn -> MapSet.new() end)

    dig_interior(points, start, trenches_pid)
  end

  defp dig_interior(points, {x, y} = curr, trenches_pid) do
    in_points? = MapSet.member?(points, curr)

    in_trenches? =
      trenches_pid
      |> Agent.get_and_update(fn trenches ->
        {
          MapSet.member?(trenches, curr),
          MapSet.put(trenches, curr)
        }
      end)

    case in_points? or in_trenches? do
      true ->
        trenches_pid

      false ->
        dig_interior(points, {x, y - 1}, trenches_pid)
        dig_interior(points, {x, y + 1}, trenches_pid)
        dig_interior(points, {x - 1, y}, trenches_pid)
        dig_interior(points, {x + 1, y}, trenches_pid)
    end
  end
end

Tests - Part 1

ExUnit.start(autorun: false)

defmodule PartOneTest do
  use ExUnit.Case, async: true
  import PartOne

  @input """
  R 6 (#70c710)
  D 5 (#0dc571)
  L 2 (#5713f0)
  D 2 (#d2c081)
  R 2 (#59c680)
  D 2 (#411b91)
  L 5 (#8ceee2)
  U 2 (#caa173)
  L 1 (#1b58a2)
  U 2 (#caa171)
  R 2 (#7807d2)
  U 3 (#a77fa3)
  L 2 (#015232)
  U 2 (#7a21e3)
  """
  @expected 62

  test "part one" do
    assert run(@input) == @expected
  end
end

ExUnit.run()

Solution - Part 1

PartOne.solve(puzzle_input)

Part Two

Code - Part 2

defmodule PartTwo do
  def solve(input) do
    IO.puts("--- Part Two ---")
    IO.puts("Result: #{run(input)}")
  end

  def run(input) do
    instructions =
      input
      |> Parser.parse()
      |> Enum.map(fn {_, _, color} ->
        {value, direction} = String.split_at(color, -1)

        value =
          value
          |> String.to_integer(16)

        direction =
          case direction do
            "0" -> "R"
            "1" -> "D"
            "2" -> "L"
            _ -> "U"
          end

        {direction, value}
      end)

    scale =
      instructions
      |> Enum.map(fn {_, value} -> value end)
      |> Enum.min()

    points =
      instructions
      |> Enum.reduce([{0, 0}], fn {direction, value}, [{x, y} | _] = acc ->
        case direction do
          "U" ->
            (y - 1)..(y - div(value, scale))
            |> Enum.reduce(acc, fn v, acc_up ->
              [{x, v} | acc_up]
            end)

          "D" ->
            (y + 1)..(y + div(value, scale))
            |> Enum.reduce(acc, fn v, acc_down ->
              [{x, v} | acc_down]
            end)

          "L" ->
            (x - 1)..(x - div(value, scale))
            |> Enum.reduce(acc, fn v, acc_left ->
              [{v, y} | acc_left]
            end)

          "R" ->
            (x + 1)..(x + div(value, scale))
            |> Enum.reduce(acc, fn v, acc_right ->
              [{v, y} | acc_right]
            end)

          _ ->
            acc
        end
      end)
      |> Enum.uniq()
      |> MapSet.new()

    {x, y} =
      points
      |> Enum.to_list()
      |> Enum.sort(:asc)
      |> List.first()

    trenches_pid =
      points
      |> dig_interior({x + 1, y + 1})

    trenches = Agent.get(trenches_pid, fn trenches -> trenches end)

    Agent.stop(trenches_pid)

    MapSet.union(points, trenches)
    |> Enum.count()
    |> then(fn value -> value * scale * scale end)
  end

  defp dig_interior(points, start) do
    {:ok, trenches_pid} = Agent.start_link(fn -> MapSet.new() end)

    dig_interior(points, start, trenches_pid)
  end

  defp dig_interior(points, {x, y} = curr, trenches_pid) do
    in_points? = MapSet.member?(points, curr)

    in_trenches? =
      trenches_pid
      |> Agent.get_and_update(fn trenches ->
        {
          MapSet.member?(trenches, curr),
          MapSet.put(trenches, curr)
        }
      end)

    case in_points? or in_trenches? do
      true ->
        trenches_pid

      false ->
        dig_interior(points, {x, y - 1}, trenches_pid)
        dig_interior(points, {x, y + 1}, trenches_pid)
        dig_interior(points, {x - 1, y}, trenches_pid)
        dig_interior(points, {x + 1, y}, trenches_pid)
    end
  end

  # Least common multiple of 2 numbers
  defp lcm(a, b) do
    div(a * b, Integer.gcd(a, b))
  end
end

Tests - Part 2

ExUnit.start(autorun: false)

defmodule PartTwoTest do
  use ExUnit.Case, async: true
  import PartTwo

  @input """
  R 6 (#70c710)
  D 5 (#0dc571)
  L 2 (#5713f0)
  D 2 (#d2c081)
  R 2 (#59c680)
  D 2 (#411b91)
  L 5 (#8ceee2)
  U 2 (#caa173)
  L 1 (#1b58a2)
  U 2 (#caa171)
  R 2 (#7807d2)
  U 3 (#a77fa3)
  L 2 (#015232)
  U 2 (#7a21e3)
  """
  @expected 952_408_144_115

  test "part two" do
    assert run(@input) == @expected
  end
end

ExUnit.run()

Solution - Part 2

PartTwo.solve(puzzle_input)