🎄 Year 2023 🔔 Day 11

Setup

map =
  File.read!("#{__DIR__}/../../../inputs/2023/day11.txt")
  |> String.split("\n", trim: true)
  |> Enum.flat_map(fn line ->
    if String.match?(line, ~r/^\.+$/) do
      [line, line]
    else
      [line]
    end
  end)
  |> Enum.map(&String.split(&1, "", trim: true))
  |> Enum.zip()
  |> Enum.flat_map(fn line ->
    line = Tuple.to_list(line) |> Enum.join("")

    if String.match?(line, ~r/^\.+$/) do
      [line, line]
    else
      [line]
    end
  end)
  |> Enum.with_index(fn line, x ->
    line
    |> String.split("", trim: true)
    |> Enum.with_index(fn char, y ->
      {{x, y}, char}
    end)
  end)
  |> Enum.concat()
  |> Map.new()

Part 1

galaxies = Enum.filter(map, fn {_coords, char} -> char == "#" end) |> Map.new()

galaxies
|> Enum.flat_map_reduce(galaxies, fn {coords, "#"}, galaxies ->
  {"#", other_galaxies} = Map.pop!(galaxies, coords)

  pairings =
    other_galaxies
    |> Enum.map(fn {other_coords, "#"} -> {coords, other_coords} end)

  {pairings, other_galaxies}
end)
|> then(fn {pairings, %{}} -> pairings end)
|> Enum.map(fn {{x1, y1}, {x2, y2}} ->
  abs(x1 - x2) + abs(y1 - y2)
end)
|> Enum.sum()

Part 2

file = File.read!("#{__DIR__}/../../../inputs/2023/day11.txt")

empty_ys =
  file
  |> String.split("\n")
  |> Enum.with_index()
  |> Enum.filter(fn {line, _y} ->
    String.match?(line, ~r/^\.+$/)
  end)
  |> Enum.map(&elem(&1, 1))

empty_xs =
  file
  |> String.split("\n", trim: true)
  |> Enum.map(&String.split(&1, "", trim: true))
  |> Enum.zip()
  |> Enum.map(&Tuple.to_list/1)
  |> Enum.map(&Enum.join(&1, ""))
  |> Enum.with_index()
  |> Enum.filter(fn {line, _y} ->
    String.match?(line, ~r/^\.+$/)
  end)
  |> Enum.map(&elem(&1, 1))

galaxies =
  file
  |> String.split("\n", trim: true)
  |> Enum.map(fn line ->
    line
    |> String.split("", trim: true)
  end)
  |> Enum.with_index(fn line, y ->
    line
    |> Enum.with_index(fn char, x ->
      {{x, y}, char}
    end)
  end)
  |> Enum.concat()
  |> Enum.filter(fn {_coords, char} -> char == "#" end)
  |> Map.new()

galaxies
|> Enum.flat_map_reduce(galaxies, fn {coords, "#"}, galaxies ->
  {"#", other_galaxies} = Map.pop!(galaxies, coords)

  pairings =
    other_galaxies
    |> Enum.map(fn {other_coords, "#"} -> {coords, other_coords} end)

  {pairings, other_galaxies}
end)
|> then(fn {pairings, %{}} -> pairings end)
|> Enum.map(fn {{x1, y1}, {x2, y2}} ->
  abs(x1 - x2) + abs(y1 - y2) +
    Enum.count(x1..x2, &Enum.member?(empty_xs, &1)) * 999_999 +
    Enum.count(y1..y2, &Enum.member?(empty_ys, &1)) * 999_999
end)
|> Enum.sum()