Day 11: Cosmic Expansion

Part one

sample_input =
  """
  ...#......
  .......#..
  #.........
  ..........
  ......#...
  .#........
  .........#
  ..........
  .......#..
  #...#.....
  """

defmodule CosmicExpansion do
  def sum_of_shortest_paths_in_expanded_universe(input, expansion \\ 2) do
    input
    |> get_galaxy_locations()
    |> expand_universe(expansion)
    |> sum_shortest_paths_between_pairs()
  end

  defp get_galaxy_locations(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.map(fn line ->
      line
      |> String.split("", trim: true)
      |> Enum.with_index()
      |> Enum.filter(fn {symbol, _col} -> symbol == "#" end)
      |> Enum.map(&elem(&1, 1))
    end)
    |> Enum.with_index()
    |> Enum.flat_map(fn {cols, row} ->
      cols |> Enum.map(&{row, &1})
    end)
  end

  defp expand_universe(locations, expansion) do
    [0, 1]
    |> Enum.map(fn axis -> {axis, Enum.map(locations, &elem(&1, axis)) |> Enum.max()} end)
    |> Enum.reduce(locations, fn {axis, max}, acc ->
      0..(max - 1)
      |> Enum.filter(&no_galaxies_on_axis(acc, &1, axis))
      |> Enum.with_index()
      |> Enum.map(fn {i, j} -> i + j * (expansion - 1) end)
      |> Enum.reduce(acc, fn i, acc -> expand_axis(acc, i, axis, expansion) end)
    end)
  end

  defp no_galaxies_on_axis(locations, i, axis),
    do: Enum.filter(locations, &(elem(&1, axis) == i)) == []

  defp expand_axis(locations, i, axis, expansion) do
    Enum.map(locations, fn location ->
      if elem(location, axis) > i do
        put_elem(location, axis, elem(location, axis) + (expansion - 1))
      else
        location
      end
    end)
  end

  defp sum_shortest_paths_between_pairs(locations) do
    for loc_1 <- locations, loc_2 <- locations, loc_1 > loc_2 do
      abs(elem(loc_1, 0) - elem(loc_2, 0)) + abs(elem(loc_1, 1) - elem(loc_2, 1))
    end
    |> Enum.sum()
  end
end

CosmicExpansion.sum_of_shortest_paths_in_expanded_universe(sample_input)
# expected 374

Path.join(__DIR__, "day-11.input")
|> File.read!()
|> CosmicExpansion.sum_of_shortest_paths_in_expanded_universe()

# 10292708

Part two

CosmicExpansion.sum_of_shortest_paths_in_expanded_universe(sample_input, 10)
# expected 1030

CosmicExpansion.sum_of_shortest_paths_in_expanded_universe(sample_input, 100)
# expected 8410

Path.join(__DIR__, "day-11.input")
|> File.read!()
|> CosmicExpansion.sum_of_shortest_paths_in_expanded_universe(1_000_000)

# 790194712336