Day 11

Mix.install([:kino_aoc])

Section

{:ok, puzzle_input} =
  KinoAOC.download_puzzle("2023", "11", System.fetch_env!("LB_ADVENT_OF_CODE_SESSION"))

{:ok,
 ".#...........#.........................................................................#...............#...........#........................\n.......................#....................................................................................................................\n...............................#...........#.................#.............................#.............................................#..\n......#............................................#........................................................................................\n......................................................................#.....#.....................#.........................................\n.................#.......#.......................................#................#.....#.....................#.............................\n.........................................#.................#................................................................................\n................................#.............................................................#..........#..................................\n#.......................................................................#..............................................#.................#..\n..........#....................................#......................................#............................................#........\n................#....................................#........#.............................................................................\n......#........................................................................#............................................................\n......................#..............#..............................................................#..........#.....#......................\n..................................................#.......................#..................................................#..........#...\n............................#...............................................................................................................\n........#...................................................................................................................................\n..................#.......................#................................................................#................................\n.............................................................#........#.....................................................................\n..............................#.......#......................................#.............#........................#.......................\n.#............................................................................................................#...........................#.\n...................................................................................................#....................#...................\n....................#......................#.............#.....#..................................................................#.........\n.................................................................................................................#..........................\n............................................................................................................................................\n....................................................................#.....................#.................................................\n.................................#..............................................#..............#............#.............#.................\n...........................#............................................#...........................................#.......................\n......................#.........................#...........................................................................................\n.......................................#....................................#...................................#...........................\n.....#." <> ...}

lines = String.split(puzzle_input, "\n", trim: true)

galaxies =
  for {line, y} <- Enum.with_index(lines),
      {d, x} <- Enum.with_index(String.to_charlist(line)),
      d == ?#,
      do: {x, y}

{sx, sy} =
  Enum.reduce(galaxies, {0, 0}, fn {x, y}, {mx, my} -> {max(x, mx), max(y, my)} end)

{139, 139}

[rows, cols] =
  Enum.reduce(galaxies, [MapSet.new(), MapSet.new()], fn {x, y}, [rows, cols] ->
    [MapSet.put(rows, y), MapSet.put(cols, x)]
  end)

empty_rows = MapSet.difference(MapSet.new(0..sy), rows) |> MapSet.to_list() |> Enum.sort()
empty_cols = MapSet.difference(MapSet.new(0..sx), cols) |> MapSet.to_list() |> Enum.sort()

{empty_rows, empty_cols}

{[23, 54, 77, 81, 87, 91, 106, 107], ~c".178SYh"}

defmodule Day11 do
  def reduce_unique_pairs(xs, init, fun) when is_function(fun, 3) do
    do_map(hd(xs), tl(xs), fun, init)
  end

  def do_map(_h, [], _fun, acc), do: acc

  def do_map(a, xs, fun, acc) do
    new =
      Enum.reduce(xs, acc, fn b, acc1 -> fun.(a, b, acc1) end)

    do_map(hd(xs), tl(xs), fun, new)
  end

  def expand({x, y}, empty_rows, empty_cols, m \\ 2) do
    {x + delta(x, empty_cols) * (m - 1), y + delta(y, empty_rows) * (m - 1)}
  end

  defp delta(p, empty) do
    Enum.count(empty, &(p > &1))
  end
end

{:module, Day11, <<70, 79, 82, 49, 0, 0, 12, ...>>, {:delta, 2}}

Part 1

galaxies
|> Enum.map(&Day11.expand(&1, empty_rows, empty_cols))
|> Day11.reduce_unique_pairs(0, fn {xa, ya}, {xb, yb}, acc ->
  acc + abs(xb - xa) + abs(yb - ya)
end)

9686930

Part 2

galaxies
|> Enum.map(&Day11.expand(&1, empty_rows, empty_cols, 1_000_000))
|> Day11.reduce_unique_pairs(0, fn {xa, ya}, {xb, yb}, acc ->
  acc + abs(xb - xa) + abs(yb - ya)
end)

630728425490