🎄 Year 2025 🔔 Day 09

Section

red_tile_locations =
  File.read!("#{__DIR__}/../../../inputs/2025/day09.txt")
  |> String.split("\n", trim: true)
  |> Enum.map(fn row ->
    row
    |> String.split(",", trim: true)
    |> Enum.map(&String.to_integer/1)
    |> List.to_tuple()
  end)

Part 1

defmodule Helper do
  def get_pairs_with_sizes(list) do
    get_pairs_with_sizes(list, [])
  end

  defp get_pairs_with_sizes([_head], result), do: Enum.concat(result)
  defp get_pairs_with_sizes([{hx, hy} = head | tail], result) do
    new_results = Enum.map(tail, fn {ex, ey} = e ->
      rectangle_size = (abs(hx - ex) + 1) * (abs(hy - ey) + 1)
      {head, e, rectangle_size}
    end)
    get_pairs_with_sizes(tail, [new_results | result])
  end
end

Helper.get_pairs_with_sizes(red_tile_locations)
|> Enum.max_by(&elem(&1, 2))
|> elem(2)

Part 2

{vertical_ranges, horizontal_ranges} =
  red_tile_locations
  |> Enum.chunk_every(2, 1, :discard)
  |> Enum.reduce({[], []}, fn [{fx, fy}, {sx, sy}], {vertical_ranges, horizontal_ranges} ->
    if fx == sx do
      nv_range = {fx, min(fy, sy)..max(fy, sy)}
      {[nv_range | vertical_ranges], horizontal_ranges}
    else
      nh_range = {min(fx, sx)..max(fx, sx), fy}
      {vertical_ranges, [nh_range | horizontal_ranges]}
    end
  end)

Helper.get_pairs_with_sizes(red_tile_locations)
|> Enum.sort_by(&elem(&1, 2), :desc)
|> Enum.find(fn {{fx, fy}, {sx, sy}, _size} ->
  max_x = max(fx, sx)
  min_x = min(fx, sx)
  max_y = max(fy, sy)
  min_y = min(fy, sy)
  iy_range = (min_y+1)..(max_y-1)
  ix_range = (min_x+1)..(max_x-1)

  # Check for gaps in top edge
  any_vertical_overlap =
    vertical_ranges
    |> Enum.any?(fn {x, y_range} ->
      x > min_x &amp;&amp; x < max_x &amp;&amp; !Range.disjoint?(y_range, iy_range)
    end)

  any_horizontal_overlap =
    horizontal_ranges
    |> Enum.any?(fn {x_range, y} ->
      y > min_y &amp;&amp; y < max_y &amp;&amp; !Range.disjoint?(x_range, ix_range)
    end)

  !any_vertical_overlap &amp;&amp; !any_horizontal_overlap
end)
|> elem(2)