Day 13

[input, folds] =
  File.read!("day13.txt")
  |> String.trim()
  |> String.split("\n\n")

input =
  input
  |> String.split("\n")
  |> Enum.map(fn line ->
    [x, y] = String.split(line, ",")

    {String.to_integer(x), String.to_integer(y)}
  end)
  |> MapSet.new()

folds =
  folds
  |> String.split("\n")
  |> Enum.map(fn
    "fold along " <> <<c>> <> "=" <> rest ->
      {String.to_atom(<<c>>), String.to_integer(rest)}
  end)

defmodule Day13 do
  def fold({orientation, pos}, set) do
    Enum.reduce(set, MapSet.new(), fn point, acc ->
      new_point = folded_coords(orientation, pos, point)

      MapSet.put(acc, new_point)
    end)
  end

  defp folded_coords(:x, col, {x, y}) when x > col, do: {abs(2 * col - x), y}
  defp folded_coords(:y, row, {x, y}) when y > row, do: {x, abs(2 * row - y)}
  defp folded_coords(_, _, point), do: point

  def draw(set) do
    set
    |> Enum.group_by(&elem(&1, 1))
    |> Enum.sort()
    |> Enum.map(fn {_, points} ->
      points
      |> Enum.map(&elem(&1, 0))
      |> Enum.sort()
      |> Enum.chunk_every(2, 1)
      |> Enum.map(fn
        [a, b] -> b - a
        _ -> 0
      end)
      |> Enum.map(&String.pad_trailing("█", &1, " "))
    end)
    |> Enum.join("\n")
  end
end

{:module, Day13, <<70, 79, 82, 49, 0, 0, 13, ...>>, {:draw, 1}}

Task 1

Day13.fold(hd(folds), input) |> MapSet.size()

802

Task 2

Enum.reduce(folds, input, &Day13.fold/2)
|> Day13.draw()
|> IO.puts()

███  █  █ █  █ ████ ████  ██  █  █ ███
█  █ █ █  █  █ █       █ █  █ █  █ █  █
█  █ ██   ████ ███    █  █    █  █ ███
███  █ █  █  █ █     █   █ ██ █  █ █  █
█ █  █ █  █  █ █    █    █  █ █  █ █  █
█  █ █  █ █  █ █    ████  ███  ██  ███

:ok