aoc 2021 day 13

Setup

Mix.install([{:kino, "~> 0.4.1"}])

input = Kino.Input.textarea("input")

Part1

defmodule Part1 do
  def fold(pos_map, {:x, fold_x}) do
    pos_map
    |> Map.new(fn
      {{x, y}, true} when x > fold_x ->
        {{fold_x * 2 - x, y}, true}

      other ->
        other
    end)
  end

  def fold(pos_map, {:y, fold_y}) do
    pos_map
    |> Map.new(fn
      {{x, y}, true} when y > fold_y ->
        {{x, fold_y * 2 - y}, true}

      other ->
        other
    end)
  end
end

lines =
  input
  |> Kino.Input.read()
  |> String.split("\n", trim: true)

{coord_lines, fold_command_lines} = Enum.split_while(lines, &(!String.starts_with?(&1, "fold")))

coords =
  coord_lines
  |> Stream.map(&String.split(&1, ","))
  |> Stream.map(&Enum.map(&1, fn s -> String.to_integer(s) end))

fold_commands =
  fold_command_lines
  |> Enum.map(fn
    "fold along x=" <> x ->
      {:x, String.to_integer(x)}

    "fold along y=" <> y ->
      {:y, String.to_integer(y)}
  end)

pos_map =
  coords
  |> Enum.reduce(%{}, fn [x, y], pos_map ->
    put_in(pos_map, [{x, y}], true)
  end)

[{xy, num} | _] = fold_commands

Part1.fold(pos_map, {xy, num})
|> Enum.count()

Part 2

{max_x, max_y} =
  pos_map
  |> Map.keys()
  |> then(fn keys ->
    {max_x, _} = Enum.max(keys)
    {_, max_y} = Enum.max_by(keys, &elem(&1, 1))
    {max_x, max_y}
  end)

defmodule Drawer do
  def draw(pos_map) do
    {max_x, max_y} = max_xy(pos_map)

    0..max_y
    |> Enum.map_join("\n", fn y ->
      0..max_x
      |> Enum.map_join(fn x ->
        if pos_map[{x, y}] do
          '#'
        else
          '.'
        end
      end)
    end)
  end

  defp max_xy(pos_map) do
    pos_map
    |> Map.keys()
    |> then(fn keys ->
      {max_x, _} = Enum.max(keys)
      {_, max_y} = Enum.max_by(keys, &elem(&1, 1))
      {max_x, max_y}
    end)
  end
end

pos_map =
  coords
  |> Enum.reduce(%{}, fn [x, y], pos_map ->
    put_in(pos_map, [{x, y}], true)
  end)

fold_commands
|> Enum.reduce(pos_map, fn command, acc ->
  Part1.fold(acc, command)
end)
|> Drawer.draw()
|> IO.puts()