Day 13

Setup

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

input = Kino.Input.textarea("Please provide your input: ")

[dots, instructions] =
  input
  |> Kino.Input.read()
  |> String.split("\n\n")

dots =
  dots
  |> String.split("\n")
  |> Enum.map(fn pair ->
    [x, y] =
      pair
      |> String.split(",")
      |> Enum.map(&String.to_integer(&1))

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

instructions =
  instructions
  |> String.split("\n")
  |> Enum.map(fn text ->
    [_fold, _along, instruction] = String.split(text, " ")
    [axis, line] = instruction |> String.split("=")
    {axis, String.to_integer(line)}
  end)

Part 1

instructions
|> Enum.take(1)
|> Enum.reduce(dots, fn {axis, line}, dots ->
  case axis do
    "y" ->
      MapSet.new(dots, fn {x, y} ->
        if y < line do
          {x, y}
        else
          {x, 2 * line - y}
        end
      end)

    "x" ->
      MapSet.new(dots, fn {x, y} ->
        if x < line do
          {x, y}
        else
          {2 * line - x, y}
        end
      end)
  end
end)
|> Enum.count()

Part 2

solution =
  instructions
  |> Enum.reduce(dots, fn {axis, line}, dots ->
    case axis do
      "y" ->
        MapSet.new(dots, fn {x, y} ->
          if y < line do
            {x, y}
          else
            {x, 2 * line - y}
          end
        end)

      "x" ->
        MapSet.new(dots, fn {x, y} ->
          if x < line do
            {x, y}
          else
            {2 * line - x, y}
          end
        end)
    end
  end)

{max_x, max_y} = Enum.max(solution)

for y <- 0..(max_y + 1) do
  line =
    for x <- 0..(max_x + 1) do
      if MapSet.member?(solution, {x, y}) do
        "#"
      else
        "."
      end
    end

  IO.puts(line)
end