# AOC2024 Day 12

## Section

```elixir
defmodule Grid2D do
  defstruct width: nil, height: nil

  @directions %{
    straight: [
      {1, 0},
      {-1, 0},
      {0, 1},
      {0, -1}
    ],
    diagonal: [
      {1, 1},
      {1, -1},
      {-1, 1},
      {-1, -1}
    ]
  }

  def new(width, height) do
    %Grid2D{width: width, height: height}
  end

  @doc """
  ## Examples
  iex> Grid2D.index_to_coord(Grid2D.new(5,5), 0)
  {0,0}

  iex> Grid2D.index_to_coord(Grid2D.new(5,5), 5)
  {0,1}

  iex> Grid2D.index_to_coord(Grid2D.new(5,5), 6)
  {1,1}

  iex> Grid2D.index_to_coord(Grid2D.new(5,5), 3)
  {3,0}

  iex> Grid2D.index_to_coord(Grid2D.new(5,5), 24)
  {4, 4}
  """
  def index_to_coord(%Grid2D{width: w}, index) do
    {rem(index, w), floor(index / w)}
  end

  @doc """
  ## Examples
  iex> Grid2D.coord_to_index( Grid2D.new(5,5), {0,0})
  0

  iex> Grid2D.coord_to_index( Grid2D.new(5,5), {4,4})
  24

  iex> Grid2D.coord_to_index( Grid2D.new(5,5), {2,2})
  12
  """
  def coord_to_index(%Grid2D{width: w}, {x, y}) do
    y * w + x
  end

  @doc """

  ## Examples
  iex> Grid2D.neighbors({0, 0}, Grid2D.new(5,5)) |> MapSet.new()
  MapSet.new([{0, 1},  {1, 1}, {1,0}])

  iex> Grid2D.neighbors({2, 2}, Grid2D.new(5,5)) |> MapSet.new()
  [{1, 1}, {1, 2}, {1, 3}, {2, 1}, {2, 3}, {3, 1}, {3, 2}, {3, 3}] |> MapSet.new()

  iex> Grid2D.neighbors({4, 4}, Grid2D.new(5,5)) |> MapSet.new
  [{3, 3}, {3, 4}, {4, 3}] |> MapSet.new
  """
  def neighbors(p, g, allowed_directions \\ :all) do
    directions(allowed_directions)
    |> Stream.map(&add(p, &1))
    |> Stream.filter(&is_in_grid?(&1, g))
  end

  @doc """
  Distance from two points in a grid

  iex> Grid2D.distance(Grid2D.new(5,5), {0,0}, {0,1}, :radial)
  1

  iex> Grid2D.distance(Grid2D.new(5,5), {0,0}, {1,1}, :radial)
  1

  iex> Grid2D.distance(Grid2D.new(5,5), {0,0}, {1,2}, :radial)
  2
  """
  def distance(grid, p1, p2, type)
  def distance(_, {x, y}, {a, b}, :radial), do: max(abs(x - a), abs(y - b))

  @doc """
  Add two points together pairwise

  iex> Grid2D.add({0,0}, {0,1})
  {0,1}

  iex> Grid2D.add({2,3}, {3,2})
  {5,5}
  """
  def add({x, y}, {j, k}), do: {x + j, y + k}

  @doc """
  Test if a point is in a grid

  iex> Grid2D.is_in_grid?({1, 2}, %Grid2D{width: 3, height: 4})
  true

  iex> Grid2D.is_in_grid?({2, 2}, %Grid2D{width: 2, height: 4})
  false
  """
  def is_in_grid?({x, y}, %Grid2D{width: w, height: h}), do: x < w and y < h and x >= 0 and y >= 0

  @spec directions(allowed :: :all | :straight | :diagonal) :: any()
  def directions(allowed \\ :all)
  def directions(:all), do: directions(:straight) ++ directions(:diagonal)
  def directions(:straight), do: @directions |> Map.get(:straight)
  def directions(:diagonal), do: @directions |> Map.get(:diagonal)

  def mul({x, y}, n), do: {n*x, n*y}
end

```

```elixir
defmodule EnumUtil do
  def pick_n(enum, n)
  def pick_n([], _), do: []
  def pick_n([a | rest], n) do
      pick_n(rest, n, n, []) ++ pick_n(rest, n, n - 1, [a])
  end
  defp pick_n([], n, _, acc) do
    if acc |> Enum.count == n do
      [acc]
    else
      []
    end
  end
  defp pick_n(_, _, 0, acc), do: [acc]
  defp pick_n([a | rest], n, r, acc) do
      pick_n(rest, n, r, acc) ++ pick_n(rest, n, r - 1, [a | acc])
  end
end
```

```elixir
input = """
"""
```

```elixir
lines = input |> String.trim() |> String.split("\n")
height = lines |> Enum.count
width = lines |> Enum.at(0) |> String.length()
tiles = lines |> Stream.with_index() |> Stream.flat_map(fn {line, i} -> line |> String.graphemes() |> Stream.with_index() |> Enum.map(fn {c, j} -> {{j, i}, c} end) end) |> Enum.into(%{})
grid = Grid2D.new(width, height)
```

```elixir
defmodule Crawl do
  def crawl([], _, _, acc), do: acc
  def crawl(points, tiles, grid, acc) do
    n = points 
      |> Enum.flat_map(fn p -> Grid2D.neighbors(p, grid, :straight) 
      |> Enum.reject(&(MapSet.member?(acc, &1) or Map.get(tiles, &1) != Map.get(tiles, p))) end)
      |> Enum.uniq()

    # IO.puts("#{inspect(n)}")
    crawl(n, tiles, grid, points |> Enum.reduce(acc, fn x, s -> MapSet.put(s, x) end))
  end

  def crawl(tiles, grid) do
    for {pt, _} <- tiles, reduce: {MapSet.new([]),[]} do
      {seen, list} ->
        if !MapSet.member?(seen, pt) do
          visited = Crawl.crawl([pt], tiles, grid, MapSet.new([pt]))
          {seen |> MapSet.union(visited), [visited | list]}
        else
          {seen, list}
        end
    end |> elem(1)
  end

  def pricep1(regions) do
    for region <- regions, reduce: 0 do
      acc ->
        area = region |> MapSet.size()
        perimeter = 4*area - (region |> MapSet.to_list() |> EnumUtil.pick_n(2) |> Stream.map(
          fn [{a,b},{x,y}] ->
            if (a == x and abs(b-y) == 1) or (b == y and abs(a-x) == 1) do
              2
            else
              0
            end
          end
        ) |> Enum.sum())
        acc + area*perimeter
    end
  end

  def rotate90({a, b}), do: {-b, a}
  def pricep2(regions, tiles, grid) do
    for region <- regions, reduce: 0 do
      acc ->
        sides = for direction <- Grid2D.directions(:straight) do
    
          for tile <- region,
            neighbor = Grid2D.add(tile, direction),
            Map.get(tiles, neighbor) != Map.get(tiles, tile),
            reduce: {MapSet.new, 0} do
            {used, total} ->
              if !MapSet.member?(used, tile) do
                scan_dir = rotate90(direction)
                line = Stream.iterate(1, &(&1 + 1))
                |> Stream.map(fn n ->
                  if rem(n, 2) == 0 do
                      {:up, div(n,2)}
                    else
                      {:down, div(n+1, 2)*-1}
                    end
                end)
                |> Stream.map(&({elem(&1, 0), Grid2D.mul(scan_dir, elem(&1, 1))}))
                |> Enum.reduce_while(
                  %{line: MapSet.new, up: true, down: true},
                  fn {dir, next}, %{line: line} = acc ->
                    next = Grid2D.add(next, tile)
    
                    is_same? = Map.get(tiles, next) == Map.get(tiles, tile)
                    next_external? = Map.get(tiles, Grid2D.add(next, direction)) != Map.get(tiles, next)
                    acc = if acc[dir] and Grid2D.is_in_grid?(next, grid) and is_same? and next_external? do
                      %{acc | line: MapSet.put(line, next)}
                    else
                      acc |> Map.put(dir, false)
                    end
      
                    {(if acc[:up] or acc[:down], do: :cont, else: :halt), acc}
                  end
                ) |> Map.get(:line)
    
                {used |> MapSet.union(line), total + 1}
              else
                {used, total}
              end
              
              end |> elem(1)
        end |> Enum.sum()
    
        area = region |> MapSet.size()
      
        acc + area*sides
    end
  end
end
```

```elixir
regions = Crawl.crawl(tiles, grid)
```

```elixir
Crawl.pricep1(regions)
```

```elixir
Crawl.pricep2(regions, tiles, grid)
```