Day 4

Part 1

input = File.stream!("4/input.txt")
# input = File.stream!("4/example.txt")

defmodule P1.Parser do
  def parse(input) do
    {_, xs, map} = Enum.reduce(input, {0, [], %{}}, fn line, {line_number, xs, map} ->
      {_, xs, map} =
        Enum.reduce(String.codepoints(line), {0, xs, map}, fn
          "X", {column, xs, map} ->
            {column + 1, [{column, line_number} | xs], map}

          letter, {column, xs, map} when letter in ["M", "A", "S"] ->
            {column + 1, xs, Map.put(map, {column, line_number}, letter)}

          _, {column, xs, map} ->
            {column + 1, xs, map}
        end)

      {line_number + 1, xs, map}
    end)
    {xs, map}
  end
end

data = input |> P1.Parser.parse

defmodule P1.Counter do
  def count({xs, letters}) do
    count(0, xs, letters)
  end

  defp count(acc, [], _) do
    acc
  end

  defp count(acc, [x | xs], map) do
    count(acc + find(x, map), xs, map)
  end

  def find(x, map) do
    Enum.reduce(
      [
        find_0(x, map),
        find_45(x, map),
        find_90(x, map),
        find_135(x, map),
        find_180(x, map),
        find_225(x, map),
        find_270(x, map),
        find_315(x, map)
      ],
      0,
      fn
        true, acc -> 
          # IO.inspect(x)
          acc + 1
        false, acc -> acc
      end
    )
  end

  def find_0({x, y}, map) do
    map[{x, y - 1}] == "M" && map[{x, y - 2}] == "A" && map[{x, y - 3}] == "S"
  end

  def find_45({x, y}, map) do
    map[{x + 1, y - 1}] == "M" && map[{x + 2, y - 2}] == "A" &&
      map[{x + 3, y - 3}] == "S"
  end

  def find_90({x, y}, map) do
    map[{x + 1, y}] == "M" && map[{x + 2, y}] == "A" && map[{x + 3, y}] == "S"
  end

  def find_135({x, y}, map) do
    map[{x + 1, y + 1}] == "M" && map[{x + 2, y + 2}] == "A" &&
      map[{x + 3, y + 3}] == "S"
  end

  def find_180({x, y}, map) do
    map[{x, y + 1}] == "M" && map[{x, y + 2}] == "A" && map[{x, y + 3}] == "S"
  end

  def find_225({x, y}, map) do
    map[{x - 1, y + 1}] == "M" && map[{x - 2, y + 2}] == "A" &&
      map[{x - 3, y + 3}] == "S"
  end

  def find_270({x, y}, map) do
    map[{x - 1, y}] == "M" && map[{x - 2, y}] == "A" && map[{x - 3, y}] == "S"
  end

  def find_315({x, y}, map) do
    map[{x - 1, y - 1}] == "M" && map[{x - 2, y - 2}] == "A" &&
      map[{x - 3, y - 3}] == "S"
  end
end

P1.Counter.count(data)

Part 2

defmodule P2.Parser do
  def parse(input) do
    {_, xs, map} = Enum.reduce(input, {0, [], %{}}, fn line, {line_number, xs, map} ->
      {_, xs, map} =
        Enum.reduce(String.codepoints(line), {0, xs, map}, fn
          "A", {column, xs, map} ->
            {column + 1, [{column, line_number} | xs], map}

          letter, {column, xs, map} when letter in ["M", "S"] ->
            {column + 1, xs, Map.put(map, {column, line_number}, letter)}

          _, {column, xs, map} ->
            {column + 1, xs, map}
        end)

      {line_number + 1, xs, map}
    end)
    {xs, map}
  end
end

defmodule P2.Counter do
  def count({xs, letters}) do
    count(0, xs, letters)
  end

  defp count(acc, [], _) do
    acc
  end

  defp count(acc, [x | xs], map) do
    count(acc + find(x, map), xs, map)
  end

  def find(x, map) do
    if (find_tl_br(x, map) && find_tr_bl(x, map)) do
      1
    else
      0
    end
  end

  def find_tl_br({x, y}, map) do
    (map[{x - 1, y - 1}] == "M" && map[{x + 1, y + 1}] == "S") ||
      (map[{x - 1, y - 1}] == "S" && map[{x + 1, y + 1}] == "M")
  end

  def find_tr_bl({x, y}, map) do
    (map[{x + 1, y - 1}] == "M" && map[{x - 1, y + 1}] == "S") ||
      (map[{x + 1, y - 1}] == "S" && map[{x - 1, y + 1}] == "M")
  end
end

data2 = input |> P2.Parser.parse

P2.Counter.count(data2)