Day 4: Printing Department

Day 4: Printing Department

Day 4: Printing Department

Part 1

defmodule AdventOfCode2025Day4Part1Solver do
  def solve(map) do
    map
    |> Enum.reduce({0, map}, &fun/2)
    |> elem(0)
  end

  def fun({{_i, _j}, ?.}, {acc, map}), do: {acc, map}
  def fun({{i, j}, ?@}, {acc, map}) do
    directions = [
      {-1, -1}, {0, -1}, {1, -1},
      {-1,  0},          {1,  0},
      {-1,  1}, {0,  1}, {1,  1}
    ]

    new_acc = directions
    |> Enum.reduce_while(0, fn {x, y}, cnt ->
      v = Map.get(map, {i + x, j + y}, ?.)
      
      (if v == ?@, do: cnt + 1, else: cnt)
      |> do_fun()
    end)
    |> Kernel.<(4)
    |> if(do: acc + 1, else: acc)

    {new_acc, map}
  end

  defp do_fun(new_cnt) when new_cnt >= 4, do: {:halt, new_cnt}
  defp do_fun(new_cnt), do: {:cont, new_cnt}
end

defmodule AdventOfCode2025Day4Part1 do
  def run(input) do
    input
    |> parse_input()
    |> AdventOfCode2025Day4Part1Solver.solve()
  end

  defp parse_input(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.with_index()
    |> Enum.reduce(%{}, fn {line, i}, acc ->
      line
      |> String.to_charlist()
      |> Enum.with_index()
      |> Enum.reduce(acc, fn {c, j}, acc ->
        Map.put(acc, {i, j}, c)
      end)
    end)
  end
end

input = """
..@@.@@@@.
@@@.@.@.@@
@@@@@.@.@@
@.@@@@..@.
@@.@@@@.@@
.@@@@@@@.@
.@.@.@.@@@
@.@@@.@@@@
.@@@@@@@@.
@.@.@@@.@.
"""

AdventOfCode2025Day4Part1.run(input)

Part 2

defmodule AdventOfCode2025Day4Part2Solver do
  def solve(map) do
    do_solve(map, 0)
  end

  defp do_solve(map, cnt) do
    removes = map |> Enum.reduce({[], map}, &fun/2) |> elem(0)
    if Enum.count(removes) == 0 do
      cnt
    else
      new_cnt = cnt + Enum.count(removes)
      new_map = removes
        |> Enum.reduce(map, fn {i, j}, acc ->
          Map.put(acc, {i, j}, ?x)
        end)

      do_solve(new_map, new_cnt)
    end
  end

  defp fun({{_i, _j}, ?.}, {acc, map}), do: {acc, map}
  defp fun({{_i, _j}, ?x}, {acc, map}), do: {acc, map}
  defp fun({{i, j}, ?@}, {acc, map}) do
    directions = [
      {-1, -1}, {0, -1}, {1, -1},
      {-1,  0},          {1,  0},
      {-1,  1}, {0,  1}, {1,  1}
    ]

    new_acc = directions
    |> Enum.reduce_while(0, fn {x, y}, cnt ->
      v = Map.get(map, {i + x, j + y}, ?.)
      
      (if v == ?@, do: cnt + 1, else: cnt)
      |> do_fun()
    end)
    |> Kernel.<(4)
    |> if(do: [{i, j} | acc], else: acc)

    {new_acc, map}
  end

  defp do_fun(new_cnt) when new_cnt >= 4, do: {:halt, new_cnt}
  defp do_fun(new_cnt), do: {:cont, new_cnt}
end

defmodule AdventOfCode2025Day4Part2 do
  def run(input) do
    input
    |> parse_input()
    |> AdventOfCode2025Day4Part2Solver.solve()
  end

  defp parse_input(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.with_index()
    |> Enum.reduce(%{}, fn {line, i}, acc ->
      line
      |> String.to_charlist()
      |> Enum.with_index()
      |> Enum.reduce(acc, fn {c, j}, acc ->
        Map.put(acc, {i, j}, c)
      end)
    end)
  end
end

input = """
..@@.@@@@.
@@@.@.@.@@
@@@@@.@.@@
@.@@@@..@.
@@.@@@@.@@
.@@@@@@@.@
.@.@.@.@@@
@.@@@.@@@@
.@@@@@@@@.
@.@.@@@.@.
"""

AdventOfCode2025Day4Part2.run(input)