Day 7: Laboratories

Day 7: Laboratories

Day 7: Laboratories

Part 1

defmodule AdventOfCode2025Day7Part1Solver do
  def solve(list_of_charlists) do
    list_of_charlists
    |> Enum.reduce({nil, 0}, &do_solve/2)
    |> elem(1)
  end

  def do_solve(charlist, {nil, 0}) do
    s_index = Enum.find_index(charlist, & &1 == ?S)
    {MapSet.new([s_index]), 0}
  end

  def do_solve(charlist, {map_set, count}) do
    Enum.reduce(map_set, {map_set, count}, fn i, {acc_map_set, acc_count} ->
      if Enum.at(charlist, i) == ?^ do
        new_map_set = acc_map_set |> MapSet.delete(i) |> MapSet.put(i - 1) |> MapSet.put(i + 1)
        {new_map_set, acc_count + 1}
      else
        {acc_map_set, acc_count}
      end
    end)
  end
end

defmodule AdventOfCode2025Day7Part1 do
  def run(input) do
    input
    |> parse_input()
    |> solve()
  end

  defp solve(list_of_charlists), do: AdventOfCode2025Day7Part1Solver.solve(list_of_charlists)

  defp parse_input(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.map(&String.to_charlist/1)
  end
end

input = """
.......S.......
...............
.......^.......
...............
......^.^......
...............
.....^.^.^.....
...............
....^.^...^....
...............
...^.^...^.^...
...............
..^...^.....^..
...............
.^.^.^.^.^...^.
...............
"""

AdventOfCode2025Day7Part1.run(input)

Part 2

defmodule AdventOfCode2025Day7Part2Solver do
  def solve(list_of_charlists) do
    list_of_charlists
    |> Enum.reduce(nil, &do_solve/2)
    |> Map.values()
    |> Enum.sum()
  end

  def do_solve(charlist, nil) do
    s_index = Enum.find_index(charlist, & &1 == ?S)
    %{s_index => 1}
  end

  def do_solve(charlist, map) do
    Enum.reduce(map, map, fn {i, count}, acc ->
      if Enum.at(charlist, i) == ?^ do
        acc |> Map.delete(i) |> Map.update(i - 1, count, & &1 + count) |> Map.update(i + 1, count, & &1 + count)
      else
        acc
      end
    end)
  end
end

defmodule AdventOfCode2025Day7Part2 do
  def run(input) do
    input
    |> parse_input()
    |> solve()
  end

  defp solve(list_of_charlists), do: AdventOfCode2025Day7Part2Solver.solve(list_of_charlists)

  defp parse_input(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.map(&String.to_charlist/1)
  end
end

input = """
.......S.......
...............
.......^.......
...............
......^.^......
...............
.....^.^.^.....
...............
....^.^...^....
...............
...^.^...^.^...
...............
..^...^.....^..
...............
.^.^.^.^.^...^.
...............
"""

AdventOfCode2025Day7Part2.run(input)