Day 3: Lobby

Day 3: Lobby

Day 3: Lobby

Part 1

defmodule AdventOfCode2025Day3Part1Solver do
  def run(list_of_lists) do
    list_of_lists
    |> Enum.reduce(0, fn list, acc ->
      upper = Enum.slice(list, 0..-2//1) |> Enum.max()
      uppser_index = Enum.find_index(list, & &1 == upper)
      lower = Enum.slice(list, (uppser_index + 1)..-1//1) |> Enum.max()

      acc + upper * 10 + lower
    end)
  end
end

defmodule AdventOfCode2025Day3Part1 do
  def run(input) do
    input
    |> parse_input()
    |> solve()
  end

  defp solve(list_of_lists) do
    AdventOfCode2025Day3Part1Solver.run(list_of_lists)
  end

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

  defp parse(line) do
    line
    |> String.to_charlist()
    |> Enum.map(& &1 - ?0)
  end
end

input = """
987654321111111
811111111111119
234234234234278
818181911112111
"""

AdventOfCode2025Day3Part1.run(input)

Part 2

defmodule AdventOfCode2025Day3Part2Solver do
  def run(list_of_lists) do
    list_of_lists
    |> Enum.reduce(0, fn list, acc ->
      12..1//-1
      |> Enum.reduce({0, -1}, 
        fn i, {acc, index} ->
          {max, new_index} = Enum.slice(list, (index + 1)..-i//1) |> find_max()
          {(10 ** (i - 1)) * max + acc, new_index + index + 1} 
      end)
      |> elem(0)
      |> Kernel.+(acc)
    end)
    
  end

  defp find_max(list) do
    max = Enum.max(list)
    index = Enum.find_index(list, & &1 == max)
    {max, index}
  end
end

defmodule AdventOfCode2025Day3Part2 do
  def run(input) do
    input
    |> parse_input()
    |> solve()
  end

  defp solve(list_of_lists) do
    AdventOfCode2025Day3Part2Solver.run(list_of_lists)
  end

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

  defp parse(line) do
    line
    |> String.to_charlist()
    |> Enum.map(& &1 - ?0)
  end
end

input = """
987654321111111
811111111111119
234234234234278
818181911112111
"""

AdventOfCode2025Day3Part2.run(input)