— Day 3: Lobby —

Mix.install([{:kino_aoc, "~> 0.1"}])

Part 1

{:ok, puzzle_input} =
  KinoAOC.download_puzzle("2025", "3", System.fetch_env!("LB_AOC_SESSION_COOKIE"))

test_input = "987654321111111\n811111111111119\n234234234234278\n818181911112111"

defmodule Lobby do
  def part_1(input) do
    input
    |> parse_input()
    |> Enum.map(&max_joltage_1/1)
    |> Enum.sum()
  end

  def parse_input(input) do
    input
    |> String.split()
    |> Enum.map(&amp;for(s <- String.graphemes(&amp;1), do: String.to_integer(s)))
  end

  def max_joltage_1(bank) do
    {_last, pop_bank} = List.pop_at(bank, -1)
    max_battery = Enum.max(pop_bank)
    max_battery_index = Enum.find_index(bank, &amp;(&amp;1 == max_battery)) + 1
    submax_battery = bank |> Enum.slice(max_battery_index..-1//1) |> Enum.max()

    Integer.undigits([max_battery, submax_battery])
  end

  def max_joltage_2(bank) do
    battery_range = 11..0//-1
    max_index = length(bank) - 1
    bank = Enum.reverse(bank)

    Enum.reduce(battery_range, {[], max_index}, fn battery_index, {joltages, max_index} ->
      slice_range = battery_index..max_index
      {max_joltage_index, max_joltage} = max_with_index(bank, slice_range)
      {[max_joltage | joltages], max_joltage_index - 1}
    end)
    |> elem(0)
    |> Enum.reverse()
    |> Integer.undigits()
  end

  @spec max_with_index([integer()], Range.t()) :: {index :: integer(), max_joltage :: integer()}
  defp max_with_index(bank, slice_range) do
    bank
    |> Enum.with_index(&amp;{&amp;2, &amp;1})
    |> Enum.slice(slice_range)
    |> Enum.reverse()
    |> Enum.max_by(&amp;elem(&amp;1, 1))
  end

  def part_2(input) do
    input
    |> parse_input()
    |> Enum.map(&amp;max_joltage_2/1)
    |> Enum.sum()
  end
end

# Lobby.part_1(test_input)
Lobby.part_1(puzzle_input)

Part 2

# Lobby.part_2(test_input)
Lobby.part_2(puzzle_input)