11_chips_and_generators.rb

CHIP = 0
GENERATOR = 1
TYPE_NAMES = %w(chip gen).map(&:freeze).freeze
TYPE_BITS = 1
TYPE_MASK = (1 << TYPE_BITS) - 1

START_FLOOR = begin
  arg = ARGV.find { |x| x.start_with?(?-) && x.include?(?s) }
  # No need to delete here, will get deleted later.
  arg ? Integer(arg[(arg.index(?s) + 1)..-1]) : 0
end

module State; refine Array do
  def moves(elevator)
    items = self[elevator]

    chips, gens = items.partition { |x| x & TYPE_MASK == CHIP }

    # We can't move an A chip and B generator together,
    # because the A chip will certainly get fried.
    # So, our choices for twos: two chips or two generators or a pair.

    # If there is a pair, it doesn't matter which one.
    paired_chip = chips.find { |c| gens.include?(c | GENERATOR) }
    pair = paired_chip && [paired_chip, paired_chip | GENERATOR]

    unpaired_gens = gens.reject { |g| chips.include?(g & ~GENERATOR) }

    # If there's a paired generator plus any other,
    # we can't move the paired generator out alone.
    movable_gens1 = (gens.size == 1 ? gens : unpaired_gens).freeze

    # Considerations for moving two generators:
    # If there are two generators, we can always move them both out.
    # If there is one generator, combination(2) will be empty.
    # If there are 3+ generators, no paired generator can move.
    movable_gens2 = gens.size == 2 ? [gens] : unpaired_gens.combination(2).to_a
    movable_gens2.freeze

    destinations = [elevator - 1, elevator + 1].select { |f| 0 <= f && f < size }

    destinations.flat_map { |dest|
      dest_chips, dest_gens = self[dest].partition { |x| x & TYPE_MASK == CHIP }
      unpaired_chips = dest_chips.reject { |c|
        dest_gens.include?(c | GENERATOR)
      }

      # If the destination floor has generators,
      # we can only move the corresponding chips in.
      movable_chips = dest_gens.empty? ? chips : chips.select { |c|
        dest_gens.include?(c | GENERATOR)
      }

      dest_gens1, dest_gens2 =
        case unpaired_chips.size
        when 0; [movable_gens1, movable_gens2]
        when 1
          gen = unpaired_chips[0] | GENERATOR
          # We must move the unpaired chip's generator in.
          movable_by_itself = movable_gens1.include?(gen)
          [
            # That generator can move in by itself if it was movable by itself.
            movable_by_itself ? [gen] : [],
            # Moves for which that generator can move in with another:
            if movable_by_itself
              if gens.size == 2
                # Note that the other generator may not have been movable by itself:
                # It may have been a pair on the source floor,
                # in which case moving it by itself would cause its chip to fry,
                # but moving both the generators together solves that problem.
                [gens]
              else
                # move it with all the other gens that would have been movable by themselves.
                (movable_gens1 - [gen]).map { |g| [g, gen] }
              end
            else
              []
            end
          ]
        when 2
          needed_gens = unpaired_chips.map { |c| c | GENERATOR }
          # We must move both unpaired chips' generators in.
          # They are unpaired on the source floor,
          # so this does not conflict with the above logic for movable_gens2.
          [
            [],
            needed_gens.all? { |g| gens.include?(g) } ? [needed_gens] : [],
          ]
        else
          # No matter what we do, no generators are moving to this floor.
          [[], []]
        end

      two_choices = movable_chips.combination(2).to_a + dest_gens2
      one_choices = movable_chips + dest_gens1

      # If there are unpaired chips, the pair's generator will fry them.
      two_choices << pair if pair && unpaired_chips.empty?

      (two_choices + one_choices.map { |item| [item] }).map { |moved|
        [moved, dest, moved.size * (dest - elevator)]
      }
    }
  end

  def pairs
    each_with_index.with_object(Hash.new(0)) { |(items, floor), h|
      items.each { |item|
        # h's key is the element of the item.
        # The value is (generator_floor << size) | chip_floor
        # Left-shift by log_2(size) would work too,
        # but that's more work to calculate.
        h[item >> TYPE_BITS] |= floor << (item & TYPE_MASK) * size
      }
    }.values.sort.freeze
  end

  def move(moved_items, from:, to:)
    map.with_index { |items, floor|
      next items + moved_items if floor == to
      next items - moved_items if floor == from
      items
    }
  end
end end

using State

def moves_to_assemble(input, verbose: false, list_moves: false)
  # moves, state, floor
  move_queue = [[0, input, START_FLOOR]]

  # [state, floor] -> [state, floor, moved_items]
  prev = {[input.pairs, START_FLOOR] => :start}

  max_moves = 0

  while (a = move_queue.shift)
    moves_so_far, state, elevator = a

    if moves_so_far > max_moves
      puts "#{moves_so_far} moves: #{move_queue.size + 1} states" if verbose
      max_moves = moves_so_far
    end

    best_positive = 0
    best_negative = -1.0 / 0.0

    cleared_floor = state.find_index(&:any?) - 1

    state.moves(elevator).sort_by(&:last).reverse_each { |moved_items, floor_moved_to, rating|
      # If you're higher than the highest *consecutive* empty floor,
      # you probably don't want to move things into it.
      next if floor_moved_to == cleared_floor

      # If I've already gotten better moves out, bail.
      next if (rating > 0 ? best_positive : best_negative) > rating

      new_state = state.move(moved_items, from: elevator, to: floor_moved_to)

      if new_state[0..-2].all?(&:empty?)
        return [nil] * (moves_so_far + 1) unless list_moves

        state_pair = state.pairs
        floor = elevator

        prev_moves = moves_so_far.times.map {
          old_state, old_floor, moved = prev[[state_pair, floor]]

          state_pair = old_state
          [moved, floor].tap { floor = old_floor }
        }

        return prev_moves.reverse << [moved_items, floor_moved_to]
      end

      # Set ratings BEFORE pruning seen states.
      # If you can reach a seen state with a +2 move,
      # you don't need to bother with any +1 move!
      best_positive = rating if rating > 0
      best_negative = rating if rating < 0

      # MOST IMPORTANT OPTIMISATION: ALL PAIRS ARE INTERCHANGEABLE
      prev_key = [new_state.pairs, floor_moved_to]
      next if prev.has_key?(prev_key)
      prev[prev_key] = list_moves && [state.pairs, elevator, moved_items]

      move_queue << [moves_so_far + 1, new_state, floor_moved_to]
    }
  end
end

def has_flag?(flag)
  ARGV.any? { |a| a.start_with?(?-) && a.include?(flag) }
end

verbose = has_flag?(?v)
part_1_only = has_flag?(?1)
list = has_flag?(?l)
show_state = has_flag?('ll')

solve = ->(input, elements) {
  element_names = elements.sort_by(&:last).map(&:first)
  name = ->(i) {
    "#{element_names[i >> TYPE_BITS]} #{TYPE_NAMES[i & TYPE_MASK]}"
  }
  moves = moves_to_assemble(input, verbose: verbose, list_moves: list)
  puts moves.size
  if list || show_state
    state = input
    floor = START_FLOOR
    moves.each_with_index { |(moved_items, floor_moved_to), i|
      puts "#{i + 1}: #{moved_items.map(&name)} -> #{floor_moved_to}" if list
      state = state.move(moved_items, from: floor, to: floor_moved_to)
      floor = floor_moved_to
      if show_state
        state.reverse_each.with_index { |items, distance_from_top|
          puts "#{state.size - distance_from_top}: #{items.map(&name)}"
        }
        puts
      end
    }
  end
}

ARGV.reject! { |a| a.start_with?(?-) }

elements = {}

input = ARGF.each_line.map { |l|
  [
    [GENERATOR, /(\w+) generator/],
    [CHIP, /(\w+)-compatible microchip/],
  ].flat_map { |type, regex|
    l.scan(regex).map { |x|
      element = elements[x[0]] ||= elements.size
      element << TYPE_BITS | type
    }
  }
}

solve[input, elements]

input[0].concat((0...4).map { |x| x + (elements.size << TYPE_BITS) })
%w(elerium dilithium).each { |e| elements[e] = elements.size }

# for the example input,
# adding the two generators to floor 1 would immediately fry the two chips there,
# so there's no point in doing part 2 for the example input
solve[input, elements] unless part_1_only