# AoC 2021 Day 23: Amphipodinator

### Source: Amphipod

#### Part 1: Given 4 rooms full of amphipods with various energy costs for movement (a=1, b=10, c=100, d=1000) and a hallway, how much energy does it take (at minimum) to sort the amphipods into their own rooms with the following conditions:

• Amphipods will move from rooms into the hallway, but will not stop on the space immediately outside of any room
• Amphipods will not enter a room other than their own
• Amphipods will not enter a room that has other kinds of amphipods in it
• Amphipods will only move from the hallway into a room

There’s an interesting caveat in the last two points, but we’ll come back to that.

So let’s start by getting these things loaded in:

@dataclass(frozen=True)
class Point:
x: int
y: int

def __repr__(self):
return f'<{self.x}, {self.y}>'

def distance(self, other: 'Point'):
return abs(other.x - self.x) + abs(other.y - self.y)

@dataclass(frozen=True)
class State:
width: int
height: int
walls: FrozenSet[Point]
amphipods: Mapping[str, FrozenSet[Point]]

@staticmethod

width = 0
height = 0
walls = set()
amphipods = {}

for y, line in enumerate(file):
height = max(y + 1, height)

for x, c in enumerate(line.rstrip('\n')):
width = max(x + 1, width)

p = Point(x, y)

if c == '#':

elif c.isalpha():
amphipods.setdefault(c, set())

return State(
width,
height,
frozenset(walls),
frozendict({
c: frozenset(points)
for c, points in amphipods.items()
})
)

def __getitem__(self, p: Union[Tuple[int, int], Point]):
if isinstance(p, tuple):
x, y = p
p = Point(x, y)

if p in self.walls:
return '#'

for c, points in self.amphipods.items():
if p in points:
return c

return ' '

def __str__(self):
map_data = []

for y in range(self.height):
for x in range(self.width):
map_data.append(self[x, y])
map_data.append('\n')

return f'State<{self.width}x{self.height}>{{\n{"".join(map_data)}}}'


I’m eventually going to use A* again, so in order to do that, I have to make sure everything is immutable and can be hashed, thus the frozenset and frozendict everywhere. I wish that were more elegant.

Next up, we want the ability to generate a list of moves from a given state. That is, check every amphipod and generate every move they could make following the three rules:

@dataclass(frozen=True)
class State:
...

def moves(self) -> Generator[Tuple[int, 'State'], None, None]:
'''Generate the possible next states that we can get into from here.'''

logging.debug('Generating moves')

for c, points in self.amphipods.items():
logging.debug(f'- Generating moves for {c=}')

for p in points:
logging.debug(f'-- Generating moves for {c=} @ {p=}')

# Move amphipods from rooms to the hallway
# Blocked amphipods won't floodfill, so this is fine
if p.y >= 2:
logging.debug('-- Dealing with a room amphipod')
for new_p in self.floodfill(p):
# An amphipod will always stop in the hallway
if new_p.y != 1:
continue

# ORIGINAL (WRONG) CONDITION:
# # An amphipod will not stop immediately outside of the room they came from
# if new_p.x == p.x:
#    continue

# An amphipod will not stop immediately outside of any room
if new_p.x in TARGET_ROOMS.values():
continue

# Otherwise, yield this as a possibility
yield (p.distance(new_p) * ENERGY_COSTS[c], self.move(p, new_p))

# An amphipod in the hallway will only go into a room
# + only it's correct room
# + only if there are no non-similar amphipods beneath it
elif p.y == 1:
logging.debug('-- Dealing with a hallway amphipod')
for new_p in self.floodfill(p):
# An amphipod will not stay in the hallway once they are in it
if new_p.y == 1:
continue

# An amphipod will only got into it's own room
if new_p.x != TARGET_ROOMS[c]:
continue

# An amphipod will not enter a room that has other kinds of amphipods in it
if new_p.y >= 2 and any(
self[new_p.x, y] not in (c, ' ')
for y in range(2, self.height - 1)
):
continue

# Otherwise, yield this as a possibility
yield (p.distance(new_p) * ENERGY_COSTS[c], self.move(p, new_p))

# This shouldn't happen...
else:
logging.critical(f'Confused amphipod: {c} @ {p}')


That’s the simulation in a nutshell, although we do need a two helper functions of course:

class State:
...

def floodfill(self, origin: Point) -> Generator[Point, None, None]:
'''Generate all points reachable from a given location.'''

scanned = set()
to_scan = queue.Queue()
to_scan.put(origin)

while not to_scan.empty():
p = to_scan.get_nowait()

if p in scanned:
continue

if p != origin:
yield p

for xd, yd in NEIGHBORS:
new_p = Point(p.x + xd, p.y + yd)

if self[new_p] == ' ':
to_scan.put(new_p)

def move(self, src: Point, dst: Point) -> 'State':
'''Return a new state with the given amphipod moved from src to dst'''

for delta_c, points in self.amphipods.items():
if src in points:
break

return State(
self.width,
self.height,
self.walls,
frozendict({
c: points if c != delta_c else points - {src} | {dst}
for c, points in self.amphipods.items()
})
)


It’s all immutable, so generate a new state for move and scan only empty spaces for floodfill. As seen in moves, we deal with the actual rules there, floodfill just needs to generate spaces that are connected and empty.

So that’s all we have:

@app.command()
def main(initial_file: typer.FileText, goal_file: typer.FileText):

q = queue.PriorityQueue()
q.put((0, initial))

visited = set()

i = 0
while not q.empty():
energy, state = q.get_nowait()

i += 1
if i % 10000 == 0:
logging.info(
f'[{i=}, qsize={q.qsize()}] '
f'{energy=} {str(state)}'
)

if state in visited:
continue
else:

if state == goal:
break

for new_energy, new_state in state.moves():
q.put((energy + new_energy, new_state))

print('Final solution:')
print(state)
print('states examined:', i)
print(energy)


For now, all we’re doing is sorting states by energy expended and trying all states until we find one that works. And it works pretty well:

$python3 amphipodinator.py main input1.txt goal1.txt Final solution: State<13x5>{ ############# # # ###A#B#C#D### #A#B#C#D# ######### } states examined: 896123 11608 # time 99305909917ns / 99.31s  But… I feel like we can do better. Remember how I mentioned A*? Let’s write a heuristic function: class State: ... def heuristic(self) -> int: '''Guess how many movies it will take to get to the final state.''' # For any amphis not in the target room, calculate a guess cost to get there # This will go to the hallway and into their room ignoring everything in their way return sum( ( p.y - 1 # To the hallway, 0 if in hallway already + abs(p.x - TARGET_ROOMS[c]) # Along the hallway the quickest way + 1 # Into the room (first spot, underestimates) ) * ENERGY_COSTS[c] for c, points in self.amphipods.items() for p in points if p.x != TARGET_ROOMS[c] )  Because the amphipods can just run right through one another, this will never overestimate (which is required to guarantee an optimal solution from A*), but it should help. Add it to the solver: @app.command() def main(initial_file: typer.FileText, goal_file: typer.FileText, heuristic: bool = False): initial = State.read(initial_file) goal = State.read(goal_file) q = queue.PriorityQueue() q.put((0, 0, initial)) visited = set() i = 0 while not q.empty(): heuristic_score, energy, state = q.get_nowait() i += 1 if i % 10000 == 0: logging.info( f'[{i=}, qsize={q.qsize()}] ' f'heuristic={heuristic_score if heuristic else "disabled"} ' f'{energy=} {str(state)}' ) if state in visited: continue else: visited.add(state) if state == goal: break for new_energy, new_state in state.moves(): if heuristic: heuristic_score = energy + new_energy + new_state.heuristic() else: heuristic_score = 0 q.put((heuristic_score, energy + new_energy, new_state)) print('Final solution:') print(state) print('states examined:', i) print(energy)  I like how the state doesn’t care at all, it’s all in the wrapping algorithm that uses it. So run it: $ python3 amphipodinator.py --heuristic main input1.txt goal1.txt
Final solution:
State<13x5>{
#############
#           #
###A#B#C#D###
#A#B#C#D#
#########
}
states examined: 156876
11608
# time 18045001792ns / 18.05s


Nice! It examined 150k versus 900k states (6x fewer) and ran 5x as fast! That will help with part 2 I’m sure. :D

#### Part 2: Add two more (given) amphipods to each room, solve for least energy.

Yup, it’s just bigger and more complicated. Nothing else changes:

\$ python3 amphipodinator.py --heuristic main input2.txt goal2.txt
Final solution:
State<13x7>{
#############
#           #
###A#B#C#D###
#A#B#C#D#
#A#B#C#D#
#A#B#C#D#
#########
}
states examined: 784637
46754
# time 158144712250ns / 158.14s


Nice! And it only had just shy of 800k states total. I know without the heuristic it would have been… somewhat more than that.

One interesting note. I originally read the rules differently. I thought that:

• amphipods would go as deep into their room as possible and stay there (this is wrong, they can go into their room and come back out if that’s more efficient)
• amphipods could stop in front of each other’s rooms, just not their own

In both cases, I still got an answer, it just wasn’t as efficient as the problem wanted. Turns out it’s better for A to be able to go in and out of their room to get out of the rest’s ways. It didn’t matter in part 1 (other than making it a hair quicker), but it did for part 2!

A neat problem. I like how relatively elegant the rules can be expressed.