My Advent of Code solutions in Python.
kevinyap.ca/2019/12/going-fast-in-advent-of-code/
advent-of-code
python
Add solution for 2024/12
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2024/day12.py
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2024/day12.py
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import fileinput
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from collections import defaultdict, deque
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from utils import Point, DIRS, N, S, E, W
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def process_plot(garden, start, visited):
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"""
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Walk through the garden using BFS from `start`, only
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visiting the same plant as present in `garden[start]`.
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Write seen locations back to `visited` so that we can
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keep track of which garden plots have already been seen.
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Returns the area, perimeter, and side count of the plot.
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"""
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plot = set()
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horizon = deque([start])
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plant = garden.get(start)
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while horizon:
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node = horizon.popleft()
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if node in plot:
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continue
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plot.add(node)
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visited.add(node)
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for n in node.neighbours():
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if n in garden and garden.get(n) == plant:
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horizon.append(n)
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# The area of the plot is simply how many locations we visited.
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area = len(plot)
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# Compute the perimeter by taking every plant location in the
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# plot, and counting how many neighbouring locations are *not*
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# also in the plot. Each of those locations contributes 1 unit
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# length to the overall perimeter of the plot.
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perimeter = sum(len(set(p.neighbours()) - plot) for p in plot)
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# Compute the number of sides. This one is a bit tricky.
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# Walk every row in the garden. For each position that is part
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# of this plot, look to see if there is a non-plot location above
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# and below it (similar to the perimeter calculation). Add this
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# to a set of "potential sides". We will need to eliminate extra
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# sides in a future step.
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row_sides = {
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N: defaultdict(set),
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S: defaultdict(set),
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}
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for y in range(-2, 150):
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for d in [N, S]:
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for x in range(-2, 150):
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p = Point(x, y)
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if p in plot and p + d not in plot:
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row_sides[d][y].add(p)
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# Do the same for every column in the garden, looking left/right.
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col_sides = {
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E: defaultdict(set),
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W: defaultdict(set)
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}
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for x in range(-2, 150):
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for d in [E, W]:
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for y in range(-2, 150):
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p = Point(x, y)
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if p in plot and p + d not in plot:
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col_sides[d][x].add(p)
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# Do another horizonal/vertical sweep of the garden. When we
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# "enter" a position that is part of a "side", increment the
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# side count. Continue to walk until we are not in that side
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# anymore. At this point, the polygon has turned or something,
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# or we might be passing over a "break" (imagine walking down
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# the horizontal legs of the letter "E" vertically). Don't start
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# counting another side until we reach an in-plot location again.
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sides = 0
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for d in [N, S]:
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for y in range(-2, 150):
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x = -2
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inside = False
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while x < 150:
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p = Point(x, y)
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if p in row_sides[d][y]:
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if not inside:
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sides += 1
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inside = True
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else:
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if inside:
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inside = False
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x += 1
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for d in [E, W]:
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for x in range(-2, 150):
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y = -2
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inside = False
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while y < 150:
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p = Point(x, y)
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if p in col_sides[d][x]:
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if not inside:
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sides += 1
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inside = True
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else:
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if inside:
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# print("exit inside")
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inside = False
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y += 1
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return area, perimeter, sides
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# Parse problem input.
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GARDEN = {}
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for y, line in enumerate(fileinput.input()):
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for x, c in enumerate(line.strip()):
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GARDEN[Point(x, y)] = c
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# Solve problem.
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part_1 = 0
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part_2 = 0
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visited = set()
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for pos in GARDEN:
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if pos not in visited:
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area, perim, sides = process_plot(GARDEN, pos, visited)
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part_1 += area * perim
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part_2 += area * sides
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print("Part 1:", part_1)
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print("Part 2:", part_2)