My Advent of Code solutions in Python.
kevinyap.ca/2019/12/going-fast-in-advent-of-code/
advent-of-code
python
Add solution for 2024/20
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2024/day20.py
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2024/day20.py
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import fileinput
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from collections import deque, Counter
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from itertools import permutations
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from utils import Point
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def bfs(board, start, end):
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horizon = deque([(start, 0)])
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seen = {}
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while horizon:
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pos, dist = horizon.popleft()
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if pos in seen:
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continue
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seen[pos] = dist
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if pos == END:
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return seen
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for n in pos.neighbours():
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if BOARD.get(n) == '#' or n not in BOARD:
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continue
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horizon.append((n, dist + 1))
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def num_cheat_paths(distances, max_cheat_dist):
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cheat_saves = Counter()
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for start, end in permutations(distances, 2):
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if distances[end] < distances[start]:
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continue
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cheat_dist = abs(start.dist_manhattan(end))
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if cheat_dist > max_cheat_dist:
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continue
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saved = distances[end] - distances[start] - cheat_dist
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cheat_saves[saved] += 1
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num_cheats = 0
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for cheat_dist, count in cheat_saves.items():
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if cheat_dist >= 100:
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num_cheats += count
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return num_cheats
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# Read problem input.
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BOARD = {}
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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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p = Point(x, y)
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if c == '#':
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BOARD[p] = '#'
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else:
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BOARD[p] = '.'
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if c == 'S':
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START = p
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elif c == 'E':
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END = p
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# Solve problem.
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distances = bfs(BOARD, START, END)
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print("Part 1:", num_cheat_paths(distances, 2))
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print("Part 2:", num_cheat_paths(distances, 20))