61 lines
1.9 KiB
Python
Executable file
61 lines
1.9 KiB
Python
Executable file
#!/usr/bin/env python
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import collections
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import copy
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import functools
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import sys
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Graph = dict[int, set[int]]
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def solve(input: str) -> int:
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def parse_ordering(input: str) -> Graph:
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graph: Graph = collections.defaultdict(set)
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for l in input.splitlines():
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lhs, rhs = map(int, l.split("|"))
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graph[lhs].add(rhs)
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return graph
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def parse_updates(input: str) -> list[list[int]]:
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return [[int(n) for n in line.split(",")] for line in input.splitlines()]
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def parse(input: str) -> tuple[Graph, list[list[int]]]:
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ordering, updates = input.split("\n\n")
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return parse_ordering(ordering), parse_updates(updates)
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def validate_update(ordering: Graph, update: list[int]) -> tuple[int, int] | None:
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for i in range(len(update)):
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for j in range(i + 1, len(update)):
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lhs, rhs = update[i], update[j]
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if lhs in ordering[rhs]:
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return i, j
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return None
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def get_middle(update: list[int]) -> int:
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assert len(update) % 2 == 1 # Sanity check
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return update[len(update) // 2]
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def fix_update(ordering: Graph, update: list[int]) -> list[int]:
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# The graph has cycles, so using a topological sort is out
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# Instead just swap the wrong pages until we fixed the update
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update = copy.copy(update)
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while (indices := validate_update(ordering, update)) is not None:
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lhs, rhs = indices
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update[lhs], update[rhs] = update[rhs], update[lhs]
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return update
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ordering, updates = parse(input)
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invalid_updates = filter(functools.partial(validate_update, ordering), updates)
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fixed_updates = map(functools.partial(fix_update, ordering), invalid_updates)
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return sum(get_middle(update) for update in fixed_updates)
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def main() -> None:
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input = sys.stdin.read()
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print(solve(input))
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if __name__ == "__main__":
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main()
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