49 lines
1.6 KiB
Python
Executable file
49 lines
1.6 KiB
Python
Executable file
#!/usr/bin/env python
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import collections
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import sys
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def solve(input: str) -> int:
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def parse(input: str) -> tuple[dict[str, list[str]], str]:
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raw_replacements, molecule = input.strip().split("\n\n")
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res: dict[str, list[str]] = collections.defaultdict(list)
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for start, end in (
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line.split(" => ") for line in raw_replacements.splitlines()
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):
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res[start].append(end)
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return res, molecule
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def inverse(input: dict[str, list[str]]) -> dict[str, str]:
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res: dict[str, str] = {}
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for key, vals in input.items():
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for val in vals:
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res[val] = key
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assert len(res) == sum(map(len, input.values())) # Sanity check
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return res
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def build(replacements: dict[str, list[str]], molecule: str) -> int:
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# We will be "building down" to the electron, so inverse the replacement mapping
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inversed_replacements = inverse(replacements)
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# Order the replacement needles to try the longest ones first
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needles = sorted(inversed_replacements.keys(), key=len, reverse=True)
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res = 0
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while molecule != "e":
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for needle in needles:
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if needle not in molecule:
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continue
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res += 1
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molecule = molecule.replace(needle, inversed_replacements[needle], 1)
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return res
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replacements, molecule = parse(input)
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return build(replacements, molecule)
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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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