43 lines
1.2 KiB
Python
43 lines
1.2 KiB
Python
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import math
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import sys
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from functools import reduce
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from itertools import combinations
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def main():
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lines = [line.rstrip() for line in sys.stdin]
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earliest = int(lines[0])
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ids = map(int, filter(lambda s: s != "x", lines[1].split(",")))
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# Part 1
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wait_times = {i: i - (earliest % i) for i in ids}
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best_id, wait = min(wait_times.items(), key=lambda item: item[1])
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print("Part 1:", best_id * wait)
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# Part 2
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# math shit
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# https://en.wikipedia.org/wiki/Chinese_remainder_theorem wtf is going on
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ids = []
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rems = []
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for i, v in enumerate(lines[1].split(",")):
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if v != "x":
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# import hint: you want to use negative indices for remainders to
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# use it with CRT here.
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rems.append(-i)
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ids.append(int(v))
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# are all bus IDs co-prime?
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assert all(math.gcd(a, b) == 1 for a, b in combinations(ids, 2))
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# CRT
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# https://www.youtube.com/watch?v=zIFehsBHB8o
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N = reduce(lambda a, b: a * b, ids, 1)
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x = 0
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for ni, ai in zip(ids, rems):
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Ni = N // ni
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x += ai * Ni * pow(Ni, -1, ni)
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x %= N
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print("Part 2:", x)
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if __name__ == "__main__":
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main()
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