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115 lines
3.2 KiB
Python
115 lines
3.2 KiB
Python
import functools
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import math
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from typing import Dict
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def main(filename: str, expected_part_1: int = None, expected_part_2: int = None):
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print(f"\n+ Running on {filename}")
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with open(filename) as f:
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notes = f.read().strip().split("\n")
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counter_part_1 = solve_part_1(notes)
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print(f"1. Found {counter_part_1}")
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if expected_part_1:
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assert expected_part_1 == counter_part_1
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counter_part_2 = solve_part_2(notes)
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print(f"2. Found {counter_part_2}")
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if expected_part_2:
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assert expected_part_2 == counter_part_2
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def solve_part_1(notes) -> int:
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earliest = int(notes[0])
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buses = map(int, filter(lambda bus: bus != "x", notes[1].split(",")))
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min_delta = math.inf
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min_bus = None
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for bus in buses:
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delta = math.ceil(earliest / bus) * bus - earliest
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if delta < min_delta:
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min_delta = delta
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min_bus = bus
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return min_bus * min_delta
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def solve_part_2(notes) -> int:
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split = notes[1].split(",")
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modulos = []
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remainders = []
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for position, bus_index in enumerate(split):
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if bus_index == "x":
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continue
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bus_index = int(bus_index)
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modulos.append(bus_index)
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remainders.append(bus_index - position)
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return chinese_remainder(modulos, remainders)
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def chinese_remainder(modulos, remainders):
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"""List of modulos, then list of remainders"""
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# https://rosettacode.org/wiki/Chinese_remainder_theorem#Python
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# License: https://www.gnu.org/licenses/old-licenses/fdl-1.2.html
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total = 0
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prod = functools.reduce(lambda a, b: a * b, modulos)
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for modulo, remainder in zip(modulos, remainders):
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p = prod // modulo
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total += remainder * mul_inv(p, modulo) * p
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return total % prod
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def mul_inv(a, b):
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# https://rosettacode.org/wiki/Chinese_remainder_theorem#Python
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# License: https://www.gnu.org/licenses/old-licenses/fdl-1.2.html
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b0 = b
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x0, x1 = 0, 1
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if b == 1:
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return 1
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while a > 1:
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q = a // b
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a, b = b, a % b
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x0, x1 = x1 - q * x0, x0
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if x1 < 0:
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x1 += b0
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return x1
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if __name__ == "__main__":
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n = [3, 5, 7]
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a = [2, 3, 2]
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print(chinese_remainder(n, a))
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def solve_part_2_iterative(notes) -> int:
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"""Works but far too slow for the real deal."""
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split = notes[1].split(",")
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buses = list(map(int, filter(lambda bus: bus != "x", split)))
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minutes = {} # type: Dict[int, int]
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biggest_bus = max(buses)
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biggest_bus_index = split.index(str(biggest_bus))
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for bus in buses:
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minutes[split.index(str(bus)) - biggest_bus_index] = bus
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found = False
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timestamp = 99999999999628
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while not found:
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timestamp += biggest_bus
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found = True
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for delta, bus in minutes.items():
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if (timestamp + delta) % bus != 0:
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found = False
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break
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return timestamp + min(minutes.keys())
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if __name__ == "__main__":
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main("inputs/day13-test1", 295, 1068781)
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main("inputs/day13-test2", None, 3417)
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main("inputs/day13-test3", None, 754018)
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main("inputs/day13-test4", None, 779210)
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main("inputs/day13-test5", None, 1261476)
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main("inputs/day13-test6", None, 1202161486)
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main("inputs/day13", 4315, 556100168221141)
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