150 lines
4.1 KiB
Python
150 lines
4.1 KiB
Python
import sys
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from itertools import combinations
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# --- Day 9: Encoding Error ---
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# With your neighbor happily enjoying their video game, you turn your attention
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# to an open data port on the little screen in the seat in front of you.
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# Though the port is non-standard, you manage to connect it to your computer
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# through the clever use of several paperclips. Upon connection, the port
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# outputs a series of numbers (your puzzle input).
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# The data appears to be encrypted with the eXchange-Masking Addition System
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# (XMAS) which, conveniently for you, is an old cypher with an important
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# weakness.
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# XMAS starts by transmitting a preamble of 25 numbers. After that, each number
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# you receive should be the sum of any two of the 25 immediately previous
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# numbers. The two numbers will have different values, and there might be more
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# than one such pair.
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# For example, suppose your preamble consists of the numbers 1 through 25 in a
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# random order. To be valid, the next number must be the sum of two of those
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# numbers:
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# 26 would be a valid next number, as it could be 1 plus 25 (or many other
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# pairs, like 2 and 24).
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# 49 would be a valid next number, as it is the sum of 24 and 25.
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# 100 would not be valid; no two of the previous 25 numbers sum to 100.
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# 50 would also not be valid; although 25 appears in the previous 25
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# numbers, the two numbers in the pair must be different.
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# Suppose the 26th number is 45, and the first number (no longer an option, as
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# it is more than 25 numbers ago) was 20. Now, for the next number to be valid,
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# there needs to be some pair of numbers among 1-19, 21-25, or 45 that add up
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# to it:
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# 26 would still be a valid next number, as 1 and 25 are still within the
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# previous 25 numbers.
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# 65 would not be valid, as no two of the available numbers sum to it.
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# 64 and 66 would both be valid, as they are the result of 19+45 and 21+45
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# respectively.
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# Here is a larger example which only considers the previous 5 numbers (and has
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# a preamble of length 5):
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# 35
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# 20
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# 15
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# 25
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# 47
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# 40
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# 62
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# 55
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# 65
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# 95
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# 102
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# 117
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# 150
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# 182
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# 127
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# 219
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# 299
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# 277
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# 309
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# 576
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# In this example, after the 5-number preamble, almost every number is the sum
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# of two of the previous 5 numbers; the only number that does not follow this
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# rule is 127.
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# The first step of attacking the weakness in the XMAS data is to find the
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# first number in the list (after the preamble) which is not the sum of two of
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# the 25 numbers before it. What is the first number that does not have this
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# property?
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with open("files/P9.txt", "r") as f:
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numbers = [int(num) for num in f.read().strip().split("\n")]
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def part_1() -> int:
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window = 25
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for i in range(window, len(numbers)):
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if all(
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[
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x + y != numbers[i]
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for x, y in combinations(numbers[i - window : i], 2)
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]
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):
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break
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print(f"The first number without the property is {numbers[i]}")
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return numbers[i]
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# --- Part Two ---
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# The final step in breaking the XMAS encryption relies on the invalid number
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# you just found: you must find a contiguous set of at least two numbers in
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# your list which sum to the invalid number from step 1.
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# Again consider the above example:
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# 35
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# 20
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# 15
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# 25
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# 47
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# 40
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# 62
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# 55
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# 65
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# 95
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# 102
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# 117
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# 150
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# 182
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# 127
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# 219
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# 299
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# 277
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# 309
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# 576
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# In this list, adding up all of the numbers from 15 through 40 produces the
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# invalid number from step 1, 127. (Of course, the contiguous set of numbers in
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# your actual list might be much longer.)
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# To find the encryption weakness, add together the smallest and largest number
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# in this contiguous range; in this example, these are 15 and 47, producing 62.
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# What is the encryption weakness in your XMAS-encrypted list of numbers?
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def part_2() -> None:
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target = invalid_number
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for i in range(len(numbers)):
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for j in range(i + 1, len(numbers)):
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if sum(numbers[i:j]) == target:
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print(
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f"The encryption weakness is {min(numbers[i:j]) + max(numbers[i:j])}"
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)
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return
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if __name__ == "__main__":
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invalid_number = part_1()
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part_2()
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