87 lines
3.2 KiB
Python
87 lines
3.2 KiB
Python
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"""
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--- Day 6: Tuning Trouble ---
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The preparations are finally complete; you and the Elves leave camp on
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foot and begin to make your way toward the star fruit grove.
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As you move through the dense undergrowth, one of the Elves gives you a
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handheld device. He says that it has many fancy features, but the most
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important one to set up right now is the communication system.
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However, because he's heard you have significant experience dealing with
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signal-based systems, he convinced the other Elves that it would be okay
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to give you their one malfunctioning device - surely you'll have no
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problem fixing it.
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As if inspired by comedic timing, the device emits a few colorful sparks.
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To be able to communicate with the Elves, the device needs to lock on
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to their signal. The signal is a series of seemingly-random characters
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that the device receives one at a time.
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To fix the communication system, you need to add a subroutine to the
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device that detects a start-of-packet marker in the datastream. In the
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protocol being used by the Elves, the start of a packet is indicated by
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a sequence of four characters that are all different.
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The device will send your subroutine a datastream buffer (your puzzle
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input); your subroutine needs to identify the first position where the
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four most recently received characters were all different. Specifically,
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it needs to report the number of characters from the beginning of the
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buffer to the end of the first such four-character marker.
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For example, suppose you receive the following datastream buffer:
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mjqjpqmgbljsphdztnvjfqwrcgsmlb
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After the first three characters (mjq) have been received, there haven't
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been enough characters received yet to find the marker. The first time a
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marker could occur is after the fourth character is received, making the
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most recent four characters mjqj. Because j is repeated, this isn't a
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marker.
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The first time a marker appears is after the seventh character arrives.
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Once it does, the last four characters received are jpqm, which are all
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different. In this case, your subroutine should report the value 7,
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because the first start-of-packet marker is complete after 7 characters
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have been processed.
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Here are a few more examples:
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bvwbjplbgvbhsrlpgdmjqwftvncz: first marker after character 5
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nppdvjthqldpwncqszvftbrmjlhg: first marker after character 6
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nznrnfrfntjfmvfwmzdfjlvtqnbhcprsg: first marker after character 10
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zcfzfwzzqfrljwzlrfnpqdbhtmscgvjw: first marker after character 11
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How many characters need to be processed before the first
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start-of-packet marker is detected?
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"""
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def main():
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with open("input.txt", "r", encoding="utf-8") as input_file:
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data_stream = input_file.read()
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read_chars = []
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for index, char in enumerate(data_stream):
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read_chars.append(char)
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# We need 4 unique characters, so as long as less than 4 chars
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# were read, just continue loading characters.
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if len(read_chars) < 4:
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continue
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# We only need the last 4 characters to compare, so if we load
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# a fifth char we can pop the oldest out of the list.
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if len(read_chars) == 5:
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read_chars.pop(0)
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if len(set(read_chars)) == 4:
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print(f"After {index+1} chars")
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break
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if __name__ == "__main__":
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main()
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