98 lines
4.0 KiB
Plaintext
98 lines
4.0 KiB
Plaintext
Advent of Code
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--- Day 1: Inverse Captcha ---
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The night before Christmas, one of Santa's Elves calls you in a panic. "The
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printer's broken! We can't print the Naughty or Nice List!" By the time you
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make it to sub-basement 17, there are only a few minutes until midnight. "We
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have a big problem," she says; "there must be almost fifty bugs in this
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system, but nothing else can print The List. Stand in this square, quick!
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There's no time to explain; if you can convince them to pay you in stars,
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you'll be able to--" She pulls a lever and the world goes blurry.
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When your eyes can focus again, everything seems a lot more pixelated than
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before. She must have sent you inside the computer! You check the system
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clock: 25 milliseconds until midnight. With that much time, you should be
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able to collect all fifty stars by December 25th.
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Collect stars by solving puzzles. Two puzzles will be made available on each
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day millisecond in the advent calendar; the second puzzle is unlocked when
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you complete the first. Each puzzle grants one star. Good luck!
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You're standing in a room with "digitization quarantine" written in LEDs
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along one wall. The only door is locked, but it includes a small interface.
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"Restricted Area - Strictly No Digitized Users Allowed."
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It goes on to explain that you may only leave by solving a captcha to prove
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you're not a human. Apparently, you only get one millisecond to solve the
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captcha: too fast for a normal human, but it feels like hours to you.
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The captcha requires you to review a sequence of digits (your puzzle input)
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and find the sum of all digits that match the next digit in the list. The
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list is circular, so the digit after the last digit is the first digit in
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the list.
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For example:
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• 1122 produces a sum of 3 (1 + 2) because the first digit (1) matches the
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second digit and the third digit (2) matches the fourth digit.
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• 1111 produces 4 because each digit (all 1) matches the next.
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• 1234 produces 0 because no digit matches the next.
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• 91212129 produces 9 because the only digit that matches the next one is
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the last digit, 9.
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What is the solution to your captcha?
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Your puzzle answer was 1144.
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--- Part Two ---
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You notice a progress bar that jumps to 50% completion. Apparently, the door
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isn't yet satisfied, but it did emit a star as encouragement. The
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instructions change:
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Now, instead of considering the next digit, it wants you to consider the
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digit halfway around the circular list. That is, if your list contains 10
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items, only include a digit in your sum if the digit 10/2 = 5 steps forward
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matches it. Fortunately, your list has an even number of elements.
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For example:
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• 1212 produces 6: the list contains 4 items, and all four digits match
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the digit 2 items ahead.
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• 1221 produces 0, because every comparison is between a 1 and a 2.
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• 123425 produces 4, because both 2s match each other, but no other digit
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has a match.
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• 123123 produces 12.
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• 12131415 produces 4.
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What is the solution to your new captcha?
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Your puzzle answer was 1194.
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Both parts of this puzzle are complete! They provide two gold stars: **
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At this point, you should return to your advent calendar and try another
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puzzle.
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If you still want to see it, you can get your puzzle input.
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References
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Visible links
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. http://adventofcode.com/
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. http://adventofcode.com/2017/about
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. http://adventofcode.com/2017/support
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. http://adventofcode.com/2017/events
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. http://adventofcode.com/2017/settings
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. http://adventofcode.com/2017/auth/logout
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. http://adventofcode.com/2017
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. http://adventofcode.com/2017
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. http://adventofcode.com/2017/leaderboard
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. http://adventofcode.com/2017/stats
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. http://adventofcode.com/2017/sponsors
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. http://adventofcode.com/2017/sponsors
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. https://en.wikipedia.org/wiki/CAPTCHA
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. http://adventofcode.com/2017
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. http://adventofcode.com/2017/day/1/input
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