86 lines
3.5 KiB
Markdown
86 lines
3.5 KiB
Markdown
## \-\-- Day 1: Inverse Captcha \-\--
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The night before Christmas, one of Santa\'s Elves calls you in a panic.
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\"The printer\'s broken! We can\'t print the *Naughty or Nice List*!\"
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By the time you make it to [sub-basement
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17]{title="Floor 17: cafeteria, printing department, and experimental organic digitization equipment."},
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there are only a few minutes until midnight. \"We have a big problem,\"
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she says; \"there must be almost *fifty* bugs in this system, but
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nothing else can print The List. Stand in this square, quick! There\'s
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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
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than before. She must have sent you inside the computer! You check the
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system clock: *25 milliseconds* until midnight. With that much time, you
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should be 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
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each ~~day~~ millisecond in the Advent calendar; the second puzzle is
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unlocked when you complete the first. Each puzzle grants *one star*.
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Good luck!
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You\'re standing in a room with \"digitization quarantine\" written in
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LEDs along one wall. The only door is locked, but it includes a small
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interface. \"Restricted Area - Strictly No Digitized Users Allowed.\"
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It goes on to explain that you may only leave by solving a
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[captcha](https://en.wikipedia.org/wiki/CAPTCHA) to prove you\'re *not*
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a human. Apparently, you only get one millisecond to solve the captcha:
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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
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input) and find the *sum* of all digits that match the *next* digit in
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the list. The list is circular, so the digit after the last digit is the
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*first* digit in the list.
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For example:
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- `1122` produces a sum of `3` (`1` + `2`) because the first digit
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(`1`) matches the second digit and the third digit (`2`) matches the
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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
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one is the last digit, `9`.
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*What is the solution* to your captcha?
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Your puzzle answer was `1158`.
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## \-\-- Part Two \-\-- {#part2}
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You notice a progress bar that jumps to 50% completion. Apparently, the
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door isn\'t yet satisfied, but it did emit a *star* as encouragement.
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The instructions change:
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Now, instead of considering the *next* digit, it wants you to consider
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the digit *halfway around* the circular list. That is, if your list
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contains `10` items, only include a digit in your sum if the digit
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`10/2 = 5` steps forward matches it. Fortunately, your list has an even
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number of elements.
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For example:
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- `1212` produces `6`: the list contains `4` items, and all four
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digits match the digit `2` items ahead.
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- `1221` produces `0`, because every comparison is between a `1` and a
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`2`.
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- `123425` produces `4`, because both `2`s match each other, but no
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other digit 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 `1132`.
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Both parts of this puzzle are complete! They provide two gold stars:
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\*\*
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At this point, you should [return to your Advent calendar](/2017) and
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try another puzzle.
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If you still want to see it, you can [get your puzzle
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input](1/input).
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