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66 lines
2.7 KiB
66 lines
2.7 KiB
1 year ago
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# Day 20: Grove Positioning System
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[https://adventofcode.com/2022/day/20](https://adventofcode.com/2022/day/20)
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## Description
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### Part One
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It's finally time to meet back up with the Elves. When you try to contact them, however, you get no reply. Perhaps you're out of range?
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You know they're headed to the grove where the _star_ fruit grows, so if you can figure out where that is, you should be able to meet back up with them.
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Fortunately, your handheld device has a file (your puzzle input) that contains the grove's coordinates! Unfortunately, the file is _encrypted_ - just in case the device were to fall into the wrong hands.
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Maybe you can <span title="You once again make a mental note to remind the Elves later not to invent their own cryptographic functions.">decrypt</span> it?
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When you were still back at the camp, you overheard some Elves talking about coordinate file encryption. The main operation involved in decrypting the file is called _mixing_.
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The encrypted file is a list of numbers. To _mix_ the file, move each number forward or backward in the file a number of positions equal to the value of the number being moved. The list is _circular_, so moving a number off one end of the list wraps back around to the other end as if the ends were connected.
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For example, to move the `1` in a sequence like `4, 5, 6, 1, 7, 8, 9`, the `1` moves one position forward: `4, 5, 6, 7, 1, 8, 9`. To move the `-2` in a sequence like `4, -2, 5, 6, 7, 8, 9`, the `-2` moves two positions backward, wrapping around: `4, 5, 6, 7, 8, -2, 9`.
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The numbers should be moved _in the order they originally appear_ in the encrypted file. Numbers moving around during the mixing process do not change the order in which the numbers are moved.
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Consider this encrypted file:
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1
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2
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-3
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3
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-2
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0
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4
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Mixing this file proceeds as follows:
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Initial arrangement:
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1, 2, -3, 3, -2, 0, 4
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1 moves between 2 and -3:
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2, 1, -3, 3, -2, 0, 4
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2 moves between -3 and 3:
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1, -3, 2, 3, -2, 0, 4
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-3 moves between -2 and 0:
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1, 2, 3, -2, -3, 0, 4
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3 moves between 0 and 4:
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1, 2, -2, -3, 0, 3, 4
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-2 moves between 4 and 1:
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1, 2, -3, 0, 3, 4, -2
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0 does not move:
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1, 2, -3, 0, 3, 4, -2
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4 moves between -3 and 0:
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1, 2, -3, 4, 0, 3, -2
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Then, the grove coordinates can be found by looking at the 1000th, 2000th, and 3000th numbers after the value `0`, wrapping around the list as necessary. In the above example, the 1000th number after `0` is _`4`_, the 2000th is _`-3`_, and the 3000th is _`2`_; adding these together produces _`3`_.
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Mix your encrypted file exactly once. _What is the sum of the three numbers that form the grove coordinates?_
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