59 lines
3.4 KiB
Text
59 lines
3.4 KiB
Text
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\--- Day 17: Spinlock ---
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Suddenly, whirling in the distance, you notice what looks like a massive, pixelated hurricane: a deadly [spinlock](https://en.wikipedia.org/wiki/Spinlock). This spinlock isn't just consuming computing power, but memory, too; vast, digital mountains are being ripped from the ground and consumed by the vortex.
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If you don't move quickly, fixing that printer will be the least of your problems.
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This spinlock's algorithm is simple but efficient, quickly consuming everything in its path. It starts with a circular buffer containing only the value `0`, which it marks as the *current position*. It then steps forward through the circular buffer some number of steps (your puzzle input) before inserting the first new value, `1`, after the value it stopped on. The inserted value becomes the *current position*. Then, it steps forward from there the same number of steps, and wherever it stops, inserts after it the second new value, `2`, and uses that as the new *current position* again.
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It repeats this process of *stepping forward*, *inserting a new value*, and *using the location of the inserted value as the new current position* a total of `*2017*` times, inserting `2017` as its final operation, and ending with a total of `2018` values (including `0`) in the circular buffer.
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For example, if the spinlock were to step `3` times per insert, the circular buffer would begin to evolve like this (using parentheses to mark the current position after each iteration of the algorithm):
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* `(0)`, the initial state before any insertions.
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* `0 (1)`: the spinlock steps forward three times (`0`, `0`, `0`), and then inserts the first value, `1`, after it. `1` becomes the current position.
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* `0 (2) 1`: the spinlock steps forward three times (`0`, `1`, `0`), and then inserts the second value, `2`, after it. `2` becomes the current position.
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* `0 2 (3) 1`: the spinlock steps forward three times (`1`, `0`, `2`), and then inserts the third value, `3`, after it. `3` becomes the current position.
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And so on:
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* `0 2 (4) 3 1`
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* `0 (5) 2 4 3 1`
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* `0 5 2 4 3 (6) 1`
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* `0 5 (7) 2 4 3 6 1`
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* `0 5 7 2 4 3 (8) 6 1`
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* `0 (9) 5 7 2 4 3 8 6 1`
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Eventually, after 2017 insertions, the section of the circular buffer near the last insertion looks like this:
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```
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1512 1134 151 (2017) 638 1513 851
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```
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Perhaps, if you can identify the value that will ultimately be *after* the last value written (`2017`), you can short-circuit the spinlock. In this example, that would be `638`.
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*What is the value after `2017`* in your completed circular buffer?
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Your puzzle answer was `1244`.
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\--- Part Two ---
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----------
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The spinlock does not short-circuit. Instead, it gets *more* angry. At least, you assume that's what happened; it's spinning significantly faster than it was a moment ago.
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You have good news and bad news.
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The good news is that you have improved calculations for how to stop the spinlock. They indicate that you actually need to identify *the value after `0`* in the current state of the circular buffer.
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The bad news is that while you were determining this, the spinlock has just finished inserting its fifty millionth value (`50000000`).
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*What is the value after `0`* the moment `50000000` is inserted?
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Your puzzle answer was `11162912`.
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Both parts of this puzzle are complete! They provide two gold stars: \*\*
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At this point, all that is left is for you to [admire your Advent calendar](/2017).
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Your puzzle input was `370`.
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