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<!DOCTYPE html>
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<title>Day 17 - Advent of Code 2017</title>
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Oh, hello! Funny seeing you here.
I appreciate your enthusiasm, but you aren't going to find much down here.
There certainly aren't clues to any of the puzzles. The best surprises don't
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Please be careful with automated requests; I'm not a massive company, and I can
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built the whole thing myself, including the design, animations, prose, and all
of the puzzles.
The puzzles are most of the work; preparing a new calendar and a new set of
puzzles each year takes all of my free time for 4-5 months. A lot of effort
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If you'd like to hang out, I'm @was.tl on Bluesky, @[email protected] on
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- Eric Wastl
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<article class="day-desc"><h2>--- Day 17: Spinlock ---</h2><p>Suddenly, whirling in the distance, you notice what looks like a massive, <span title="You know, as opposed to all those non-pixelated hurricanes you see on TV.">pixelated hurricane</span>: a deadly <a href="https://en.wikipedia.org/wiki/Spinlock">spinlock</a>. 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.</p>
<p>If you don't move quickly, fixing that printer will be the least of your problems.</p>
<p>This spinlock's algorithm is simple but efficient, quickly consuming everything in its path. It starts with a circular buffer containing only the value <code>0</code>, which it marks as the <em>current position</em>. It then steps forward through the circular buffer some number of steps (your puzzle input) before inserting the first new value, <code>1</code>, after the value it stopped on. The inserted value becomes the <em>current position</em>. Then, it steps forward from there the same number of steps, and wherever it stops, inserts after it the second new value, <code>2</code>, and uses that as the new <em>current position</em> again.</p>
<p>It repeats this process of <em>stepping forward</em>, <em>inserting a new value</em>, and <em>using the location of the inserted value as the new current position</em> a total of <code><em>2017</em></code> times, inserting <code>2017</code> as its final operation, and ending with a total of <code>2018</code> values (including <code>0</code>) in the circular buffer.</p>
<p>For example, if the spinlock were to step <code>3</code> 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):</p>
<ul>
<li><code>(0)</code>, the initial state before any insertions.</li>
<li><code>0 (1)</code>: the spinlock steps forward three times (<code>0</code>, <code>0</code>, <code>0</code>), and then inserts the first value, <code>1</code>, after it. <code>1</code> becomes the current position.</li>
<li><code>0 (2) 1</code>: the spinlock steps forward three times (<code>0</code>, <code>1</code>, <code>0</code>), and then inserts the second value, <code>2</code>, after it. <code>2</code> becomes the current position.</li>
<li><code>0 2 (3) 1</code>: the spinlock steps forward three times (<code>1</code>, <code>0</code>, <code>2</code>), and then inserts the third value, <code>3</code>, after it. <code>3</code> becomes the current position.</li>
</ul>
<p>And so on:</p>
<ul>
<li><code>0 2 (4) 3 1</code></li>
<li><code>0 (5) 2 4 3 1</code></li>
<li><code>0 5 2 4 3 (6) 1</code></li>
<li><code>0 5 (7) 2 4 3 6 1</code></li>
<li><code>0 5 7 2 4 3 (8) 6 1</code></li>
<li><code>0 (9) 5 7 2 4 3 8 6 1</code></li>
</ul>
<p>Eventually, after 2017 insertions, the section of the circular buffer near the last insertion looks like this:</p>
<pre><code>1512 1134 151 (2017) 638 1513 851</code></pre>
<p>Perhaps, if you can identify the value that will ultimately be <em>after</em> the last value written (<code>2017</code>), you can short-circuit the spinlock. In this example, that would be <code>638</code>.</p>
<p><em>What is the value after <code>2017</code></em> in your completed circular buffer?</p>
</article>
<p>Your puzzle input is <code class="puzzle-input">394</code>.</p>
<form method="post" action="17/answer"><input type="hidden" name="level" value="1"/><p>Answer: <input type="text" name="answer" autocomplete="off"/> <input type="submit" value="[Submit]"/></p></form>
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