problems/day17.html

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  16 Oh, hello!  Funny seeing you here.
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  18 I appreciate your enthusiasm, but you aren't going to find much down here.
  19 There certainly aren't clues to any of the puzzles.  The best surprises don't
  20 even appear in the source until you unlock them for real.
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  22 Please be careful with automated requests; I'm not Google, and I can only take
  23 so much traffic.  Please be considerate so that everyone gets to play.
  24
  25 If you're curious about how Advent of Code works, it's running on some custom
  26 Perl code. Other than a few integrations (auth, analytics, ads, social media),
  27 I built the whole thing myself, including the design, animations, prose, and
  28 all of the puzzles.
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  30 The puzzles probably took the longest; the easiest ones took an hour or two
  31 each, but the harder ones took 4-5 hours, and a few even longer than that. A
  32 lot of effort went into building this thing - I hope you're enjoying playing it
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  35 If you'd like to hang out, I'm @ericwastl on Twitter.
  36
  37 - Eric Wastl
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  90 <header><div><h1 class="title-global"><a href="/">Advent of Code</a></h1><nav><ul><li><a href="/2017/about">[About]</a></li><li><a href="/2017/support">[AoC++]</a></li><li><a href="/2017/events">[Events]</a></li><li><a href="/2017/settings">[Settings]</a></li><li><a href="/2017/auth/logout">[Log Out]</a></li></ul></nav><div class="user">Neil Smith <span class="supporter">(AoC++)</span> <span class="star-count">36*</span></div></div><div><h1 class="title-event">&nbsp;&nbsp;&nbsp;<span class="title-event-wrap">0x0000|</span><a href="/2017">2017</a><span class="title-event-wrap"></span></h1><nav><ul><li><a href="/2017">[Calendar]</a></li><li><a href="/2017/leaderboard">[Leaderboard]</a></li><li><a href="/2017/stats">[Stats]</a></li><li><a href="/2017/sponsors">[Sponsors]</a></li></ul></nav></div></header>
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  99 <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>
 100 <p>If you don't move quickly, fixing that printer will be the least of your problems.</p>
 101 <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>
 102 <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>
 103 <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>
 104 <ul>
 105 <li><code>(0)</code>, the initial state before any insertions.</li>
 106 <li><code>0&nbsp;(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>
 107 <li><code>0&nbsp;(2)&nbsp;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>
 108 <li><code>0&nbsp;&nbsp;2&nbsp;(3)&nbsp;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>
 109 </ul>
 110 <p>And so on:</p>
 111 <ul>
 112 <li><code>0&nbsp;&nbsp;2&nbsp;(4)&nbsp;3&nbsp;&nbsp;1</code></li>
 113 <li><code>0&nbsp;(5)&nbsp;2&nbsp;&nbsp;4&nbsp;&nbsp;3&nbsp;&nbsp;1</code></li>
 114 <li><code>0&nbsp;&nbsp;5&nbsp;&nbsp;2&nbsp;&nbsp;4&nbsp;&nbsp;3&nbsp;(6)&nbsp;1</code></li>
 115 <li><code>0&nbsp;&nbsp;5&nbsp;(7)&nbsp;2&nbsp;&nbsp;4&nbsp;&nbsp;3&nbsp;&nbsp;6&nbsp;&nbsp;1</code></li>
 116 <li><code>0&nbsp;&nbsp;5&nbsp;&nbsp;7&nbsp;&nbsp;2&nbsp;&nbsp;4&nbsp;&nbsp;3&nbsp;(8)&nbsp;6&nbsp;&nbsp;1</code></li>
 117 <li><code>0&nbsp;(9)&nbsp;5&nbsp;&nbsp;7&nbsp;&nbsp;2&nbsp;&nbsp;4&nbsp;&nbsp;3&nbsp;&nbsp;8&nbsp;&nbsp;6&nbsp;&nbsp;1</code></li>
 118 </ul>
 119 <p>Eventually, after 2017 insertions, the section of the circular buffer near the last insertion looks like this:</p>
 120 <pre><code>1512  1134  151 (2017) 638  1513  851</code></pre>
 121 <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>
 122 <p><em>What is the value after <code>2017</code></em> in your completed circular buffer?</p>
 123 </article>
 124 <p>Your puzzle answer was <code>1025</code>.</p><article class="day-desc"><h2>--- Part Two ---</h2><p>The spinlock does not short-circuit.  Instead, it gets <em>more</em> angry. At least, you assume that's what happened; it's spinning significantly faster than it was a moment ago.</p>
 125 <p>You have good news and bad news.</p>
 126 <p>The good news is that you have improved calculations for how to stop the spinlock. They indicate that you actually need to identify <em>the value after <code>0</code></em> in the current state of the circular buffer.</p>
 127 <p>The bad news is that while you were determining this, the spinlock has just finished inserting its fifty millionth value (<code>50000000</code>).</p>
 128 <p><em>What is the value after <code>0</code></em> the moment <code>50000000</code> is inserted?</p>
 129 </article>
 130 <p>Your puzzle answer was <code>37803463</code>.</p><p class="day-success">Both parts of this puzzle are complete! They provide two gold stars: **</p>
 131 <p>At this point, you should <a href="/2017">return to your advent calendar</a> and try another puzzle.</p>
 132 <p>Your puzzle input was <code class="puzzle-input">366</code>.</p>
 133 <p>You can also <span class="share">[Share<span class="share-content">on
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