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+<title>Day 16 - Advent of Code 2016</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
+even appear in the source until you unlock them for real.
+
+Please be careful with automated requests; I'm not Google, and I can only take
+so much traffic. Please be considerate so that everyone gets to play.
+
+If you're curious about how Advent of Code works, it's running on some custom
+Perl code. Other than a few integrations (auth, analytics, ads, social media),
+I built the whole thing myself, including the design, animations, prose, and
+all of the puzzles.
+
+The puzzles probably took the longest; the easiest ones were around 45 minutes
+each, but the harder ones took 2-3 hours, and a few even longer than that. A
+lot of effort went into building this thing - I hope you're enjoying playing it
+as much as I enjoyed making it for you!
+
+If you'd like to hang out, I'm @ericwastl on Twitter.
+
+- Eric Wastl
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+-->
+<body>
+<header><div><h1 class="title-global"><a href="/">Advent of Code</a></h1><nav><ul><li><a href="/2016/about">[About]</a></li><li><a href="/2016/support">[AoC++]</a></li><li><a href="/2016/events">[Events]</a></li><li><a href="/2016/settings">[Settings]</a></li><li><a href="/2016/auth/logout">[Log Out]</a></li></ul></nav><div class="user">Neil Smith <span class="supporter">(AoC++)</span> <span class="star-count">32*</span></div></div><div><h1 class="title-event"> <span class="title-event-wrap">int y=</span><a href="/2016">2016</a><span class="title-event-wrap">;</span></h1><nav><ul><li><a href="/2016">[Calendar]</a></li><li><a href="/2016/leaderboard">[Leaderboard]</a></li><li><a href="/2016/stats">[Stats]</a></li><li><a href="/2016/sponsors">[Sponsors]</a></li></ul></nav></div></header>
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+<div id="sponsor"><div class="quiet">Our <a href="/2016/sponsors">sponsors</a> help make AoC possible:</div><p><a href="http://www.aandkrentals.net/" target="_blank" onclick="if(ga)ga('send','event','sponsor','click',this.href);">A&K Rentals</a> - Affordable, high-quality homes just north of Kansas City.</p></div>
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+<main>
+<article class="day-desc"><h2>--- Day 16: Dragon Checksum ---</h2><p>You're done scanning this part of the network, but you've left traces of your presence. You need to <span title="If I ever find one of my disks overwritten with a dragon curve, I'll know it was you.">overwrite some disks</span> with random-looking data to cover your tracks and update the local security system with a new checksum for those disks.</p>
+<p>For the data to not be suspiscious, it needs to have certain properties; purely random data will be detected as tampering. To generate appropriate random data, you'll need to use a modified <a href="https://en.wikipedia.org/wiki/Dragon_curve">dragon curve</a>.</p>
+<p>Start with an appropriate initial state (your puzzle input). Then, so long as you don't have enough data yet to fill the disk, repeat the following steps:</p>
+<ul>
+<li>Call the data you have at this point "a".</li>
+<li>Make a copy of "a"; call this copy "b".</li>
+<li>Reverse the order of the characters in "b".</li>
+<li>In "b", replace all instances of <code>0</code> with <code>1</code> and all <code>1</code>s with <code>0</code>.</li>
+<li>The resulting data is "a", then a single <code>0</code>, then "b".</li>
+</ul>
+<p>For example, after a single step of this process,</p>
+<ul>
+<li><code>1</code> becomes <code>100</code>.</li>
+<li><code>0</code> becomes <code>001</code>.</li>
+<li><code>11111</code> becomes <code>11111000000</code>.</li>
+<li><code>111100001010</code> becomes <code>1111000010100101011110000</code>.</li>
+</ul>
+<p>Repeat these steps until you have enough data to fill the desired disk.</p>
+<p>Once the data has been generated, you also need to create a checksum of that data. Calculate the checksum <em>only</em> for the data that fits on the disk, even if you generated more data than that in the previous step.</p>
+<p>The checksum for some given data is created by considering each non-overlapping <em>pair</em> of characters in the input data. If the two characters match (<code>00</code> or <code>11</code>), the next checksum character is a <code>1</code>. If the characters do not match (<code>01</code> or <code>10</code>), the next checksum character is a <code>0</code>. This should produce a new string which is exactly half as long as the original. If the length of the checksum is <em>even</em>, repeat the process until you end up with a checksum with an <em>odd</em> length.</p>
+<p>For example, suppose we want to fill a disk of length <code>12</code>, and when we finally generate a string of at least length <code>12</code>, the first <code>12</code> characters are <code>110010110100</code>. To generate its checksum:</p>
+<ul>
+<li>Consider each pair: <code>11</code>, <code>00</code>, <code>10</code>, <code>11</code>, <code>01</code>, <code>00</code>.</li>
+<li>These are same, same, different, same, different, same, producing <code>110101</code>.</li>
+<li>The resulting string has length <code>6</code>, which is <em>even</em>, so we repeat the process.</li>
+<li>The pairs are <code>11</code> (same), <code>01</code> (different), <code>01</code> (different).</li>
+<li>This produces the checksum <code>100</code>, which has an <em>odd</em> length, so we stop.</li>
+</ul>
+<p>Therefore, the checksum for <code>110010110100</code> is <code>100</code>.</p>
+<p>Combining all of these steps together, suppose you want to fill a disk of length <code>20</code> using an initial state of <code>10000</code>:</p>
+<ul>
+<li>Because <code>10000</code> is too short, we first use the modified dragon curve to make it longer.</li>
+<li>After one round, it becomes <code>10000011110</code> (<code>11</code> characters), still too short.</li>
+<li>After two rounds, it becomes <code>10000011110010000111110</code> (<code>23</code> characters), which is enough.</li>
+<li>Since we only need <code>20</code>, but we have <code>23</code>, we get rid of all but the first <code>20</code> characters: <code>10000011110010000111</code>.</li>
+<li>Next, we start calculating the checksum; after one round, we have <code>0111110101</code>, which <code>10</code> characters long (<em>even</em>), so we continue.</li>
+<li>After two rounds, we have <code>01100</code>, which is <code>5</code> characters long (<em>odd</em>), so we are done.</li>
+</ul>
+<p>In this example, the correct checksum would therefore be <code>01100</code>.</p>
+<p>The first disk you have to fill has length <code>272</code>. Using the initial state in your puzzle input, <em>what is the correct checksum</em>?</p>
+</article>
+<p>Your puzzle answer was <code>10100011010101011</code>.</p><article class="day-desc"><h2>--- Part Two ---</h2><p>The second disk you have to fill has length <code>35651584</code>. Again using the initial state in your puzzle input, <em>what is the correct checksum</em> for this disk?</p>
+</article>
+<p>Your puzzle answer was <code>01010001101011001</code>.</p><p class="day-success">Both parts of this puzzle are complete! They provide two gold stars: **</p>
+<p>At this point, you should <a href="/2016">return to your advent calendar</a> and try another puzzle.</p>
+<p>Your puzzle input was <code class="puzzle-input">11100010111110100</code>.</p>
+<p>You can also <span class="share">[Share<span class="share-content">on
+ <a href="https://twitter.com/intent/tweet?text=I%27ve+completed+%22Dragon+Checksum%22+%2D+Day+16+%2D+Advent+of+Code+2016&url=http%3A%2F%2Fadventofcode%2Ecom%2F2016%2Fday%2F16&related=ericwastl&hashtags=AdventOfCode" target="_blank">Twitter</a>
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+></span>]</span> this puzzle.</p>
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