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+<title>Day 21 - 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
+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 took an hour or two
+each, but the harder ones took 4-5 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="/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">42*</span></div></div><div><h1 class="title-event"> <span class="title-event-wrap"></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>
+
+<div id="sidebar">
+<div id="sponsor"><div class="quiet">Our <a href="/2017/sponsors">sponsors</a> help make Advent of Code possible:</div><p><a href="http://kx.com/" target="_blank" onclick="if(ga)ga('send','event','sponsor','click',this.href);" rel="noopener">Kx Systems</a> - kdb+, the in-memory time series technology standard</p></div>
+<p class="quiet">By popular demand, there are now AoC-themed objects available (until Jan. 3rd)! Get them shipped <a href="https://teespring.com/advent-of-code" target="_blank">from the US</a> or <a href="https://teespring.com/advent-of-code-eu" target="_blank">from Europe</a>.</p>
+
+</div><!--/sidebar-->
+
+<main>
+<article class="day-desc"><h2>--- Day 21: Fractal Art ---</h2><p>You find a program trying to generate some art. It uses a strange process that involves <span title="This technique is also often used on TV.">repeatedly enhancing</span> the detail of an image through a set of rules.</p>
+<p>The image consists of a two-dimensional square grid of pixels that are either on (<code>#</code>) or off (<code>.</code>). The program always begins with this pattern:</p>
+<pre><code>.#.
+..#
+###
+</code></pre>
+<p>Because the pattern is both <code>3</code> pixels wide and <code>3</code> pixels tall, it is said to have a <em>size</em> of <code>3</code>.</p>
+<p>Then, the program repeats the following process:</p>
+<ul>
+<li>If the size is evenly divisible by <code>2</code>, break the pixels up into <code>2x2</code> squares, and convert each <code>2x2</code> square into a <code>3x3</code> square by following the corresponding <em>enhancement rule</em>.</li>
+<li>Otherwise, the size is evenly divisible by <code>3</code>; break the pixels up into <code>3x3</code> squares, and convert each <code>3x3</code> square into a <code>4x4</code> square by following the corresponding <em>enhancement rule</em>.</li>
+</ul>
+<p>Because each square of pixels is replaced by a larger one, the image gains pixels and so its <em>size</em> increases.</p>
+<p>The artist's book of enhancement rules is nearby (your puzzle input); however, it seems to be missing rules. The artist explains that sometimes, one must <em>rotate</em> or <em>flip</em> the input pattern to find a match. (Never rotate or flip the output pattern, though.) Each pattern is written concisely: rows are listed as single units, ordered top-down, and separated by slashes. For example, the following rules correspond to the adjacent patterns:</p>
+<pre><code>../.# = ..
+ .#
+
+ .#.
+.#./..#/### = ..#
+ ###
+
+ #..#
+#..#/..../#..#/.##. = ....
+ #..#
+ .##.
+</code></pre>
+<p>When searching for a rule to use, rotate and flip the pattern as necessary. For example, all of the following patterns match the same rule:</p>
+<pre><code>.#. .#. #.. ###
+..# #.. #.# ..#
+### ### ##. .#.
+</code></pre>
+<p>Suppose the book contained the following two rules:</p>
+<pre><code>../.# => ##./#../...
+.#./..#/### => #..#/..../..../#..#
+</code></pre>
+<p>As before, the program begins with this pattern:</p>
+<pre><code>.#.
+..#
+###
+</code></pre>
+<p>The size of the grid (<code>3</code>) is not divisible by <code>2</code>, but it is divisible by <code>3</code>. It divides evenly into a single square; the square matches the second rule, which produces:</p>
+<pre><code>#..#
+....
+....
+#..#
+</code></pre>
+<p>The size of this enhanced grid (<code>4</code>) is evenly divisible by <code>2</code>, so that rule is used. It divides evenly into four squares:</p>
+<pre><code>#.|.#
+..|..
+--+--
+..|..
+#.|.#
+</code></pre>
+<p>Each of these squares matches the same rule (<code>../.# => ##./#../...</code>), three of which require some flipping and rotation to line up with the rule. The output for the rule is the same in all four cases:</p>
+<pre><code>##.|##.
+#..|#..
+...|...
+---+---
+##.|##.
+#..|#..
+...|...
+</code></pre>
+<p>Finally, the squares are joined into a new grid:</p>
+<pre><code>##.##.
+#..#..
+......
+##.##.
+#..#..
+......
+</code></pre>
+<p>Thus, after <code>2</code> iterations, the grid contains <code>12</code> pixels that are <em>on</em>.</p>
+<p><em>How many pixels stay on</em> after <code>5</code> iterations?</p>
+</article>
+<p>Your puzzle answer was <code>158</code>.</p><article class="day-desc"><h2>--- Part Two ---</h2><p><em>How many pixels stay on</em> after <code>18</code> iterations?</p>
+</article>
+<p>Your puzzle answer was <code>2301762</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="/2017">return to your advent calendar</a> and try another puzzle.</p>
+<p>If you still want to see it, you can <a href="21/input" target="_blank">get your puzzle input</a>.</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+%22Fractal+Art%22+%2D+Day+21+%2D+Advent+of+Code+2017&url=http%3A%2F%2Fadventofcode%2Ecom%2F2017%2Fday%2F21&related=ericwastl&hashtags=AdventOfCode" target="_blank">Twitter</a>
+ <a href="https://plus.google.com/share?url=http%3A%2F%2Fadventofcode%2Ecom%2F2017%2Fday%2F21" target="_blank">Google+</a>
+ <a href="http://www.reddit.com/submit?url=http%3A%2F%2Fadventofcode%2Ecom%2F2017%2Fday%2F21&title=I%27ve+completed+%22Fractal+Art%22+%2D+Day+21+%2D+Advent+of+Code+2017" target="_blank">Reddit</a
+></span>]</span> this puzzle.</p>
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