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+<meta charset="utf-8"/>
+<title>Day 6 - Advent of Code 2021</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 a massive company, 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, social media), I
+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
+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="/2021/about">[About]</a></li><li><a href="/2021/events">[Events]</a></li><li><a href="https://teespring.com/stores/advent-of-code" target="_blank">[Shop]</a></li><li><a href="/2021/settings">[Settings]</a></li><li><a href="/2021/auth/logout">[Log Out]</a></li></ul></nav><div class="user">Neil Smith <a href="/2021/support" class="supporter-badge" title="Advent of Code Supporter">(AoC++)</a> <span class="star-count">12*</span></div></div><div><h1 class="title-event"> <span class="title-event-wrap">sub y{</span><a href="/2021">2021</a><span class="title-event-wrap">}</span></h1><nav><ul><li><a href="/2021">[Calendar]</a></li><li><a href="/2021/support">[AoC++]</a></li><li><a href="/2021/sponsors">[Sponsors]</a></li><li><a href="/2021/leaderboard">[Leaderboard]</a></li><li><a href="/2021/stats">[Stats]</a></li></ul></nav></div></header>
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+<div id="sponsor"><div class="quiet">Our <a href="/2021/sponsors">sponsors</a> help make Advent of Code possible:</div><div class="sponsor"><a href="https://retool.com/?utm_source=sponsor&utm_medium=event&utm_campaign=adventofcode" target="_blank" onclick="if(ga)ga('send','event','sponsor','sidebar',this.href);" rel="noopener">Retool</a> - Build internal apps remarkably fast. Drag and drop a form together, and have it POST back to your API in minutes. Write JavaScript anywhere to customize. Deploy instantly with access controls and audit logs.</div></div>
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+<script>window.addEventListener('click', function(e,s,r){if(e.target.nodeName==='CODE'&&e.detail===3){s=window.getSelection();s.removeAllRanges();r=document.createRange();r.selectNodeContents(e.target);s.addRange(r);}});</script>
+<article class="day-desc"><h2>--- Day 6: Lanternfish ---</h2><p>The sea floor is getting steeper. Maybe the sleigh keys got carried this way?</p>
+<p>A massive school of glowing <a href="https://en.wikipedia.org/wiki/Lanternfish" target="_blank">lanternfish</a> swims past. They must spawn quickly to reach such large numbers - maybe <em>exponentially</em> quickly? You should model their growth rate to be sure.</p>
+<p>Although you know nothing about this specific species of lanternfish, you make some guesses about their attributes. Surely, <span title="I heard you like lanternfish.">each lanternfish creates a new lanternfish</span> once every <em>7</em> days.</p>
+<p>However, this process isn't necessarily synchronized between every lanternfish - one lanternfish might have 2 days left until it creates another lanternfish, while another might have 4. So, you can model each fish as a single number that represents <em>the number of days until it creates a new lanternfish</em>.</p>
+<p>Furthermore, you reason, a <em>new</em> lanternfish would surely need slightly longer before it's capable of producing more lanternfish: two more days for its first cycle.</p>
+<p>So, suppose you have a lanternfish with an internal timer value of <code>3</code>:</p>
+<ul>
+<li>After one day, its internal timer would become <code>2</code>.</li>
+<li>After another day, its internal timer would become <code>1</code>.</li>
+<li>After another day, its internal timer would become <code>0</code>.</li>
+<li>After another day, its internal timer would reset to <code>6</code>, and it would create a <em>new</em> lanternfish with an internal timer of <code>8</code>.</li>
+<li>After another day, the first lanternfish would have an internal timer of <code>5</code>, and the second lanternfish would have an internal timer of <code>7</code>.</li>
+</ul>
+<p>A lanternfish that creates a new fish resets its timer to <code>6</code>, <em>not <code>7</code></em> (because <code>0</code> is included as a valid timer value). The new lanternfish starts with an internal timer of <code>8</code> and does not start counting down until the next day.</p>
+<p>Realizing what you're trying to do, the submarine automatically produces a list of the ages of several hundred nearby lanternfish (your puzzle input). For example, suppose you were given the following list:</p>
+<pre><code>3,4,3,1,2</code></pre>
+<p>This list means that the first fish has an internal timer of <code>3</code>, the second fish has an internal timer of <code>4</code>, and so on until the fifth fish, which has an internal timer of <code>2</code>. Simulating these fish over several days would proceed as follows:</p>
+<pre><code>Initial state: 3,4,3,1,2
+After 1 day: 2,3,2,0,1
+After 2 days: 1,2,1,6,0,8
+After 3 days: 0,1,0,5,6,7,8
+After 4 days: 6,0,6,4,5,6,7,8,8
+After 5 days: 5,6,5,3,4,5,6,7,7,8
+After 6 days: 4,5,4,2,3,4,5,6,6,7
+After 7 days: 3,4,3,1,2,3,4,5,5,6
+After 8 days: 2,3,2,0,1,2,3,4,4,5
+After 9 days: 1,2,1,6,0,1,2,3,3,4,8
+After 10 days: 0,1,0,5,6,0,1,2,2,3,7,8
+After 11 days: 6,0,6,4,5,6,0,1,1,2,6,7,8,8,8
+After 12 days: 5,6,5,3,4,5,6,0,0,1,5,6,7,7,7,8,8
+After 13 days: 4,5,4,2,3,4,5,6,6,0,4,5,6,6,6,7,7,8,8
+After 14 days: 3,4,3,1,2,3,4,5,5,6,3,4,5,5,5,6,6,7,7,8
+After 15 days: 2,3,2,0,1,2,3,4,4,5,2,3,4,4,4,5,5,6,6,7
+After 16 days: 1,2,1,6,0,1,2,3,3,4,1,2,3,3,3,4,4,5,5,6,8
+After 17 days: 0,1,0,5,6,0,1,2,2,3,0,1,2,2,2,3,3,4,4,5,7,8
+After 18 days: 6,0,6,4,5,6,0,1,1,2,6,0,1,1,1,2,2,3,3,4,6,7,8,8,8,8
+</code></pre>
+<p>Each day, a <code>0</code> becomes a <code>6</code> and adds a new <code>8</code> to the end of the list, while each other number decreases by 1 if it was present at the start of the day.</p>
+<p>In this example, after 18 days, there are a total of <code>26</code> fish. After 80 days, there would be a total of <code><em>5934</em></code>.</p>
+<p>Find a way to simulate lanternfish. <em>How many lanternfish would there be after 80 days?</em></p>
+</article>
+<p>Your puzzle answer was <code>352195</code>.</p><article class="day-desc"><h2 id="part2">--- Part Two ---</h2><p>Suppose the lanternfish live forever and have unlimited food and space. Would they take over the entire ocean?</p>
+<p>After 256 days in the example above, there would be a total of <code><em>26984457539</em></code> lanternfish!</p>
+<p><em>How many lanternfish would there be after 256 days?</em></p>
+</article>
+<p>Your puzzle answer was <code>1600306001288</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="/2021">return to your Advent calendar</a> and try another puzzle.</p>
+<p>If you still want to see it, you can <a href="6/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+%22Lanternfish%22+%2D+Day+6+%2D+Advent+of+Code+2021&url=https%3A%2F%2Fadventofcode%2Ecom%2F2021%2Fday%2F6&related=ericwastl&hashtags=AdventOfCode" target="_blank">Twitter</a>
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+></span>]</span> this puzzle.</p>
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