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<title>Day 6 - Advent of Code 2021</title>
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</head><div id="readability-page-1" class="page"><article><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>
<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>
<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>
<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
<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>396210</code>.</p><article><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>1770823541496</code>.</p></div>