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16 Oh, hello! Funny seeing you here.
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.
22 Please be careful with automated requests; I'm not a massive company, and I can
23 only take so much traffic. Please be considerate so that everyone gets to play.
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, social media), I
27 built the whole thing myself, including the design, animations, prose, and all
30 The puzzles are most of the work; preparing a new calendar and a new set of
31 puzzles each year takes all of my free time for 4-5 months. A lot of effort
32 went into building this thing - I hope you're enjoying playing it as much as I
33 enjoyed making it for you!
35 If you'd like to hang out, I'm @ericwastl on Twitter.
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98 <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>
99 <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>
100 <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>
101 <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>
102 <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>
103 <p>So, suppose you have a lanternfish with an internal timer value of
<code>3</code>:
</p>
105 <li>After one day, its internal timer would become
<code>2</code>.
</li>
106 <li>After another day, its internal timer would become
<code>1</code>.
</li>
107 <li>After another day, its internal timer would become
<code>0</code>.
</li>
108 <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>
109 <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>
111 <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>
112 <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>
113 <pre><code>3,
4,
3,
1,
2</code></pre>
114 <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>
115 <pre><code>Initial state:
3,
4,
3,
1,
2
116 After
1 day:
2,
3,
2,
0,
1
117 After
2 days:
1,
2,
1,
6,
0,
8
118 After
3 days:
0,
1,
0,
5,
6,
7,
8
119 After
4 days:
6,
0,
6,
4,
5,
6,
7,
8,
8
120 After
5 days:
5,
6,
5,
3,
4,
5,
6,
7,
7,
8
121 After
6 days:
4,
5,
4,
2,
3,
4,
5,
6,
6,
7
122 After
7 days:
3,
4,
3,
1,
2,
3,
4,
5,
5,
6
123 After
8 days:
2,
3,
2,
0,
1,
2,
3,
4,
4,
5
124 After
9 days:
1,
2,
1,
6,
0,
1,
2,
3,
3,
4,
8
125 After
10 days:
0,
1,
0,
5,
6,
0,
1,
2,
2,
3,
7,
8
126 After
11 days:
6,
0,
6,
4,
5,
6,
0,
1,
1,
2,
6,
7,
8,
8,
8
127 After
12 days:
5,
6,
5,
3,
4,
5,
6,
0,
0,
1,
5,
6,
7,
7,
7,
8,
8
128 After
13 days:
4,
5,
4,
2,
3,
4,
5,
6,
6,
0,
4,
5,
6,
6,
6,
7,
7,
8,
8
129 After
14 days:
3,
4,
3,
1,
2,
3,
4,
5,
5,
6,
3,
4,
5,
5,
5,
6,
6,
7,
7,
8
130 After
15 days:
2,
3,
2,
0,
1,
2,
3,
4,
4,
5,
2,
3,
4,
4,
4,
5,
5,
6,
6,
7
131 After
16 days:
1,
2,
1,
6,
0,
1,
2,
3,
3,
4,
1,
2,
3,
3,
3,
4,
4,
5,
5,
6,
8
132 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
133 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
135 <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>
136 <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>
137 <p>Find a way to simulate lanternfish.
<em>How many lanternfish would there be after
80 days?
</em></p>
139 <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>
140 <p>After
256 days in the example above, there would be a total of
<code><em>26984457539</em></code> lanternfish!
</p>
141 <p><em>How many lanternfish would there be after
256 days?
</em></p>
143 <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>
144 <p>At this point, you should
<a href=
"/2021">return to your Advent calendar
</a> and try another puzzle.
</p>
145 <p>If you still want to see it, you can
<a href=
"6/input" target=
"_blank">get your puzzle input
</a>.
</p>
146 <p>You can also
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"share">[Share
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"share-content">on
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149 ></span>]
</span> this puzzle.
</p>
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