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<!DOCTYPE html>
<html lang="en-us">
<head>
<meta charset="utf-8"/>
<title>Day 2 - 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
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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@hachyderm.io on Mastodon and
@ericwastl on Twitter.
- Eric Wastl
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<article class="day-desc"><h2>--- Day 2: Dive! ---</h2><p>Now, you need to figure out how to <span title="Tank, I need a pilot program for a B212 helicopter.">pilot this thing</span>.</p>
<p>It seems like the submarine can take a series of commands like <code>forward 1</code>, <code>down 2</code>, or <code>up 3</code>:</p>
<ul>
<li><code>forward X</code> increases the horizontal position by <code>X</code> units.</li>
<li><code>down X</code> <em>increases</em> the depth by <code>X</code> units.</li>
<li><code>up X</code> <em>decreases</em> the depth by <code>X</code> units.</li>
</ul>
<p>Note that since you're on a submarine, <code>down</code> and <code>up</code> affect your <em>depth</em>, and so they have the opposite result of what you might expect.</p>
<p>The submarine seems to already have a planned course (your puzzle input). You should probably figure out where it's going. For example:</p>
<pre><code>forward 5
down 5
forward 8
up 3
down 8
forward 2
</code></pre>
<p>Your horizontal position and depth both start at <code>0</code>. The steps above would then modify them as follows:</p>
<ul>
<li><code>forward 5</code> adds <code>5</code> to your horizontal position, a total of <code>5</code>.</li>
<li><code>down 5</code> adds <code>5</code> to your depth, resulting in a value of <code>5</code>.</li>
<li><code>forward 8</code> adds <code>8</code> to your horizontal position, a total of <code>13</code>.</li>
<li><code>up 3</code> decreases your depth by <code>3</code>, resulting in a value of <code>2</code>.</li>
<li><code>down 8</code> adds <code>8</code> to your depth, resulting in a value of <code>10</code>.</li>
<li><code>forward 2</code> adds <code>2</code> to your horizontal position, a total of <code>15</code>.</li>
</ul>
<p>After following these instructions, you would have a horizontal position of <code>15</code> and a depth of <code>10</code>. (Multiplying these together produces <code><em>150</em></code>.)</p>
<p>Calculate the horizontal position and depth you would have after following the planned course. <em>What do you get if you multiply your final horizontal position by your final depth?</em></p>
</article>
<p>Your puzzle answer was <code>1670340</code>.</p><article class="day-desc"><h2 id="part2">--- Part Two ---</h2><p>Based on your calculations, the planned course doesn't seem to make any sense. You find the submarine manual and discover that the process is actually slightly more complicated.</p>
<p>In addition to horizontal position and depth, you'll also need to track a third value, <em>aim</em>, which also starts at <code>0</code>. The commands also mean something entirely different than you first thought:</p>
<ul>
<li><code>down X</code> <em>increases</em> your aim by <code>X</code> units.</li>
<li><code>up X</code> <em>decreases</em> your aim by <code>X</code> units.</li>
<li><code>forward X</code> does two things:<ul>
<li>It increases your horizontal position by <code>X</code> units.</li>
<li>It increases your depth by your aim <em>multiplied by</em> <code>X</code>.</li>
</ul></li>
</ul>
<p>Again note that since you're on a submarine, <code>down</code> and <code>up</code> do the opposite of what you might expect: "down" means aiming in the positive direction.</p>
<p>Now, the above example does something different:</p>
<ul>
<li><code>forward 5</code> adds <code>5</code> to your horizontal position, a total of <code>5</code>. Because your aim is <code>0</code>, your depth does not change.</li>
<li><code>down 5</code> adds <code>5</code> to your aim, resulting in a value of <code>5</code>.</li>
<li><code>forward 8</code> adds <code>8</code> to your horizontal position, a total of <code>13</code>. Because your aim is <code>5</code>, your depth increases by <code>8*5=40</code>.</li>
<li><code>up 3</code> decreases your aim by <code>3</code>, resulting in a value of <code>2</code>.</li>
<li><code>down 8</code> adds <code>8</code> to your aim, resulting in a value of <code>10</code>.</li>
<li><code>forward 2</code> adds <code>2</code> to your horizontal position, a total of <code>15</code>. Because your aim is <code>10</code>, your depth increases by <code>2*10=20</code> to a total of <code>60</code>.</li>
</ul>
<p>After following these new instructions, you would have a horizontal position of <code>15</code> and a depth of <code>60</code>. (Multiplying these produces <code><em>900</em></code>.)</p>
<p>Using this new interpretation of the commands, calculate the horizontal position and depth you would have after following the planned course. <em>What do you get if you multiply your final horizontal position by your final depth?</em></p>
</article>
<p>Your puzzle answer was <code>1954293920</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="2/input" target="_blank">get your puzzle input</a>.</p>
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