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<!DOCTYPE html>
<html lang="en-us">
<head>
<meta charset="utf-8"/>
<title>Day 5 - 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
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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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<main>
<article class="day-desc"><h2>--- Day 5: Hydrothermal Venture ---</h2><p>You come across a field of <a href="https://en.wikipedia.org/wiki/Hydrothermal_vent" target="_blank">hydrothermal vents</a> on the ocean floor! These vents constantly produce large, opaque clouds, so it would be best to avoid them if possible.</p>
<p>They tend to form in <em>lines</em>; the submarine helpfully produces a list of nearby <span title="Maybe they're Bresenham vents.">lines of vents</span> (your puzzle input) for you to review. For example:</p>
<pre><code>0,9 -> 5,9
8,0 -> 0,8
9,4 -> 3,4
2,2 -> 2,1
7,0 -> 7,4
6,4 -> 2,0
0,9 -> 2,9
3,4 -> 1,4
0,0 -> 8,8
5,5 -> 8,2
</code></pre>
<p>Each line of vents is given as a line segment in the format <code>x1,y1 -> x2,y2</code> where <code>x1</code>,<code>y1</code> are the coordinates of one end the line segment and <code>x2</code>,<code>y2</code> are the coordinates of the other end. These line segments include the points at both ends. In other words:</p>
<ul>
<li>An entry like <code>1,1 -> 1,3</code> covers points <code>1,1</code>, <code>1,2</code>, and <code>1,3</code>.</li>
<li>An entry like <code>9,7 -> 7,7</code> covers points <code>9,7</code>, <code>8,7</code>, and <code>7,7</code>.</li>
</ul>
<p>For now, <em>only consider horizontal and vertical lines</em>: lines where either <code>x1 = x2</code> or <code>y1 = y2</code>.</p>
<p>So, the horizontal and vertical lines from the above list would produce the following diagram:</p>
<pre><code>.......1..
..1....1..
..1....1..
.......1..
.112111211
..........
..........
..........
..........
222111....
</code></pre>
<p>In this diagram, the top left corner is <code>0,0</code> and the bottom right corner is <code>9,9</code>. Each position is shown as <em>the number of lines which cover that point</em> or <code>.</code> if no line covers that point. The top-left pair of <code>1</code>s, for example, comes from <code>2,2 -> 2,1</code>; the very bottom row is formed by the overlapping lines <code>0,9 -> 5,9</code> and <code>0,9 -> 2,9</code>.</p>
<p>To avoid the most dangerous areas, you need to determine <em>the number of points where at least two lines overlap</em>. In the above example, this is anywhere in the diagram with a <code>2</code> or larger - a total of <code><em>5</em></code> points.</p>
<p>Consider only horizontal and vertical lines. <em>At how many points do at least two lines overlap?</em></p>
</article>
<p>Your puzzle answer was <code>6225</code>.</p><article class="day-desc"><h2 id="part2">--- Part Two ---</h2><p>Unfortunately, considering only horizontal and vertical lines doesn't give you the full picture; you need to also consider <em>diagonal lines</em>.</p>
<p>Because of the limits of the hydrothermal vent mapping system, the lines in your list will only ever be horizontal, vertical, or a diagonal line at exactly 45 degrees. In other words:</p>
<ul>
<li>An entry like <code>1,1 -> 3,3</code> covers points <code>1,1</code>, <code>2,2</code>, and <code>3,3</code>.</li>
<li>An entry like <code>9,7 -> 7,9</code> covers points <code>9,7</code>, <code>8,8</code>, and <code>7,9</code>.</li>
</ul>
<p>Considering all lines from the above example would now produce the following diagram:</p>
<pre><code>1.1....11.
.111...2..
..2.1.111.
...1.2.2..
.112313211
...1.2....
..1...1...
.1.....1..
1.......1.
222111....
</code></pre>
<p>You still need to determine <em>the number of points where at least two lines overlap</em>. In the above example, this is still anywhere in the diagram with a <code>2</code> or larger - now a total of <code><em>12</em></code> points.</p>
<p>Consider all of the lines. <em>At how many points do at least two lines overlap?</em></p>
</article>
<p>Your puzzle answer was <code>22116</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="5/input" target="_blank">get your puzzle input</a>.</p>
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