problems/day09.html

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   5 <title>Day 9 - Advent of Code 2021</title>
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  16 Oh, hello!  Funny seeing you here.
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  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.
  21
  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.
  24
  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
  28 of the puzzles.
  29
  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!
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  35 If you'd like to hang out, I'm @ericwastl on Twitter.
  36
  37 - Eric Wastl
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  90 <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">20*</span></div></div><div><h1 class="title-event">&nbsp;&nbsp;&nbsp;<span class="title-event-wrap">$year=</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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  98 <article class="day-desc"><h2>--- Day 9: Smoke Basin ---</h2><p>These caves seem to be <a href="https://en.wikipedia.org/wiki/Lava_tube" target="_blank">lava tubes</a>. Parts are even still volcanically active; small hydrothermal vents release smoke into the caves that slowly <span title="This was originally going to be a puzzle about watersheds, but we're already under water.">settles like rain</span>.</p>
  99 <p>If you can model how the smoke flows through the caves, you might be able to avoid it and be that much safer. The submarine generates a heightmap of the floor of the nearby caves for you (your puzzle input).</p>
 100 <p>Smoke flows to the lowest point of the area it's in. For example, consider the following heightmap:</p>
 101 <pre><code>2<em>1</em>9994321<em>0</em>
 102 3987894921
 103 98<em>5</em>6789892
 104 8767896789
 105 989996<em>5</em>678
 106 </code></pre>
 107 <p>Each number corresponds to the height of a particular location, where <code>9</code> is the highest and <code>0</code> is the lowest a location can be.</p>
 108 <p>Your first goal is to find the <em>low points</em> - the locations that are lower than any of its adjacent locations. Most locations have four adjacent locations (up, down, left, and right); locations on the edge or corner of the map have three or two adjacent locations, respectively. (Diagonal locations do not count as adjacent.)</p>
 109 <p>In the above example, there are <em>four</em> low points, all highlighted: two are in the first row (a <code>1</code> and a <code>0</code>), one is in the third row (a <code>5</code>), and one is in the bottom row (also a <code>5</code>). All other locations on the heightmap have some lower adjacent location, and so are not low points.</p>
 110 <p>The <em>risk level</em> of a low point is <em>1 plus its height</em>. In the above example, the risk levels of the low points are <code>2</code>, <code>1</code>, <code>6</code>, and <code>6</code>. The sum of the risk levels of all low points in the heightmap is therefore <code><em>15</em></code>.</p>
 111 <p>Find all of the low points on your heightmap. <em>What is the sum of the risk levels of all low points on your heightmap?</em></p>
 112 </article>
 113 <p>Your puzzle answer was <code>524</code>.</p><article class="day-desc"><h2 id="part2">--- Part Two ---</h2><p>Next, you need to find the largest basins so you know what areas are most important to avoid.</p>
 114 <p>A <em>basin</em> is all locations that eventually flow downward to a single low point. Therefore, every low point has a basin, although some basins are very small. Locations of height <code>9</code> do not count as being in any basin, and all other locations will always be part of exactly one basin.</p>
 115 <p>The <em>size</em> of a basin is the number of locations within the basin, including the low point. The example above has four basins.</p>
 116 <p>The top-left basin, size <code>3</code>:</p>
 117 <pre><code><em>21</em>99943210
 118 <em>3</em>987894921
 119 9856789892
 120 8767896789
 121 9899965678
 122 </code></pre>
 123 <p>The top-right basin, size <code>9</code>:</p>
 124 <pre><code>21999<em>43210</em>
 125 398789<em>4</em>9<em>21</em>
 126 985678989<em>2</em>
 127 8767896789
 128 9899965678
 129 </code></pre>
 130 <p>The middle basin, size <code>14</code>:</p>
 131 <pre><code>2199943210
 132 39<em>878</em>94921
 133 9<em>85678</em>9892
 134 <em>87678</em>96789
 135 9<em>8</em>99965678
 136 </code></pre>
 137 <p>The bottom-right basin, size <code>9</code>:</p>
 138 <pre><code>2199943210
 139 3987894921
 140 9856789<em>8</em>92
 141 876789<em>678</em>9
 142 98999<em>65678</em>
 143 </code></pre>
 144 <p>Find the three largest basins and multiply their sizes together. In the above example, this is <code>9 * 14 * 9 = <em>1134</em></code>.</p>
 145 <p><em>What do you get if you multiply together the sizes of the three largest basins?</em></p>
 146 </article>
 147 <p>Your puzzle answer was <code>1235430</code>.</p><p class="day-success">Both parts of this puzzle are complete! They provide two gold stars: **</p>
 148 <p>At this point, you should <a href="/2021">return to your Advent calendar</a> and try another puzzle.</p>
 149 <p>If you still want to see it, you can <a href="9/input" target="_blank">get your puzzle input</a>.</p>
 150 <p>You can also <span class="share">[Share<span class="share-content">on
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