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+<title>Day 22 - Advent of Code 2018</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
+even appear in the source until you unlock them for real.
+
+Please be careful with automated requests; I'm not Google, 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, ads, 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; the easiest ones take 3-4 hours each, but the
+harder ones take 6-8 hours, and a few even longer than that. 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 on Twitter.
+
+- Eric Wastl
+
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+-->
+<body>
+<header><div><h1 class="title-global"><a href="/">Advent of Code</a></h1><nav><ul><li><a href="/2018/about">[About]</a></li><li><a href="/2018/events">[Events]</a></li><li><a href="https://teespring.com/adventofcode-2019" target="_blank">[Shop]</a></li><li><a href="/2018/settings">[Settings]</a></li><li><a href="/2018/auth/logout">[Log Out]</a></li></ul></nav><div class="user">Neil Smith <a href="/2018/support" class="supporter-badge" title="Advent of Code Supporter">(AoC++)</a> <span class="star-count">44*</span></div></div><div><h1 class="title-event"> <span class="title-event-wrap">{year=></span><a href="/2018">2018</a><span class="title-event-wrap">}</span></h1><nav><ul><li><a href="/2018">[Calendar]</a></li><li><a href="/2018/support">[AoC++]</a></li><li><a href="/2018/sponsors">[Sponsors]</a></li><li><a href="/2018/leaderboard">[Leaderboard]</a></li><li><a href="/2018/stats">[Stats]</a></li></ul></nav></div></header>
+
+<div id="sidebar">
+<div id="sponsor"><div class="quiet">Our <a href="/2018/sponsors">sponsors</a> help make Advent of Code possible:</div><div class="sponsor"><a href="https://www.wearedevelopers.com/world-congress/" target="_blank" onclick="if(ga)ga('send','event','sponsor','click',this.href);" rel="noopener">WeAreDevelopers</a> - Use "AOC-25", save EUR 25 and join 10^4 devs on June 6-7 at the WeAreDevelopers World Congress in Berlin ticket.get(now)</div></div>
+</div><!--/sidebar-->
+
+<main>
+<article class="day-desc"><h2>--- Day 22: Mode Maze ---</h2><p>This is it, your final stop: the year <span title="Yes, really: there is no year zero.">-483</span>. It's snowing and dark outside; the only light you can see is coming from a small cottage in the distance. You make your way there and knock on the door.</p>
+<p>A portly man with a large, white beard answers the door and invites you inside. For someone living near the North Pole in -483, he must not get many visitors, but he doesn't act surprised to see you. Instead, he offers you some milk and cookies.</p>
+<p>After talking for a while, he asks a favor of you. His friend hasn't come back in a few hours, and he's not sure where he is. Scanning the region briefly, you discover one life signal in a cave system nearby; his friend must have taken shelter there. The man asks if you can go there to retrieve his friend.</p>
+<p>The cave is divided into square <em>regions</em> which are either dominantly <em>rocky</em>, <em>narrow</em>, or <em>wet</em> (called its <em>type</em>). Each region occupies exactly one <em>coordinate</em> in <code>X,Y</code> format where <code>X</code> and <code>Y</code> are integers and zero or greater. (Adjacent regions can be the same type.)</p>
+<p>The scan (your puzzle input) is not very detailed: it only reveals the <em>depth</em> of the cave system and the <em>coordinates of the target</em>. However, it does not reveal the type of each region. The mouth of the cave is at <code>0,0</code>.</p>
+<p>The man explains that due to the unusual geology in the area, there is a method to determine any region's type based on its <em>erosion level</em>. The erosion level of a region can be determined from its <em>geologic index</em>. The geologic index can be determined using the first rule that applies from the list below:</p>
+<ul>
+<li>The region at <code>0,0</code> (the mouth of the cave) has a geologic index of <code>0</code>.</li>
+<li>The region at the coordinates of the target has a geologic index of <code>0</code>.</li>
+<li>If the region's <code>Y</code> coordinate is <code>0</code>, the geologic index is its <code>X</code> coordinate times <code>16807</code>.</li>
+<li>If the region's <code>X</code> coordinate is <code>0</code>, the geologic index is its <code>Y</code> coordinate times <code>48271</code>.</li>
+<li>Otherwise, the region's geologic index is the result of multiplying the erosion <em>levels</em> of the regions at <code>X-1,Y</code> and <code>X,Y-1</code>.</li>
+</ul>
+<p>A region's <em>erosion level</em> is its <em>geologic index</em> plus the cave system's <em>depth</em>, all <a href="https://en.wikipedia.org/wiki/Modulo_operation">modulo</a> <code>20183</code>. Then:</p>
+<ul>
+<li>If the <em>erosion level modulo <code>3</code></em> is <code>0</code>, the region's type is <em>rocky</em>.</li>
+<li>If the <em>erosion level modulo <code>3</code></em> is <code>1</code>, the region's type is <em>wet</em>.</li>
+<li>If the <em>erosion level modulo <code>3</code></em> is <code>2</code>, the region's type is <em>narrow</em>.</li>
+</ul>
+<p>For example, suppose the cave system's depth is <code>510</code> and the target's coordinates are <code>10,10</code>. Using <code>%</code> to represent the modulo operator, the cavern would look as follows:</p>
+<ul>
+<li>At <code>0,0</code>, the geologic index is <code>0</code>. The erosion level is <code>(0 + 510) % 20183 = 510</code>. The type is <code>510 % 3 = 0</code>, <em>rocky</em>.</li>
+<li>At <code>1,0</code>, because the <code>Y</code> coordinate is <code>0</code>, the geologic index is <code>1 * 16807 = 16807</code>. The erosion level is <code>(16807 + 510) % 20183 = 17317</code>. The type is <code>17317 % 3 = 1</code>, <em>wet</em>.</li>
+<li>At <code>0,1</code>, because the <code>X</code> coordinate is <code>0</code>, the geologic index is <code> 1 * 48271 = 48271</code>. The erosion level is <code>(48271 + 510) % 20183 = 8415</code>. The type is <code>8415 % 3 = 0</code>, <em>rocky</em>.</li>
+<li>At <code>1,1</code>, neither coordinate is <code>0</code> and it is not the coordinate of the target, so the geologic index is the erosion level of <code>0,1</code> (<code>8415</code>) times the erosion level of <code>1,0</code> (<code>17317</code>), <code>8415 * 17317 = 145722555</code>. The erosion level is <code>(145722555 + 510) % 20183 = 1805</code>. The type is <code>1805 % 3 = 2</code>, <em>narrow</em>.</li>
+<li>At <code>10,10</code>, because they are the target's coordinates, the geologic index is <code>0</code>. The erosion level is <code>(0 + 510) % 20183 = 510</code>. The type is <code>510 % 3 = 0</code>, <em>rocky</em>.</li>
+</ul>
+<p>Drawing this same cave system with rocky as <code>.</code>, wet as <code>=</code>, narrow as <code>|</code>, the mouth as <code>M</code>, the target as <code>T</code>, with <code>0,0</code> in the top-left corner, <code>X</code> increasing to the right, and <code>Y</code> increasing downward, the top-left corner of the map looks like this:</p>
+<pre><code><em>M</em>=.|=.|.|=.|=|=.
+.|=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===<em>T</em>===||
+=|||...|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+</code></pre>
+<p>Before you go in, you should determine the <em>risk level</em> of the area. For the rectangle that has a top-left corner of region <code>0,0</code> and a bottom-right corner of the region containing the target, add up the risk level of each individual region: <code>0</code> for rocky regions, <code>1</code> for wet regions, and <code>2</code> for narrow regions.</p>
+<p>In the cave system above, because the mouth is at <code>0,0</code> and the target is at <code>10,10</code>, adding up the risk level of all regions with an <code>X</code> coordinate from <code>0</code> to <code>10</code> and a <code>Y</code> coordinate from <code>0</code> to <code>10</code>, this total is <code><em>114</em></code>.</p>
+<p><em>What is the total risk level for the smallest rectangle that includes <code>0,0</code> and the target's coordinates?</em></p>
+</article>
+<p>Your puzzle answer was <code>8575</code>.</p><article class="day-desc"><h2 id="part2">--- Part Two ---</h2><p>Okay, it's time to go rescue the man's friend.</p>
+<p>As you leave, he hands you some tools: a <em>torch</em> and some <em>climbing gear</em>. You can't equip both tools at once, but you can choose to use <em>neither</em>.</p>
+<p>Tools can only be used in certain regions:</p>
+<ul>
+<li>In <em>rocky</em> regions, you can use the <em>climbing gear</em> or the <em>torch</em>. You cannot use <em>neither</em> (you'll likely slip and fall).</li>
+<li>In <em>wet</em> regions, you can use the <em>climbing gear</em> or <em>neither</em> tool. You cannot use the <em>torch</em> (if it gets wet, you won't have a light source).</li>
+<li>In <em>narrow</em> regions, you can use the <em>torch</em> or <em>neither</em> tool. You cannot use the <em>climbing gear</em> (it's too bulky to fit).</li>
+</ul>
+<p>You start at <code>0,0</code> (the mouth of the cave) with <em>the torch equipped</em> and must reach the target coordinates as quickly as possible. The regions with negative <code>X</code> or <code>Y</code> are solid rock and cannot be traversed. The fastest route might involve entering regions beyond the <code>X</code> or <code>Y</code> coordinate of the target.</p>
+<p>You can <em>move to an adjacent region</em> (up, down, left, or right; never diagonally) if your currently equipped tool allows you to enter that region. Moving to an adjacent region takes <em>one minute</em>. (For example, if you have the <em>torch</em> equipped, you can move between <em>rocky</em> and <em>narrow</em> regions, but cannot enter <em>wet</em> regions.)</p>
+<p>You can <em>change your currently equipped tool or put both away</em> if your new equipment would be valid for your current region. Switching to using the <em>climbing gear</em>, <em>torch</em>, or <em>neither</em> always takes <em>seven minutes</em>, regardless of which tools you start with. (For example, if you are in a <em>rocky</em> region, you can switch from the <em>torch</em> to the <em>climbing gear</em>, but you cannot switch to <em>neither</em>.)</p>
+<p>Finally, once you reach the target, you need the <em>torch</em> equipped before you can find him in the dark. The target is always in a <em>rocky</em> region, so if you arrive there with <em>climbing gear</em> equipped, you will need to spend seven minutes switching to your torch.</p>
+<p>For example, using the same cave system as above, starting in the top left corner (<code>0,0</code>) and moving to the bottom right corner (the target, <code>10,10</code>) as quickly as possible, one possible route is as follows, with your current position marked <code>X</code>:</p>
+<pre><code>Initially:
+<em>X</em>=.|=.|.|=.|=|=.
+.|=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||...|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Down:
+M=.|=.|.|=.|=|=.
+<em>X</em>|=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||...|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Right:
+M=.|=.|.|=.|=|=.
+.<em>X</em>=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||...|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Switch from using the torch to neither tool:
+M=.|=.|.|=.|=|=.
+.<em>X</em>=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||...|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Right 3:
+M=.|=.|.|=.|=|=.
+.|=|<em>X</em>|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||...|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Switch from using neither tool to the climbing gear:
+M=.|=.|.|=.|=|=.
+.|=|<em>X</em>|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||...|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Down 7:
+M=.|=.|.|=.|=|=.
+.|=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..<em>X</em>==..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||...|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Right:
+M=.|=.|.|=.|=|=.
+.|=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..=<em>X</em>=..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||...|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Down 3:
+M=.|=.|.|=.|=|=.
+.|=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||.<em>X</em>.|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Right:
+M=.|=.|.|=.|=|=.
+.|=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||..<em>X</em>|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Down:
+M=.|=.|.|=.|=|=.
+.|=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||...|==..|=.|
+=.=|=.<em>X</em>..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Right 4:
+M=.|=.|.|=.|=|=.
+.|=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||...|==..|=.|
+=.=|=.=..=<em>X</em>||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Up 2:
+M=.|=.|.|=.|=|=.
+.|=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===<em>X</em>===||
+=|||...|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+
+Switch from using the climbing gear to the torch:
+M=.|=.|.|=.|=|=.
+.|=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===<em>X</em>===||
+=|||...|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+</code></pre>
+<p>This is tied with other routes as the <em>fastest way to reach the target</em>: <code><em>45</em></code> minutes. In it, <code>21</code> minutes are spent switching tools (three times, seven minutes each) and the remaining <code>24</code> minutes are spent moving.</p>
+<p><em>What is the fewest number of minutes you can take to reach the target?</em></p>
+</article>
+<p>Your puzzle answer was <code>999</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="/2018">return to your Advent calendar</a> and try another puzzle.</p>
+<p>If you still want to see it, you can <a href="22/input" target="_blank">get your puzzle input</a>.</p>
+<p>You can also <span class="share">[Share<span class="share-content">on
+ <a href="https://twitter.com/intent/tweet?text=I%27ve+completed+%22Mode+Maze%22+%2D+Day+22+%2D+Advent+of+Code+2018&url=https%3A%2F%2Fadventofcode%2Ecom%2F2018%2Fday%2F22&related=ericwastl&hashtags=AdventOfCode" target="_blank">Twitter</a>
+ <a href="http://www.reddit.com/submit?url=https%3A%2F%2Fadventofcode%2Ecom%2F2018%2Fday%2F22&title=I%27ve+completed+%22Mode+Maze%22+%2D+Day+22+%2D+Advent+of+Code+2018" target="_blank">Reddit</a
+></span>]</span> this puzzle.</p>
+</main>
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