advent_of_code/2022/day_24/puzzle_text.md

<article class="day-desc"><h2>--- Day 24: Blizzard Basin ---</h2><p>With everything replanted for next year (and with elephants and monkeys to tend the grove), you and the Elves leave for the extraction point.</p>
<p>Partway up the mountain that shields the grove is a flat, open area that serves as the extraction point. It's a bit of a climb, but nothing the expedition can't handle.</p>
<p>At least, that would normally be true; now that the mountain is covered in snow, things have become more difficult than the Elves are used to.</p>
<p>As the expedition reaches a valley that must be traversed to reach the extraction site, you find that strong, turbulent winds are pushing small <em>blizzards</em> of snow and sharp ice around the valley. It's a good thing everyone packed warm clothes! To make it across safely, you'll need to find a way to avoid them.</p>
<p>Fortunately, it's easy to see all of this from the entrance to the valley, so you make a map of the valley and the blizzards (your puzzle input). For example:</p>
<pre><code>#.#####
#.....#
#&gt;....#
#.....#
#...v.#
#.....#
#####.#
</code></pre>
<p>The walls of the valley are drawn as <code>#</code>; everything else is ground. Clear ground - where there is currently no blizzard - is drawn as <code>.</code>. Otherwise, blizzards are drawn with an arrow indicating their direction of motion: up (<code>^</code>), down (<code>v</code>), left (<code>&lt;</code>), or right (<code>&gt;</code>).</p>
<p>The above map includes two blizzards, one moving right (<code>&gt;</code>) and one moving down (<code>v</code>). In one minute, each blizzard moves one position in the direction it is pointing:</p>
<pre><code>#.#####
#.....#
#.&gt;...#
#.....#
#.....#
#...v.#
#####.#
</code></pre>
<p>Due to <span title="I think, anyway. Do I look like a theoretical blizzacist?">conservation of blizzard energy</span>, as a blizzard reaches the wall of the valley, a new blizzard forms on the opposite side of the valley moving in the same direction. After another minute, the bottom downward-moving blizzard has been replaced with a new downward-moving blizzard at the top of the valley instead:</p>
<pre><code>#.#####
#...v.#
#..&gt;..#
#.....#
#.....#
#.....#
#####.#
</code></pre>
<p>Because blizzards are made of tiny snowflakes, they pass right through each other. After another minute, both blizzards temporarily occupy the same position, marked <code>2</code>:</p>
<pre><code>#.#####
#.....#
#...2.#
#.....#
#.....#
#.....#
#####.#
</code></pre>
<p>After another minute, the situation resolves itself, giving each blizzard back its personal space:</p>
<pre><code>#.#####
#.....#
#....&gt;#
#...v.#
#.....#
#.....#
#####.#
</code></pre>
<p>Finally, after yet another minute, the rightward-facing blizzard on the right is replaced with a new one on the left facing the same direction:</p>
<pre><code>#.#####
#.....#
#&gt;....#
#.....#
#...v.#
#.....#
#####.#
</code></pre>
<p>This process repeats at least as long as you are observing it, but probably forever.</p>
<p>Here is a more complex example:</p>
<pre><code>#.######
#&gt;&gt;.&lt;^&lt;#
#.&lt;..&lt;&lt;#
#&gt;v.&gt;&lt;&gt;#
#&lt;^v^^&gt;#
######.#
</code></pre>
<p>Your expedition begins in the only non-wall position in the top row and needs to reach the only non-wall position in the bottom row. On each minute, you can <em>move</em> up, down, left, or right, or you can <em>wait</em> in place. You and the blizzards act <em>simultaneously</em>, and you cannot share a position with a blizzard.</p>
<p>In the above example, the fastest way to reach your goal requires <code><em>18</em></code> steps. Drawing the position of the expedition as <code>E</code>, one way to achieve this is:</p>
<pre><code>Initial state:
#<em>E</em>######
#&gt;&gt;.&lt;^&lt;#
#.&lt;..&lt;&lt;#
#&gt;v.&gt;&lt;&gt;#
#&lt;^v^^&gt;#
######.#

Minute 1, move down:
#.######
#<em>E</em>&gt;3.&lt;.#
#&lt;..&lt;&lt;.#
#&gt;2.22.#
#&gt;v..^&lt;#
######.#

Minute 2, move down:
#.######
#.2&gt;2..#
#<em>E</em>^22^&lt;#
#.&gt;2.^&gt;#
#.&gt;..&lt;.#
######.#

Minute 3, wait:
#.######
#&lt;^&lt;22.#
#<em>E</em>2&lt;.2.#
#&gt;&lt;2&gt;..#
#..&gt;&lt;..#
######.#

Minute 4, move up:
#.######
#<em>E</em>&lt;..22#
#&lt;&lt;.&lt;..#
#&lt;2.&gt;&gt;.#
#.^22^.#
######.#

Minute 5, move right:
#.######
#2<em>E</em>v.&lt;&gt;#
#&lt;.&lt;..&lt;#
#.^&gt;^22#
#.2..2.#
######.#

Minute 6, move right:
#.######
#&gt;2<em>E</em>&lt;.&lt;#
#.2v^2&lt;#
#&gt;..&gt;2&gt;#
#&lt;....&gt;#
######.#

Minute 7, move down:
#.######
#.22^2.#
#&lt;v<em>E</em>&lt;2.#
#&gt;&gt;v&lt;&gt;.#
#&gt;....&lt;#
######.#

Minute 8, move left:
#.######
#.&lt;&gt;2^.#
#.<em>E</em>&lt;&lt;.&lt;#
#.22..&gt;#
#.2v^2.#
######.#

Minute 9, move up:
#.######
#&lt;<em>E</em>2&gt;&gt;.#
#.&lt;&lt;.&lt;.#
#&gt;2&gt;2^.#
#.v&gt;&lt;^.#
######.#

Minute 10, move right:
#.######
#.2<em>E</em>.&gt;2#
#&lt;2v2^.#
#&lt;&gt;.&gt;2.#
#..&lt;&gt;..#
######.#

Minute 11, wait:
#.######
#2^<em>E</em>^2&gt;#
#&lt;v&lt;.^&lt;#
#..2.&gt;2#
#.&lt;..&gt;.#
######.#

Minute 12, move down:
#.######
#&gt;&gt;.&lt;^&lt;#
#.&lt;<em>E</em>.&lt;&lt;#
#&gt;v.&gt;&lt;&gt;#
#&lt;^v^^&gt;#
######.#

Minute 13, move down:
#.######
#.&gt;3.&lt;.#
#&lt;..&lt;&lt;.#
#&gt;2<em>E</em>22.#
#&gt;v..^&lt;#
######.#

Minute 14, move right:
#.######
#.2&gt;2..#
#.^22^&lt;#
#.&gt;2<em>E</em>^&gt;#
#.&gt;..&lt;.#
######.#

Minute 15, move right:
#.######
#&lt;^&lt;22.#
#.2&lt;.2.#
#&gt;&lt;2&gt;<em>E</em>.#
#..&gt;&lt;..#
######.#

Minute 16, move right:
#.######
#.&lt;..22#
#&lt;&lt;.&lt;..#
#&lt;2.&gt;&gt;<em>E</em>#
#.^22^.#
######.#

Minute 17, move down:
#.######
#2.v.&lt;&gt;#
#&lt;.&lt;..&lt;#
#.^&gt;^22#
#.2..2<em>E</em>#
######.#

Minute 18, move down:
#.######
#&gt;2.&lt;.&lt;#
#.2v^2&lt;#
#&gt;..&gt;2&gt;#
#&lt;....&gt;#
######<em>E</em>#
</code></pre>
<p><em>What is the fewest number of minutes required to avoid the blizzards and reach the goal?</em></p>