Jake Pullen
46 lines
2.9 KiB
Markdown
46 lines
2.9 KiB
Markdown
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<article class="day-desc"><h2>--- Day 20: Grove Positioning System ---</h2><p>It's finally time to meet back up with the Elves. When you try to contact them, however, you get no reply. Perhaps you're out of range?</p>
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<p>You know they're headed to the grove where the <em class="star">star</em> fruit grows, so if you can figure out where that is, you should be able to meet back up with them.</p>
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<p>Fortunately, your handheld device has a file (your puzzle input) that contains the grove's coordinates! Unfortunately, the file is <em>encrypted</em> - just in case the device were to fall into the wrong hands.</p>
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<p>Maybe you can <span title="You once again make a mental note to remind the Elves later not to invent their own cryptographic functions.">decrypt</span> it?</p>
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<p>When you were still back at the camp, you overheard some Elves talking about coordinate file encryption. The main operation involved in decrypting the file is called <em>mixing</em>.</p>
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<p>The encrypted file is a list of numbers. To <em>mix</em> the file, move each number forward or backward in the file a number of positions equal to the value of the number being moved. The list is <em>circular</em>, so moving a number off one end of the list wraps back around to the other end as if the ends were connected.</p>
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<p>For example, to move the <code>1</code> in a sequence like <code>4, 5, 6, <em>1</em>, 7, 8, 9</code>, the <code>1</code> moves one position forward: <code>4, 5, 6, 7, <em>1</em>, 8, 9</code>. To move the <code>-2</code> in a sequence like <code>4, <em>-2</em>, 5, 6, 7, 8, 9</code>, the <code>-2</code> moves two positions backward, wrapping around: <code>4, 5, 6, 7, 8, <em>-2</em>, 9</code>.</p>
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<p>The numbers should be moved <em>in the order they originally appear</em> in the encrypted file. Numbers moving around during the mixing process do not change the order in which the numbers are moved.</p>
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<p>Consider this encrypted file:</p>
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<pre><code>1
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2
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-3
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3
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-2
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0
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4
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</code></pre>
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<p>Mixing this file proceeds as follows:</p>
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<pre><code>Initial arrangement:
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1, 2, -3, 3, -2, 0, 4
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1 moves between 2 and -3:
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2, 1, -3, 3, -2, 0, 4
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2 moves between -3 and 3:
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1, -3, 2, 3, -2, 0, 4
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-3 moves between -2 and 0:
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1, 2, 3, -2, -3, 0, 4
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3 moves between 0 and 4:
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1, 2, -2, -3, 0, 3, 4
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-2 moves between 4 and 1:
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1, 2, -3, 0, 3, 4, -2
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0 does not move:
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1, 2, -3, 0, 3, 4, -2
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4 moves between -3 and 0:
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1, 2, -3, 4, 0, 3, -2
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</code></pre>
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<p>Then, the grove coordinates can be found by looking at the 1000th, 2000th, and 3000th numbers after the value <code>0</code>, wrapping around the list as necessary. In the above example, the 1000th number after <code>0</code> is <code><em>4</em></code>, the 2000th is <code><em>-3</em></code>, and the 3000th is <code><em>2</em></code>; adding these together produces <code><em>3</em></code>.</p>
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<p>Mix your encrypted file exactly once. <em>What is the sum of the three numbers that form the grove coordinates?</em></p>
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