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<!DOCTYPE html>
<html lang="en-us">
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<title>Day 10 - Advent of Code 2021</title>
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<article class="day-desc"><h2>--- Day 10: Syntax Scoring ---</h2><p>You ask the submarine to determine the best route out of the deep-sea cave, but it only replies:</p>
<pre><code>Syntax error in navigation subsystem on line: <span title="Some days, that's just how it is.">all of them</span></code></pre>
<p><em>All of them?!</em> The damage is worse than you thought. You bring up a copy of the navigation subsystem (your puzzle input).</p>
<p>The navigation subsystem syntax is made of several lines containing <em>chunks</em>. There are one or more chunks on each line, and chunks contain zero or more other chunks. Adjacent chunks are not separated by any delimiter; if one chunk stops, the next chunk (if any) can immediately start. Every chunk must <em>open</em> and <em>close</em> with one of four legal pairs of matching characters:</p>
<ul>
<li>If a chunk opens with <code>(</code>, it must close with <code>)</code>.</li>
<li>If a chunk opens with <code>[</code>, it must close with <code>]</code>.</li>
<li>If a chunk opens with <code>{</code>, it must close with <code>}</code>.</li>
<li>If a chunk opens with <code><</code>, it must close with <code>></code>.</li>
</ul>
<p>So, <code>()</code> is a legal chunk that contains no other chunks, as is <code>[]</code>. More complex but valid chunks include <code>([])</code>, <code>{()()()}</code>, <code><([{}])></code>, <code>[<>({}){}[([])<>]]</code>, and even <code>(((((((((())))))))))</code>.</p>
<p>Some lines are <em>incomplete</em>, but others are <em>corrupted</em>. Find and discard the corrupted lines first.</p>
<p>A corrupted line is one where a chunk <em>closes with the wrong character</em> - that is, where the characters it opens and closes with do not form one of the four legal pairs listed above.</p>
<p>Examples of corrupted chunks include <code>(]</code>, <code>{()()()></code>, <code>(((()))}</code>, and <code><([]){()}[{}])</code>. Such a chunk can appear anywhere within a line, and its presence causes the whole line to be considered corrupted.</p>
<p>For example, consider the following navigation subsystem:</p>
<pre><code>[({(<(())[]>[[{[]{<()<>>
[(()[<>])]({[<{<<[]>>(
{([(<{}[<>[]}>{[]{[(<()>
(((({<>}<{<{<>}{[]{[]{}
[[<[([]))<([[{}[[()]]]
[{[{({}]{}}([{[{{{}}([]
{<[[]]>}<{[{[{[]{()[[[]
[<(<(<(<{}))><([]([]()
<{([([[(<>()){}]>(<<{{
<{([{{}}[<[[[<>{}]]]>[]]
</code></pre>
<p>Some of the lines aren't corrupted, just incomplete; you can ignore these lines for now. The remaining five lines are corrupted:</p>
<ul>
<li><code>{([(<{}[<>[]}>{[]{[(<()></code> - Expected <code>]</code>, but found <code>}</code> instead.</li>
<li><code>[[<[([]))<([[{}[[()]]]</code> - Expected <code>]</code>, but found <code>)</code> instead.</li>
<li><code>[{[{({}]{}}([{[{{{}}([]</code> - Expected <code>)</code>, but found <code>]</code> instead.</li>
<li><code>[<(<(<(<{}))><([]([]()</code> - Expected <code>></code>, but found <code>)</code> instead.</li>
<li><code><{([([[(<>()){}]>(<<{{</code> - Expected <code>]</code>, but found <code>></code> instead.</li>
</ul>
<p>Stop at the first incorrect closing character on each corrupted line.</p>
<p>Did you know that syntax checkers actually have contests to see who can get the high score for syntax errors in a file? It's true! To calculate the syntax error score for a line, take the <em>first illegal character</em> on the line and look it up in the following table:</p>
<ul>
<li><code>)</code>: <code>3</code> points.</li>
<li><code>]</code>: <code>57</code> points.</li>
<li><code>}</code>: <code>1197</code> points.</li>
<li><code>></code>: <code>25137</code> points.</li>
</ul>
<p>In the above example, an illegal <code>)</code> was found twice (<code>2*3 = <em>6</em></code> points), an illegal <code>]</code> was found once (<code><em>57</em></code> points), an illegal <code>}</code> was found once (<code><em>1197</em></code> points), and an illegal <code>></code> was found once (<code><em>25137</em></code> points). So, the total syntax error score for this file is <code>6+57+1197+25137 = <em>26397</em></code> points!</p>
<p>Find the first illegal character in each corrupted line of the navigation subsystem. <em>What is the total syntax error score for those errors?</em></p>
</article>
<p>Your puzzle answer was <code>339411</code>.</p><article class="day-desc"><h2 id="part2">--- Part Two ---</h2><p>Now, discard the corrupted lines. The remaining lines are <em>incomplete</em>.</p>
<p>Incomplete lines don't have any incorrect characters - instead, they're missing some closing characters at the end of the line. To repair the navigation subsystem, you just need to figure out <em>the sequence of closing characters</em> that complete all open chunks in the line.</p>
<p>You can only use closing characters (<code>)</code>, <code>]</code>, <code>}</code>, or <code>></code>), and you must add them in the correct order so that only legal pairs are formed and all chunks end up closed.</p>
<p>In the example above, there are five incomplete lines:</p>
<ul>
<li><code>[({(<(())[]>[[{[]{<()<>></code> - Complete by adding <code>}}]])})]</code>.</li>
<li><code>[(()[<>])]({[<{<<[]>>(</code> - Complete by adding <code>)}>]})</code>.</li>
<li><code>(((({<>}<{<{<>}{[]{[]{}</code> - Complete by adding <code>}}>}>))))</code>.</li>
<li><code>{<[[]]>}<{[{[{[]{()[[[]</code> - Complete by adding <code>]]}}]}]}></code>.</li>
<li><code><{([{{}}[<[[[<>{}]]]>[]]</code> - Complete by adding <code>])}></code>.</li>
</ul>
<p>Did you know that autocomplete tools <em>also</em> have contests? It's true! The score is determined by considering the completion string character-by-character. Start with a total score of <code>0</code>. Then, for each character, multiply the total score by 5 and then increase the total score by the point value given for the character in the following table:</p>
<ul>
<li><code>)</code>: <code>1</code> point.</li>
<li><code>]</code>: <code>2</code> points.</li>
<li><code>}</code>: <code>3</code> points.</li>
<li><code>></code>: <code>4</code> points.</li>
</ul>
<p>So, the last completion string above - <code>])}></code> - would be scored as follows:</p>
<ul>
<li>Start with a total score of <code>0</code>.</li>
<li>Multiply the total score by 5 to get <code>0</code>, then add the value of <code>]</code> (2) to get a new total score of <code>2</code>.</li>
<li>Multiply the total score by 5 to get <code>10</code>, then add the value of <code>)</code> (1) to get a new total score of <code>11</code>.</li>
<li>Multiply the total score by 5 to get <code>55</code>, then add the value of <code>}</code> (3) to get a new total score of <code>58</code>.</li>
<li>Multiply the total score by 5 to get <code>290</code>, then add the value of <code>></code> (4) to get a new total score of <code>294</code>.</li>
</ul>
<p>The five lines' completion strings have total scores as follows:</p>
<ul>
<li><code>}}]])})]</code> - <code>288957</code> total points.</li>
<li><code>)}>]})</code> - <code>5566</code> total points.</li>
<li><code>}}>}>))))</code> - <code>1480781</code> total points.</li>
<li><code>]]}}]}]}></code> - <code>995444</code> total points.</li>
<li><code>])}></code> - <code>294</code> total points.</li>
</ul>
<p>Autocomplete tools are an odd bunch: the winner is found by <em>sorting</em> all of the scores and then taking the <em>middle</em> score. (There will always be an odd number of scores to consider.) In this example, the middle score is <code><em>288957</em></code> because there are the same number of scores smaller and larger than it.</p>
<p>Find the completion string for each incomplete line, score the completion strings, and sort the scores. <em>What is the middle score?</em></p>
</article>
<p>Your puzzle answer was <code>2289754624</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>
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