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<title>Day 4 - Advent of Code 2021</title>
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</head><div id="readability-page-1" class="page"><article><h2>--- Day 4: Giant Squid ---</h2><p>You're already almost 1.5km (almost a mile) below the surface of the ocean, already so deep that you can't see any sunlight. What you <em>can</em> see, however, is a giant squid that has attached itself to the outside of your submarine.</p>
<p>Maybe it wants to play <a href="https://en.wikipedia.org/wiki/Bingo_(American_version)" target="_blank">bingo</a>?</p>
<p>Bingo is played on a set of boards each consisting of a 5x5 grid of numbers. Numbers are chosen at random, and the chosen number is <em>marked</em> on all boards on which it appears. (Numbers may not appear on all boards.) If all numbers in any row or any column of a board are marked, that board <em>wins</em>. (Diagonals don't count.)</p>
<p>The submarine has a <em>bingo subsystem</em> to help passengers (currently, you and the giant squid) pass the time. It automatically generates a random order in which to draw numbers and a random set of boards (your puzzle input). For example:</p>
<pre><code>7,4,9,5,11,17,23,2,0,14,21,24,10,16,13,6,15,25,12,22,18,20,8,19,3,26,1
22 13 17 11 0
8 2 23 4 24
21 9 14 16 7
6 10 3 18 5
1 12 20 15 19
3 15 0 2 22
9 18 13 17 5
19 8 7 25 23
20 11 10 24 4
14 21 16 12 6
14 21 17 24 4
10 16 15 9 19
18 8 23 26 20
22 11 13 6 5
2 0 12 3 7
</code></pre>
<p>After the first five numbers are drawn (<code>7</code>, <code>4</code>, <code>9</code>, <code>5</code>, and <code>11</code>), there are no winners, but the boards are marked as follows (shown here adjacent to each other to save space):</p>
<pre><code>22 13 17 <em>11</em> 0 3 15 0 2 22 14 21 17 24 <em>4</em>
8 2 23 <em>4</em> 24 <em>9</em> 18 13 17 <em>5</em> 10 16 15 <em>9</em> 19
21 <em>9</em> 14 16 <em>7</em> 19 8 <em>7</em> 25 23 18 8 23 26 20
6 10 3 18 <em>5</em> 20 <em>11</em> 10 24 <em>4</em> 22 <em>11</em> 13 6 <em>5</em>
1 12 20 15 19 14 21 16 12 6 2 0 12 3 <em>7</em>
</code></pre>
<p>After the next six numbers are drawn (<code>17</code>, <code>23</code>, <code>2</code>, <code>0</code>, <code>14</code>, and <code>21</code>), there are still no winners:</p>
<pre><code>22 13 <em>17</em> <em>11</em> <em>0</em> 3 15 <em>0</em> <em>2</em> 22 <em>14</em> <em>21</em> <em>17</em> 24 <em>4</em>
8 <em>2</em> <em>23</em> <em>4</em> 24 <em>9</em> 18 13 <em>17</em> <em>5</em> 10 16 15 <em>9</em> 19
<em>21</em> <em>9</em> <em>14</em> 16 <em>7</em> 19 8 <em>7</em> 25 <em>23</em> 18 8 <em>23</em> 26 20
6 10 3 18 <em>5</em> 20 <em>11</em> 10 24 <em>4</em> 22 <em>11</em> 13 6 <em>5</em>
1 12 20 15 19 <em>14</em> <em>21</em> 16 12 6 <em>2</em> <em>0</em> 12 3 <em>7</em>
</code></pre>
<p>Finally, <code>24</code> is drawn:</p>
<pre><code>22 13 <em>17</em> <em>11</em> <em>0</em> 3 15 <em>0</em> <em>2</em> 22 <em>14</em> <em>21</em> <em>17</em> <em>24</em> <em>4</em>
8 <em>2</em> <em>23</em> <em>4</em> <em>24</em> <em>9</em> 18 13 <em>17</em> <em>5</em> 10 16 15 <em>9</em> 19
<em>21</em> <em>9</em> <em>14</em> 16 <em>7</em> 19 8 <em>7</em> 25 <em>23</em> 18 8 <em>23</em> 26 20
6 10 3 18 <em>5</em> 20 <em>11</em> 10 <em>24</em> <em>4</em> 22 <em>11</em> 13 6 <em>5</em>
1 12 20 15 19 <em>14</em> <em>21</em> 16 12 6 <em>2</em> <em>0</em> 12 3 <em>7</em>
</code></pre>
<p>At this point, the third board <em>wins</em> because it has at least one complete row or column of marked numbers (in this case, the entire top row is marked: <code><em>14 21 17 24 4</em></code>).</p>
<p>The <em>score</em> of the winning board can now be calculated. Start by finding the <em>sum of all unmarked numbers</em> on that board; in this case, the sum is <code>188</code>. Then, multiply that sum by <em>the number that was just called</em> when the board won, <code>24</code>, to get the final score, <code>188 * 24 = <em>4512</em></code>.</p>
<p>To guarantee victory against the giant squid, figure out which board will win first. <em>What will your final score be if you choose that board?</em></p>
</article><p>Your puzzle answer was <code>60368</code>.</p><article><h2 id="part2">--- Part Two ---</h2><p>On the other hand, it might be wise to try a different strategy: <span title="That's 'cuz a submarine don't pull things' antennas out of their sockets when they lose. Giant squid are known to do that.">let the giant squid win</span>.</p>
<p>You aren't sure how many bingo boards a giant squid could play at once, so rather than waste time counting its arms, the safe thing to do is to <em>figure out which board will win last</em> and choose that one. That way, no matter which boards it picks, it will win for sure.</p>
<p>In the above example, the second board is the last to win, which happens after <code>13</code> is eventually called and its middle column is completely marked. If you were to keep playing until this point, the second board would have a sum of unmarked numbers equal to <code>148</code> for a final score of <code>148 * 13 = <em>1924</em></code>.</p>
<p>Figure out which board will win last. <em>Once it wins, what would its final score be?</em></p>
</article><p>Your puzzle answer was <code>17435</code>.</p></div>
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