80 lines
4.1 KiB
Markdown
80 lines
4.1 KiB
Markdown
# Day 4: Giant Squid
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[https://adventofcode.com/2021/day/4](https://adventofcode.com/2021/day/4)
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## Description
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### Part One
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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 _can_ see, however, is a giant squid that has attached itself to the outside of your submarine.
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Maybe it wants to play [bingo](https://en.wikipedia.org/wiki/Bingo_(American_version))?
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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 _marked_ 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 _wins_. (Diagonals don't count.)
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The submarine has a _bingo subsystem_ 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:
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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
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22 13 17 11 0
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8 2 23 4 24
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21 9 14 16 7
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6 10 3 18 5
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1 12 20 15 19
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3 15 0 2 22
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9 18 13 17 5
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19 8 7 25 23
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20 11 10 24 4
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14 21 16 12 6
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14 21 17 24 4
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10 16 15 9 19
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18 8 23 26 20
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22 11 13 6 5
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2 0 12 3 7
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After the first five numbers are drawn (`7`, `4`, `9`, `5`, and `11`), there are no winners, but the boards are marked as follows (shown here adjacent to each other to save space):
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22 13 17 11 0 3 15 0 2 22 14 21 17 24 4
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8 2 23 4 24 9 18 13 17 5 10 16 15 9 19
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21 9 14 16 7 19 8 7 25 23 18 8 23 26 20
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6 10 3 18 5 20 11 10 24 4 22 11 13 6 5
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1 12 20 15 19 14 21 16 12 6 2 0 12 3 7
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After the next six numbers are drawn (`17`, `23`, `2`, `0`, `14`, and `21`), there are still no winners:
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22 13 17 11 0 3 15 0 2 22 14 21 17 24 4
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8 2 23 4 24 9 18 13 17 5 10 16 15 9 19
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21 9 14 16 7 19 8 7 25 23 18 8 23 26 20
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6 10 3 18 5 20 11 10 24 4 22 11 13 6 5
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1 12 20 15 19 14 21 16 12 6 2 0 12 3 7
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Finally, `24` is drawn:
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22 13 17 11 0 3 15 0 2 22 14 21 17 24 4
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8 2 23 4 24 9 18 13 17 5 10 16 15 9 19
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21 9 14 16 7 19 8 7 25 23 18 8 23 26 20
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6 10 3 18 5 20 11 10 24 4 22 11 13 6 5
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1 12 20 15 19 14 21 16 12 6 2 0 12 3 7
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At this point, the third board _wins_ because it has at least one complete row or column of marked numbers (in this case, the entire top row is marked: _`14 21 17 24 4`_).
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The _score_ of the winning board can now be calculated. Start by finding the _sum of all unmarked numbers_ on that board; in this case, the sum is `188`. Then, multiply that sum by _the number that was just called_ when the board won, `24`, to get the final score, `188 * 24 = 4512`.
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To guarantee victory against the giant squid, figure out which board will win first. _What will your final score be if you choose that board?_
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### Part Two
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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>.
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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 _figure out which board will win last_ and choose that one. That way, no matter which boards it picks, it will win for sure.
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In the above example, the second board is the last to win, which happens after `13` 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 `148` for a final score of `148 * 13 = 1924`.
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Figure out which board will win last. _Once it wins, what would its final score be?_
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