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132 lines
4.0 KiB
Markdown
132 lines
4.0 KiB
Markdown
3 years ago
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https://adventofcode.com/2021/day/1
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## \--- Day 1: Sonar Sweep ---
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You're minding your own business on a ship at sea when the overboard alarm
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goes off! You rush to see if you can help. Apparently, one of the Elves
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tripped and accidentally sent the sleigh keys flying into the ocean!
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Before you know it, you're inside a submarine the Elves keep ready for
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situations like this. It's covered in Christmas lights (because of course it
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is), and it even has an experimental antenna that should be able to track the
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keys if you can boost its signal strength high enough; there's a little meter
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that indicates the antenna's signal strength by displaying 0-50 _stars_.
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Your instincts tell you that in order to save Christmas, you'll need to get
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all _fifty stars_ by December 25th.
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Collect stars by solving puzzles. Two puzzles will be made available on each
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day in the Advent calendar; the second puzzle is unlocked when you complete
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the first. Each puzzle grants _one star_. Good luck!
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As the submarine drops below the surface of the ocean, it automatically
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performs a sonar sweep of the nearby sea floor. On a small screen, the sonar
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sweep report (your puzzle input) appears: each line is a measurement of the
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sea floor depth as the sweep looks further and further away from the
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submarine.
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For example, suppose you had the following report:
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[code]
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199
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200
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208
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210
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200
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207
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240
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269
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260
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263
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[/code]
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This report indicates that, scanning outward from the submarine, the sonar
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sweep found depths of `199`, `200`, `208`, `210`, and so on.
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The first order of business is to figure out how quickly the depth increases,
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just so you know what you're dealing with - you never know if the keys will
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get carried into deeper water by an ocean current or a fish or something.
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To do this, count _the number of times a depth measurement increases_ from the
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previous measurement. (There is no measurement before the first measurement.)
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In the example above, the changes are as follows:
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[code]
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199 (N/A - no previous measurement)
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200 ( _increased_ )
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208 ( _increased_ )
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210 ( _increased_ )
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200 (decreased)
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207 ( _increased_ )
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240 ( _increased_ )
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269 ( _increased_ )
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260 (decreased)
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263 ( _increased_ )
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[/code]
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In this example, there are _`7`_ measurements that are larger than the
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previous measurement.
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_How many measurements are larger than the previous measurement?_
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Your puzzle answer was `1754`.
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## \--- Part Two ---
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Considering every single measurement isn't as useful as you expected: there's
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just too much noise in the data.
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Instead, consider sums of a _three-measurement sliding window_. Again
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considering the above example:
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[code]
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199 A
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200 A B
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208 A B C
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210 B C D
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200 E C D
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207 E F D
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240 E F G
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269 F G H
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260 G H
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263 H
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[/code]
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Start by comparing the first and second three-measurement windows. The
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measurements in the first window are marked `A` (`199`, `200`, `208`); their
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sum is `199 + 200 + 208 = 607`. The second window is marked `B` (`200`, `208`,
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`210`); its sum is `618`. The sum of measurements in the second window is
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larger than the sum of the first, so this first comparison _increased_.
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Your goal now is to count _the number of times the sum of measurements in this
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sliding window increases_ from the previous sum. So, compare `A` with `B`,
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then compare `B` with `C`, then `C` with `D`, and so on. Stop when there
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aren't enough measurements left to create a new three-measurement sum.
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In the above example, the sum of each three-measurement window is as follows:
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[code]
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A: 607 (N/A - no previous sum)
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B: 618 ( _increased_ )
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C: 618 (no change)
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D: 617 (decreased)
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E: 647 ( _increased_ )
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F: 716 ( _increased_ )
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G: 769 ( _increased_ )
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H: 792 ( _increased_ )
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[/code]
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In this example, there are _`5`_ sums that are larger than the previous sum.
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Consider sums of a three-measurement sliding window. _How many sums are larger
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than the previous sum?_
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