85 lines
2.9 KiB
Markdown
85 lines
2.9 KiB
Markdown
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# 2021 Day 01: Sonar Sweep
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Copyright (c) Eric Wastl
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#### [Direct Link](https://adventofcode.com/2021/day/01)
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## Part 1
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As the submarine drops below the surface of the ocean, it automatically performs a sonar sweep of the nearby sea floor. On a small screen, the sonar sweep report (your puzzle input) appears: each line is a measurement of the sea floor depth as the sweep looks further and further away from the submarine.
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For example, suppose you had the following report:
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```
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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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```
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This report indicates that, scanning outward from the submarine, the sonar 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, just so you know what you're dealing with - you never know if the keys will 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 previous measurement. (There is no measurement before the first measurement.) In the example above, the changes are as follows:
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```
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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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```
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In this example, there are **`7`** measurements that are larger than the previous measurement.
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**How many measurements are larger than the previous measurement?**
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## Part 2
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Considering every single measurement isn't as useful as you expected: there's just too much noise in the data.
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Instead, consider sums of a **three-measurement sliding window**. Again considering the above example:
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```
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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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```
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Start by comparing the first and second three-measurement windows. The measurements in the first window are marked `A` (`199`, `200`, `208`); their sum is `199 + 200 + 208 = 607`. The second window is marked `B` (`200`, `208`, `210`); its sum is `618`. The sum of measurements in the second window is 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 sliding window increases** from the previous sum. So, compare `A` with `B`, then compare `B` with `C`, then `C` with `D`, and so on. Stop when there 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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```
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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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```
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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 than the previous sum?**
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