71 lines
2.8 KiB
Markdown
71 lines
2.8 KiB
Markdown
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# 2021 Day 09: Smoke Basin
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Copyright (c) Eric Wastl
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#### [Direct Link](https://adventofcode.com/2021/day/09)
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These caves seem to be [lava tubes](https://en.wikipedia.org/wiki/Lava_tube). Parts are even still volcanically active; small hydrothermal vents release smoke into the caves that slowly settles like rain.
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If you can model how the smoke flows through the caves, you might be able to avoid it and be that much safer. The submarine generates a heightmap of the floor of the nearby caves for you (your puzzle input).
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Smoke flows to the lowest point of the area it's in. For example, consider the following heightmap:
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```
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2199943210
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3987894921
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9856789892
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8767896789
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9899965678
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```
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Each number corresponds to the height of a particular location, where `9` is the highest and `0` is the lowest a location can be.
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Your first goal is to find the **low points** - the locations that are lower than any of its adjacent locations. Most locations have four adjacent locations (up, down, left, and right); locations on the edge or corner of the map have three or two adjacent locations, respectively. (Diagonal locations do not count as adjacent.)
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In the above example, there are **four** low points, all highlighted: two are in the first row (a `1` and a `0`), one is in the third row (a `5`), and one is in the bottom row (also a `5`). All other locations on the heightmap have some lower adjacent location, and so are not low points.
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The **risk level** of a low point is **1 plus its height**. In the above example, the risk levels of the low points are `2`, `1`, `6`, and `6`. The sum of the risk levels of all low points in the heightmap is therefore **`15`**.
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Find all of the low points on your heightmap. **What is the sum of the risk levels of all low points on your heightmap?**
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--- Part Two ---
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Next, you need to find the largest basins so you know what areas are most important to avoid.
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A **basin** is all locations that eventually flow downward to a single low point. Therefore, every low point has a basin, although some basins are very small. Locations of height `9` do not count as being in any basin, and all other locations will always be part of exactly one basin.
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The **size** of a basin is the number of locations within the basin, including the low point. The example above has four basins.
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The top-left basin, size `3`:
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```
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2199943210
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3987894921
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9856789892
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8767896789
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9899965678
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```
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The top-right basin, size `9`:
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```
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2199943210
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3987894921
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9856789892
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8767896789
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9899965678
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```
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The middle basin, size `14`:
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```
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2199943210
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3987894921
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9856789892
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8767896789
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9899965678
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```
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The bottom-right basin, size `9`:
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```
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2199943210
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3987894921
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9856789892
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8767896789
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9899965678
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```
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Find the three largest basins and multiply their sizes together. In the above example, this is `9 * 14 * 9 = 1134`.
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**What do you get if you multiply together the sizes of the three largest basins?**
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