2021 Day 09

2021/09/README.md

@@ -0,0 +1,71 @@
+# 2021 Day 09: Smoke Basin
+Copyright (c) Eric Wastl
+#### [Direct Link](https://adventofcode.com/2021/day/09)
+
+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.
+
+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).
+
+Smoke flows to the lowest point of the area it's in. For example, consider the following heightmap:
+
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+```
+
+Each number corresponds to the height of a particular location, where `9` is the highest and `0` is the lowest a location can be.
+
+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.)
+
+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.
+
+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`**.
+
+Find all of the low points on your heightmap. **What is the sum of the risk levels of all low points on your heightmap?**
+
+--- Part Two ---
+
+Next, you need to find the largest basins so you know what areas are most important to avoid.
+
+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.
+
+The **size** of a basin is the number of locations within the basin, including the low point. The example above has four basins.
+
+The top-left basin, size `3`:
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+```
+The top-right basin, size `9`:
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+```
+The middle basin, size `14`:
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+```
+The bottom-right basin, size `9`:
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+```
+Find the three largest basins and multiply their sizes together. In the above example, this is `9 * 14 * 9 = 1134`.
+
+**What do you get if you multiply together the sizes of the three largest basins?**

2021/09/code.py

@@ -0,0 +1,58 @@
+# SPDX-License-Identifier: MIT
+# Copyright (c) 2021 Akumatic
+#
+# https://adventofcode.com/2021/day/09
+
+def read_file(filename: str = "input.txt") -> tuple:
+    with open(f"{__file__.rstrip('code.py')}{filename}", "r") as f:
+        lines =  [line for line in f.read().strip().split("\n")]
+    max_x = len(lines[0])
+    max_y = len(lines)
+    numbers = list()
+    for line in lines:
+        numbers += [int(x) for x in line]
+    return (max_x, max_y, numbers)
+
+def get_neighbors(idx: int, max_x: int, max_y: int) -> list:
+    neighbors = list()
+    if idx >= max_x:
+        neighbors.append(idx - max_x)
+    if idx < (max_x * max_y - max_x):
+        neighbors.append(idx + max_x)
+    if idx % max_x != 0:
+        neighbors.append(idx - 1)
+    if idx % max_x != (max_x - 1):
+        neighbors.append(idx + 1)
+    return neighbors
+
+def get_low_points(max_x, max_y, grid) -> list:
+    low_points = []
+    for i in range(len(grid)):
+        if all(grid[n] > grid[i] for n in get_neighbors(i, max_x, max_y)):
+            low_points.append(i)
+    return low_points
+
+def expand(point, max_x, max_y, grid) -> set:
+    result = set()
+    l = {point}
+    while len(l) != 0:
+        p = l.pop()
+        result.add(p)
+        neighbors = {n for n in get_neighbors(p, max_x, max_y) if grid[n] != 9 and n not in result}
+        l = l.union(neighbors)
+    return result
+
+def part1(max_x, max_y, grid) -> int:
+    low_points = get_low_points(max_x, max_y, grid)
+    return sum([grid[point] for point in low_points]) + len(low_points)
+
+def part2(max_x, max_y, grid) -> int:
+    low_points = get_low_points(max_x, max_y, grid)
+    basin_sizes = [len(expand(point, max_x, max_y, grid)) for point in low_points]
+    basin_sizes.sort(reverse=True)
+    return basin_sizes[0] * basin_sizes[1] * basin_sizes[2]
+    
+if __name__ == "__main__":
+    vals = read_file()
+    print(f"Part 1: {part1(*vals)}")
+    print(f"Part 2: {part2(*vals)}")

2021/09/input.txt

@@ -0,0 +1,100 @@
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2021/09/solution.txt

@@ -0,0 +1,2 @@
+Part 1: 504
+Part 2: 1558722

2021/09/test_code.py

@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: MIT
+# Copyright (c) 2021 Akumatic
+
+from code import part1, part2, read_file
+
+def test():
+    vals = read_file("test_input.txt")
+    assert part1(*vals) == 15
+    print("Passed Part 1")
+    assert part2(*vals) == 1134
+    print("Passed Part 2")
+
+if __name__ == "__main__":
+    test()

2021/09/test_input.txt

@@ -0,0 +1,5 @@
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678

2021/README.md

@@ -23,7 +23,7 @@ Collect stars by solving puzzles. Two puzzles will be made available on each day
 | 06 | :white_check_mark: | :white_check_mark: | [Solution](06/code.py) | [Day 06](https://adventofcode.com/2021/day/6) |
 | 07 | :white_check_mark: | :white_check_mark: | [Solution](07/code.py) | [Day 07](https://adventofcode.com/2021/day/7) |
 | 08 | :white_check_mark: | :white_check_mark: | [Solution](08/code.py) | [Day 08](https://adventofcode.com/2021/day/8) |
-| 09 |  |  |  |  |
+| 09 | :white_check_mark: | :white_check_mark: | [Solution](09/code.py) | [Day 09](https://adventofcode.com/2021/day/9) |
 | 10 |  |  |  |  |
 | 11 |  |  |  |  |
 | 12 |  |  |  |  |