2021 day 9.

src/holt59/aoc/2021/day9.py

@@ -1,13 +1,44 @@
 import sys
-from collections import defaultdict
-from dataclasses import dataclass
+from math import prod
 
-lines = sys.stdin.read().splitlines()
+values = [[int(c) for c in row] for row in sys.stdin.read().splitlines()]
+n_rows, n_cols = len(values), len(values[0])
+
+
+def neighbors(point: tuple[int, int]):
+    i, j = point
+    for di, dj in ((-1, 0), (+1, 0), (0, -1), (0, +1)):
+        if 0 <= i + di < n_rows and 0 <= j + dj < n_cols:
+            yield (i + di, j + dj)
+
+
+def basin(start: tuple[int, int]) -> set[tuple[int, int]]:
+    visited: set[tuple[int, int]] = set()
+    queue = [start]
+
+    while queue:
+        i, j = queue.pop()
+
+        if (i, j) in visited or values[i][j] == 9:
+            continue
+
+        visited.add((i, j))
+        queue.extend(neighbors((i, j)))
+
+    return visited
+
+
+low_points = [
+    (i, j)
+    for i in range(n_rows)
+    for j in range(n_cols)
+    if all(values[ti][tj] > values[i][j] for ti, tj in neighbors((i, j)))
+]
 
 # part 1
-answer_1 = ...
+answer_1 = sum(values[i][j] + 1 for i, j in low_points)
 print(f"answer 1 is {answer_1}")
 
 # part 2
-answer_2 = ...
+answer_2 = prod(sorted(len(basin(point)) for point in low_points)[-3:])
 print(f"answer 2 is {answer_2}")