From a1a099c7123f149fa01ee596afd07e97b9fae584 Mon Sep 17 00:00:00 2001
From: dece <shgck@pistache.land>
Date: Thu, 9 Dec 2021 14:25:21 +0100
Subject: [PATCH] Day 9

---
 2021/day9.py | 53 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)
 create mode 100644 2021/day9.py

diff --git a/2021/day9.py b/2021/day9.py
new file mode 100644
index 0000000..240318a
--- /dev/null
+++ b/2021/day9.py
@@ -0,0 +1,53 @@
+import sys
+
+
+def main():
+    lines = [line.rstrip() for line in sys.stdin]
+    nc = len(lines[0])
+    nr = len(lines)
+    m = [list(map(int, line)) for line in lines]
+
+    # Part 1
+    low_points = []
+    rlevels = 0
+    for x, row in enumerate(m):
+        for y, val in enumerate(row):
+            if (
+                (x == 0 or m[x - 1][y] > val)
+                and (x == nr - 1 or m[x + 1][y] > val)
+                and (y == 0 or m[x][y - 1] > val)
+                and (y == nc - 1 or m[x][y + 1] > val)
+            ):
+                low_points.append((x, y))  # for part 2
+                rlevels += val + 1
+    print(rlevels)
+
+    # Part 2
+    basins = [expand_basin(m, (x, y), nc, nr) for x, y in low_points]
+    a, b, c = sorted(map(len, basins))[-3:]
+    print(a * b * c)
+
+
+def expand_basin(m, start, nc, nr):
+    basin = []
+    sx, sy = start
+    explo = [(sx, sy)]
+    while explo:
+        pos = explo.pop(0)
+        x, y = pos
+        if m[x][y] == 9:
+            continue
+        basin.append(pos)
+        if x > 0 and (n := (x - 1, y)) not in basin and n not in explo:
+            explo.append(n)
+        if x < nr - 1 and (n := (x + 1, y)) not in basin and n not in explo:
+            explo.append(n)
+        if y > 0 and (n := (x, y - 1)) not in basin and n not in explo:
+            explo.append(n)
+        if y < nc - 1 and (n := (x, y + 1)) not in basin and n not in explo:
+            explo.append(n)
+    return basin
+
+
+if __name__ == "__main__":
+    main()