From 95c244102fdc06268e0ca1a4227148d94081de8a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Anders=20Ytterstr=C3=B6m?= <yttan@fastmail.se>
Date: Thu, 9 Dec 2021 11:00:28 +0100
Subject: [PATCH] Solve 2021:9 "Smoke Basin" part 2

Yet again I got stuck in the common AOC trap: The example input worked,
but not the actual input.

2 things got me stuck.

> The size of a basin is the number of locations within the basin, including the low point. The example above has four basins.

1. I missed the obvious part to only check the low points.

2. Based on the example, I asumed that the adjacent locations would
   increase by one to count. This is wrong: What matters is that their
   height is a larger value.

Way too long time for a simple problem. Never read puzzles sloppy.
---
 2021-python/solutions/day_09.py  | 46 ++++++++++++++++++++++++++++----
 2021-python/tests/test_day_09.py |  4 +--
 2 files changed, 43 insertions(+), 7 deletions(-)

diff --git a/2021-python/solutions/day_09.py b/2021-python/solutions/day_09.py
index 451805c..a09480c 100644
--- a/2021-python/solutions/day_09.py
+++ b/2021-python/solutions/day_09.py
@@ -1,4 +1,5 @@
 from solutions import BaseSolution
+from typing import List, Tuple
 
 
 class Solution(BaseSolution):
@@ -11,7 +12,37 @@ class Solution(BaseSolution):
         return [[int(c) for c in r] for r in data.split()]
 
     def solve(self, puzzle_input):
-        lp = 0
+        lp = self._lowpoints(puzzle_input)
+        return sum(lpv[1] + 1 for lpv in lp)
+
+    def solve_again(self, puzzle_input):
+        offsets = (-1, 0), (0, 1), (1, 0), (0, -1)
+        my = len(puzzle_input)
+        mx = len(puzzle_input[0])
+
+        basins = []
+
+        def basin(y, x, value, seen=set()):
+            if value < 9:
+                seen.add((value, (y, x)))
+                for oy, ox in offsets:
+                    sy = y + oy
+                    sx = x + ox
+                    if any((sy < 0, sx < 0, sy >= my, sx >= mx)):
+                        continue
+                    s = puzzle_input[sy][sx]
+                    if s > value:
+                        seen = basin(sy, sx, s, seen)
+            return seen
+
+        for lp, value in self._lowpoints(puzzle_input):
+            y, x = lp
+            basins.append(len(basin(y, x, value, set())))
+        p1, p2, p3, *_ = sorted(basins, reverse=True)
+        return p1 * p2 * p3
+
+    def _lowpoints(self, puzzle_input):
+        lp = []
         lpi = len(puzzle_input)
         for v, row in enumerate(puzzle_input):
             lr = len(row)
@@ -22,13 +53,18 @@ class Solution(BaseSolution):
                 s3 = puzzle_input[v - 1][i] if v > 0 else 11
                 s4 = puzzle_input[v + 1][i] if v < lpi - 1 else 11
                 if all(x < s for s in [s1, s2, s3, s4]):
-                    lp += 1 + x
+                    lp.append(((v, i), x))
         return lp
 
-    def solve_again(self, puzzle_input):
-        return True
-
 
 if __name__ == "__main__":
     solution = Solution()
     solution.show_results()
+
+"""
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+"""
diff --git a/2021-python/tests/test_day_09.py b/2021-python/tests/test_day_09.py
index 69e0225..118b12d 100644
--- a/2021-python/tests/test_day_09.py
+++ b/2021-python/tests/test_day_09.py
@@ -29,8 +29,8 @@ class Day09TestCase(unittest.TestCase):
     def test_solve_first_part(self):
         assert self.solution.solve(self.puzzle_input) == 15
 
-    # def test_solve_second_part(self):
-    #     assert self.solution.solve_again(self.puzzle_input) == True
+    def test_solve_second_part(self):
+        assert self.solution.solve_again(self.puzzle_input) == 9 * 14 * 9  # 891684
 
 
 if __name__ == "__main__":