From cfe67b7889885977e5b978b10ebf9738c8dbc236 Mon Sep 17 00:00:00 2001
From: Stefan Harmuth <pennywise@drock.de>
Date: Wed, 14 Dec 2022 19:35:58 +0100
Subject: [PATCH] day12 - improved

---
 day12.py | 38 +++++++++++++++++++++++++++++---------
 1 file changed, 29 insertions(+), 9 deletions(-)

diff --git a/day12.py b/day12.py
index a8dbc88..25b5527 100644
--- a/day12.py
+++ b/day12.py
@@ -1,3 +1,4 @@
+from collections import deque
 from heapq import heappush, heappop
 from tools.aoc import AOCDay
 from tools.coordinate import Coordinate, DistanceAlgorithm
@@ -47,6 +48,32 @@ def get_dijkstra_path(height_map: Grid, start: Coordinate, end: Coordinate) -> i
         return steps
 
 
+def get_bfs_path(height_map: Grid, end: Coordinate) -> int:
+    v = {end: None}
+    q = deque()
+    q.append(end)
+    while q:
+        cur = q.popleft()
+        cur_v = height_map.get(cur)
+        if cur_v == 0:
+            end = cur
+            break
+
+        for c in height_map.getNeighboursOf(cur, includeDiagonal=False):
+            if c in v or height_map.get(c) + 1 < cur_v:
+                continue
+
+            v[c] = cur
+            q.append(c)
+
+    steps = 0
+    while end in v:
+        steps += 1
+        end = v[end]
+
+    return steps
+
+
 class Day(AOCDay):
     inputs = [
         [
@@ -84,15 +111,8 @@ class Day(AOCDay):
         return get_dijkstra_path(*self.get_map())
 
     def part2(self) -> Any:
-        min_steps = float("inf")
-        height_map, start, end = self.get_map()
-        for c in height_map.getActiveCells():
-            if height_map.get(c) == 0:
-                steps_from_here = get_dijkstra_path(height_map, c, end)
-                if steps_from_here is not None and steps_from_here + 1 < min_steps:
-                    min_steps = steps_from_here + 1
-
-        return min_steps - 1
+        height_map, _, end = self.get_map()
+        return get_bfs_path(height_map, end) - 1
 
 
 if __name__ == '__main__':