From b23f24c567bed25913a870d205a568ec815b0dee Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Mon, 16 Dec 2024 18:16:05 +0100
Subject: [PATCH] Implement 2024 day 16 part 2

---
 2024/src/aoc/days/day16.py | 52 ++++++++++++++++++++++++++------------
 1 file changed, 36 insertions(+), 16 deletions(-)

diff --git a/2024/src/aoc/days/day16.py b/2024/src/aoc/days/day16.py
index 05ebdf0..3ecbd33 100644
--- a/2024/src/aoc/days/day16.py
+++ b/2024/src/aoc/days/day16.py
@@ -2,7 +2,7 @@
 
 import numpy
 
-from . import SeparateRunner
+from . import CombinedRunner
 
 TURNS = (
     (-1, 1),
@@ -10,9 +10,9 @@
 )
 
 
-class DayRunner(SeparateRunner):
+class DayRunner(CombinedRunner):
     @classmethod
-    def part1(cls, input: str) -> int:
+    def run_both(cls, input: str) -> tuple[int, int]:
         grid = numpy.array([list(line) for line in input.strip().split("\n")])
 
         y, x = numpy.where(grid == "S")
@@ -20,37 +20,57 @@ def part1(cls, input: str) -> int:
 
         todo = [(0, x, y, 1, 0)]
         best = {
-            (x, y, 1, 0): 0,
+            (x, y, 1, 0): (0, []),
         }
 
-        def enqueue(dist, x, y, dx, dy):
+        def enqueue(dist, x, y, dx, dy, cx, cy, cdx, cdy):
             if grid[y, x] == "#":
                 return
 
-            if (x, y, dx, dy) not in best or best[x, y, dx, dy] > dist:
-                best[x, y, dx, dy] = dist
+            if (x, y, dx, dy) not in best or best[x, y, dx, dy][0] > dist:
+                best[x, y, dx, dy] = (dist, [(cx, cy, cdx, cdy)])
                 heapq.heappush(todo, (dist, x, y, dx, dy))
+            elif best[x, y, dx, dy][0] == dist:
+                best[x, y, dx, dy][1].append((cx, cy, cdx, cdy))
+            
+        shortest_dist = None
+        finishes = set()
 
         while todo:
             dist, x, y, dx, dy = heapq.heappop(todo)
 
-            if best[x, y, dx, dy] < dist:
+            if best[x, y, dx, dy][0] < dist:
                 continue
 
+            if shortest_dist is not None and shortest_dist < dist:
+                break
+
             if grid[y, x] == "E":
-                return dist
+                shortest_dist = dist
+                finishes.add((x, y, dx, dy))
 
-            enqueue(dist + 1, x + dx, y + dy, dx, dy)
-            enqueue(dist + 2001, x - dx, y - dy, dx, dy)
+            enqueue(dist + 1, x + dx, y + dy, dx, dy, x, y, dx, dy)
+            enqueue(dist + 2001, x - dx, y - dy, dx, dy, x, y, dx, dy)
 
             for tx, ty in TURNS:
                 ndx = dy * ty
                 ndy = dx * ty
 
-                enqueue(dist + 1001, x + ndx, y + ndy, ndx, ndy)
+                enqueue(dist + 1001, x + ndx, y + ndy, ndx, ndy, x, y, dx, dy)
 
-        raise ValueError("Did not find path to exit")
+        assert shortest_dist is not None, "Should find a path to the exit"
 
-    @classmethod
-    def part2(cls, input: str) -> int:
-        pass
+        visited_tiles = {(x, y) for x, y, _, _ in finishes}
+        todo2 = [f for f in finishes]
+        visited_states = set(todo2)
+
+        while todo2:
+            state = todo2.pop()
+
+            for prev in best[state][1]:
+                if prev not in visited_states:
+                    visited_states.add(prev)
+                    visited_tiles.add((prev[0], prev[1]))
+                    todo2.append(prev)
+
+        return shortest_dist, len(visited_tiles)