Day 6 part 2 solution.

day6/solver.py

@@ -1,10 +1,15 @@
 import sys
+from copy import copy, deepcopy
+import operator as op
 from typing import Optional, Self, TypeAlias
 import logging
 
 
 logger = logging.getLogger()
 
+class SimulationLooping(Exception):
+    ...
+
 
 class Direction:
     steps = [(0, -1), (1, 0), (0, 1), (-1, 0)]
@@ -25,9 +30,18 @@ class Direction:
     def __str__(self):
         return self.chars[self.angle]
 
+    def __repr__(self):
+        return f"<Direction angle={self.angle} facing={str(self)}>"
+
     def turn(self, amount=1):
         return Direction((self.angle + amount) % 4)
 
+    def __hash__(self):
+        return hash(self.angle)
+
+    def __eq__(self, other):
+        return self.angle == other.angle
+
 
 class Position:
     __slots__ = ('x', 'y')
@@ -68,30 +82,43 @@ class LabMap:
         """
         return pos in self.obstacles
 
+    def __deepcopy__(self, memo):
+        return LabMap(self.width, self.height, copy(self.obstacles))
+
 
 class Guard:
     def __init__(self, pos: Optional[Position], direction: Direction):
         self.pos = pos
         self.direction = direction
 
-        # TODO: If cycles are a concern, make this (pos, direction)
-        self.visited: set[Position] = set()
+        self.visited: set[tuple[Position, Direction]] = set()
         if pos is not None:
-            self.visited.add(pos)
+            self.visited.add((pos, direction))
 
     def visit(self, pos: Position):
         self.pos = pos
-        self.visited.add(pos)
+        key = (pos, self.direction)
+        seen_before = key in self.visited
+        self.visited.add(key)
+        return seen_before
+
+    def get_path(self):
+        return set(map(op.itemgetter(0), self.visited))
 
     def leave(self):
         self.pos = None
 
     def total_visited(self):
-        return len(self.visited)
+        return len(self.get_path())
 
     def turn(self):
         self.direction = self.direction.turn()
 
+    def __deepcopy__(self, memo):
+        guard = Guard(self.pos, self.direction)
+        guard.visited = copy(self.visited)
+        return guard
+
 
 class Simulation:
     def __init__(self, map: LabMap, guards: list[Guard]):
@@ -100,6 +127,12 @@ class Simulation:
 
         self.ticker = 0
 
+    def __deepcopy__(self, memo):
+        sim = Simulation(deepcopy(self.map, memo), deepcopy(self.guards, memo))
+        sim.ticker = self.ticker
+        return sim
+
+
     @classmethod
     def from_string(cls, simstring: str) -> Self:
         """
@@ -164,6 +197,7 @@ class Simulation:
         """
         Tick the simulation.
         """
+        looping = None
         for guard in self.guards:
             if (pos := guard.pos) is not None:
                 pos = pos.move(guard.direction)
@@ -171,11 +205,15 @@ class Simulation:
                     if self.map.is_obstacle(pos):
                         guard.turn()
                     else:
-                        guard.visit(pos)
+                        looper = guard.visit(pos)
+                        looping = looper if looping is None else looping & looper
                 else:
                     guard.leave()
         self.ticker += 1
 
+        if looping:
+            raise SimulationLooping("All guards are looping!")
+
         # print(f"Tick {self.ticker}")
         # print(self.to_string())
 
@@ -186,6 +224,33 @@ class Simulation:
             self.tick()
 
 
+def find_good_obstacles(sim: Simulation):
+    """
+    Examine alternate universes to compute a list of good obstacle locations.
+
+    NOTE: This is far from the most efficient approach.
+    We could also memoise guard paths from each alt universe simulation
+    to 'fast-forward' time between changes.
+    """
+    guard = sim.guards[0]
+    good_obstacles = []
+    tried_obstacles = set()
+
+    while any(guard.pos is not None for guard in sim.guards):
+        sim_alt = deepcopy(sim)
+        sim.tick()
+        pos = guard.pos
+        if pos is not None and pos not in tried_obstacles:
+            sim_alt.map.obstacles.add(pos)
+            try:
+                sim_alt.run()
+            except SimulationLooping:
+                good_obstacles.append(pos)
+            tried_obstacles.add(pos)
+
+    return good_obstacles
+
+
 def main():
     with open(sys.argv[1]) as file:
         map = file.read().strip()
@@ -196,6 +261,12 @@ def main():
     print(f"Simulation complete after {simulation.ticker} steps.")
     print(f"Total nodes visited: {simulation.total_visited()}")
 
+    print("Loading Simulation again")
+    simulation = Simulation.from_string(map)
+    print("Searching alternate universes for good obtacle placement.")
+    result = find_good_obstacles(simulation)
+    print(f"Found {len(result)} good placements for obstacles!")
+
 
 if __name__ == '__main__':
     main()

day6/test.py

@@ -1,4 +1,4 @@
-from solver import Simulation
+from solver import Simulation, find_good_obstacles
 
 
 test_data = r"""
@@ -22,6 +22,14 @@ def test_visited():
     assert visited == 41
     print("Visited test passed")
 
+def test_loop_obstacles():
+    print("Beginning loop inducing obstacle placement test")
+    sim = Simulation.from_string(test_data)
+    places = find_good_obstacles(sim)
+    assert len(places) == 6
+    print(f"Loop inducing obstacle placement test complete")
+
 
 if __name__ == '__main__':
     test_visited()
+    test_loop_obstacles()