Day 6 part 2 solution.

day6/solver.py

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+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)]
+    chars = ['^', '>', 'v', '<']
+
+    def __init__(self, angle: int):
+        self.angle = angle
+
+    @classmethod
+    def parse(cls, char: str):
+        angle = cls.chars.index(char)
+        return cls(angle)
+
+    @property
+    def step(self):
+        return self.steps[self.angle]
+
+    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')
+
+    def __init__(self, x: int, y: int):
+        self.x = x
+        self.y = y
+
+    def __repr__(self):
+        return f"<Position x={self.x} y={self.y}>"
+
+    def move(self, direction: Direction):
+        xdiff, ydiff = direction.step 
+        return Position(self.x + xdiff, self.y + ydiff)
+
+    def __hash__(self):
+        return hash((self.x, self.y))
+
+    def __eq__(self, other):
+        return self.x == other.x and self.y == other.y
+
+
+class LabMap:
+    def __init__(self, width: int, height: int, obstacles: set[Position]):
+        self.width = width
+        self.height = height
+        self.obstacles = obstacles
+
+    def in_map(self, pos: Position):
+        """
+        Test whether a given position is in the map.
+        """
+        return (0 <= pos.x < self.width) and (0 <= pos.y < self.height)
+
+    def is_obstacle(self, pos: Position):
+        """
+        Test whether the position is an obstacle.
+        """
+        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
+
+        self.visited: set[tuple[Position, Direction]] = set()
+        if pos is not None:
+            self.visited.add((pos, direction))
+
+    def visit(self, pos: Position):
+        self.pos = 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.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]):
+        self.map = map
+        self.guards = guards
+
+        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:
+        """
+        Create a Simulation from a starting string.
+        """
+        if not simstring:
+            raise ValueError("Empty simulation string provided")
+
+        obstacles = []
+        guards = []
+        lines = simstring.splitlines()
+        width = len(lines[0])
+        height = len(lines)
+        for i, line in enumerate(simstring.splitlines()):
+            line = line.strip()
+            if not line:
+                continue
+            for j, char in enumerate(line):
+                match char:
+                    case '#':
+                        pos = Position(j, i)
+                        obstacles.append(pos)
+                    case '>' | 'v' | '<' | '^':
+                        pos = Position(j, i)
+                        direction = Direction.parse(char)
+                        guard = Guard(pos, direction)
+                        guards.append(guard)
+                    case '.':
+                        pass
+                    case _:
+                        logger.warning(f"Unknown map char: '{char}'")
+
+        map = LabMap(width, height, set(obstacles))
+        return cls(map, guards)
+
+    def to_string(self) -> str:
+        """
+        Create a simulation map string.
+        Mainly for debugging.
+        """
+        lines = []
+
+        # Initialise the list
+        for _ in range(self.map.height):
+            line = self.map.width * ['.']
+            lines.append(line)
+
+        for obstacle in self.map.obstacles:
+            lines[obstacle.y][obstacle.x] = '#'
+
+        for guard in self.guards:
+            pos = guard.pos
+            if pos is not None:
+                lines[pos.y][pos.x] = str(guard.direction)
+
+        return '\n'.join(''.join(line) for line in lines)
+
+    def total_visited(self) -> int:
+        return sum(guard.total_visited() for guard in self.guards)
+
+    def tick(self):
+        """
+        Tick the simulation.
+        """
+        looping = None
+        for guard in self.guards:
+            if (pos := guard.pos) is not None:
+                pos = pos.move(guard.direction)
+                if self.map.in_map(pos):
+                    if self.map.is_obstacle(pos):
+                        guard.turn()
+                    else:
+                        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())
+
+    def run(self):
+        if not self.guards:
+            raise ValueError("Simulation has no guards to move!")
+        while any(guard.pos is not None for guard in self.guards):
+            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()
+    print("Loading Simulation")
+    simulation = Simulation.from_string(map)
+    print("Running Simulation")
+    simulation.run()
+    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

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+from solver import Simulation, find_good_obstacles
+
+
+test_data = r"""
+....#.....
+.........#
+..........
+..#.......
+.......#..
+..........
+.#..^.....
+........#.
+#.........
+......#...
+""".strip()
+
+def test_visited():
+    print("Beginning visited test")
+    sim = Simulation.from_string(test_data)
+    sim.run()
+    visited = sim.total_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()