six.py

eg = """....#.....
.........#
..........
..#.......
.......#..
..........
.#..^.....
........#.
#.........
......#...
"""

class LoopError(Exception):
    def __init__(self, *args):
        super().__init__(*args)

class Guard:
    facing: str
    x: int
    y: int
    visited: set
    turns: dict

    def __init__(self, facing, x, y):
        self.facing = facing
        self.x = x
        self.y = y
        self.visited = set()
        self.visited.add((y,x))
        self.turns = {}

    def check(self):
        pos = (self.y,self.x)
        if pos in self.turns.keys():
            if len(self.turns[pos]) > 2:
                raise LoopError
            self.turns[pos].append(self.facing)
        else:
            self.turns[pos] = [self.facing]
    
def walk(guard, grid, debug):
    if len(guard.visited) > 0 and debug:
        print(f"turning {guard.facing} at ({guard.y},{guard.x}) - visited {len(guard.visited)} spaces")
    guard.check()
    if guard.facing == "up":
        for i in reversed(range(0, guard.y)):
            if grid[i][guard.x] == "#":
                guard.facing = "right"
                return walk(guard, grid, debug)
            else:
                guard.y = i
                guard.visited.add((i, guard.x))
    elif guard.facing == "down":
        for i in range(guard.y, len(grid)):
            if grid[i][guard.x] == "#":
                guard.facing = "left"
                return walk(guard, grid, debug)
            else:
                guard.y = i
                guard.visited.add((i, guard.x))
    elif guard.facing == "right":
        for i in range(guard.x, len(grid)):
            if grid[guard.y][i] == "#":
                guard.facing = "down"
                return walk(guard, grid, debug)
            else:
                guard.x = i
                guard.visited.add((guard.y, i))
    elif guard.facing == "left":
        for i in reversed(range(0, guard.x)):
            if grid[guard.y][i] == "#":
                guard.facing = "up"
                return walk(guard, grid, debug)
            else:
                guard.x = i
                guard.visited.add((guard.y, i))

def one(source, debug):
    lines = source.splitlines()
    blockers = []
    guard = (-1,-1)
    for y in range(0, len(lines)):
        line = lines[y]
        for x in range(0, len(line)):
            if line[x] == "^":
                guard = Guard(facing="up", x=x, y=y)
            elif line[x] == "#":
                blockers.append((y,x))
    if debug:
        print(blockers)
    walk(guard, lines, debug)
    print(f"guard at ({guard.y},{guard.x}) ({guard.facing}) has visited {len(guard.visited)} spaces")
        
one(eg, True)
print("---")
with open("six.txt", "r") as file:
    one(file.read(), False)
print("---")

def two(source):
    lines = source.splitlines()
    blockers = []
    guard = (-1,-1)
    for y in range(0, len(lines)):
        line = lines[y]
        for x in range(0, len(line)):
            if line[x] == "^":
                guard = Guard(facing="up", x=x, y=y)
            elif line[x] == "#":
                blockers.append((y,x))

    start = (guard.y, guard.x)
    walk(guard, lines, False)
    count = 0
    nodes = set(guard.visited)
    nodes.remove(start)
    for y,x in nodes:
        grid = lines.copy()
        line = grid[y]
        grid[y] = line[:x] + '#' + line[x+1:]
        try:
            guard = Guard(facing="up", x=start[1], y=start[0])
            walk(guard, grid, False)
        except LoopError:
            count += 1
            #print(f"obstruction at ({y},{x}) caused loop")
    print(f"found {count} loops")


two(eg)
print("---")
with open("six.txt", "r") as file:
    two(file.read())