117 lines
3 KiB
Python
117 lines
3 KiB
Python
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#!/usr/bin/env python
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import enum
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import itertools
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import sys
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from typing import NamedTuple
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class Point(NamedTuple):
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x: int
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y: int
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class Direction(enum.StrEnum):
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UP = "^"
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RIGHT = ">"
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DOWN = "v"
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LEFT = "<"
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def step(self, p: Point) -> Point:
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dx: int
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dy: int
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match self:
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case Direction.UP:
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dx, dy = -1, 0
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case Direction.RIGHT:
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dx, dy = 0, 1
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case Direction.DOWN:
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dx, dy = 1, 0
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case Direction.LEFT:
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dx, dy = 0, -1
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return Point(p.x + dx, p.y + dy)
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def turn_right(self) -> "Direction":
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match self:
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case Direction.UP:
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return Direction.RIGHT
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case Direction.RIGHT:
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return Direction.DOWN
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case Direction.DOWN:
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return Direction.LEFT
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case Direction.LEFT:
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return Direction.UP
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class Guard(NamedTuple):
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pos: Point
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dir: Direction
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def patrol_step(self, blockers: set[Point]) -> "Guard":
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next_step = self.dir.step(self.pos)
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if next_step not in blockers:
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return Guard(next_step, self.dir)
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return Guard(self.pos, self.dir.turn_right())
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def solve(input: str) -> int:
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def parse(input: list[str]) -> tuple[Guard, set[Point]]:
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guard: Guard | None = None
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blockers: set[Point] = set()
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for x, line in enumerate(input):
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for y, c in enumerate(line):
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if c == ".":
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continue
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if c == "#":
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blockers.add(Point(x, y))
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continue
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guard = Guard(Point(x, y), Direction(c))
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assert guard is not None # Sanity check
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return guard, blockers
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def loops(guard: Guard, blockers: set[Point], dims: Point) -> int:
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max_x, max_y = dims
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visited: set[Guard] = set()
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while True:
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if not (0 <= guard.pos.x < max_x):
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break
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if not (0 <= guard.pos.y < max_y):
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break
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if guard in visited:
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return True
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visited.add(guard)
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guard = guard.patrol_step(blockers)
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return False
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def count_obstructions(guard: Guard, blockers: set[Point], dims: Point) -> int:
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res = 0
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max_x, max_y = dims
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for x, y in itertools.product(range(max_x), range(max_y)):
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candidate = Point(x, y)
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if candidate in blockers:
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continue
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if candidate == guard.pos:
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continue
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if not loops(guard, blockers | {candidate}, dims):
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continue
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res += 1
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return res
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lines = input.splitlines()
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guard, blockers = parse(lines)
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max_x, max_y = len(lines), len(lines[0])
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return count_obstructions(guard, blockers, Point(max_x, max_y))
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def main() -> None:
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input = sys.stdin.read()
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print(solve(input))
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if __name__ == "__main__":
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main()
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