import time
from collections import defaultdict


class RateLimiter:
    """Fixed-window limiter keyed by an arbitrary string (user id or client IP)."""

    def __init__(self, max_requests: int, window_seconds: float):
        self.max_requests = max_requests
        self.window_seconds = window_seconds
        self._hits: dict[str, list[float]] = defaultdict(list)

    def allow(self, key: str) -> bool:
        now = time.monotonic()
        window_start = now - self.window_seconds
        hits = self._hits[key]
        while hits and hits[0] < window_start:
            hits.pop(0)
        if len(hits) >= self.max_requests:
            return False
        hits.append(now)
        return True