problems

problems/hard/rate-limiter/solution.py

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+"""
+Rate Limiter
+
+You're building an API gateway for a SaaS platform. To prevent abuse
+and ensure fair usage, you need to implement a rate limiter that tracks
+requests per user using a sliding window algorithm.
+
+The limiter should allow at most `max_requests` per user within any
+`window_seconds` time period.
+
+Example 1:
+    limiter = RateLimiter(max_requests=3, window_seconds=60)
+    limiter.allow_request("user1", timestamp=0)   # True (1st request)
+    limiter.allow_request("user1", timestamp=30)  # True (2nd request)
+    limiter.allow_request("user1", timestamp=45)  # True (3rd request)
+    limiter.allow_request("user1", timestamp=50)  # False (limit reached)
+    limiter.allow_request("user1", timestamp=61)  # True (1st expired)
+
+Example 2:
+    limiter = RateLimiter(max_requests=2, window_seconds=10)
+    limiter.allow_request("user1", timestamp=0)   # True
+    limiter.allow_request("user2", timestamp=0)   # True (different user)
+    limiter.allow_request("user1", timestamp=5)   # True
+    limiter.allow_request("user1", timestamp=8)   # False
+
+Constraints:
+    - max_requests >= 1
+    - window_seconds >= 1
+    - Timestamps are non-negative integers (seconds)
+    - Timestamps are non-decreasing per user
+    - user_id is a non-empty string
+"""
+
+
+class RateLimiter:
+    """Sliding window rate limiter for API request throttling."""
+
+    def __init__(self, max_requests: int, window_seconds: int):
+        """Initialize with request limit and time window."""
+        pass  # Your implementation here
+
+    def allow_request(self, user_id: str, timestamp: int) -> bool:
+        """Return True if request allowed, False if rate limited."""
+        pass  # Your implementation here
+
+    def get_remaining(self, user_id: str, timestamp: int) -> int:
+        """Return remaining requests allowed for user at timestamp."""
+        pass  # Your implementation here
+