Payment API Rate Limiter

Problem Context

You are building a high-throughput payment API that needs to protect against abuse and ensure fair usage. Your task is to implement a Sliding Window Rate Limiter that tracks API requests per user and determines whether each incoming request should be allowed or rejected.

Problem Description

Implement a RateLimiter class using the sliding window algorithm. The rate limiter should allow a maximum of maxRequests requests per windowSizeInSeconds for each unique user.

The sliding window ensures that at any point in time, no user has made more than maxRequests requests in the last windowSizeInSeconds seconds.

Class Interface

class RateLimiter:
    def __init__(self, maxRequests: int, windowSizeInSeconds: int):
        # Initialize the rate limiter
        pass

    def allowRequest(self, userId: str, timestamp: int) -> bool:
        # Returns True if the request is allowed, False otherwise
        pass

Input / Output Format

Constructor:

• maxRequests — maximum number of requests allowed per window
• windowSizeInSeconds — the duration (in seconds) of the sliding window

allowRequest(userId, timestamp):

• userId — string identifier for the requesting user
• timestamp — integer representing the current time in seconds (non-decreasing per user)
• Returns True if the request is allowed, False if the user has exceeded their limit

Approach Hints

• Use a dictionary of deques to store timestamps of recent requests per user.
• On each call to allowRequest, evict timestamps that fall outside the sliding window (i.e., timestamp - windowSizeInSeconds).
• If the remaining count is less than maxRequests, allow the request and record the timestamp.
• Timestamps are guaranteed to be non-decreasing for a given sequence of calls.