Logger Rate Limiter

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Description

Design a logger system that receives a stream of messages along with their timestamps. Each unique message should only be printed at most every 10 seconds (i.e. a message printed at timestamp t will prevent other identical messages from being printed until timestamp t + 10).

All messages will come in chronological order. Several messages may arrive at the same timestamp.

Implement the Logger class:

• Logger() Initializes the logger object.
• bool shouldPrintMessage(int timestamp, string message) Returns true if the message should be printed in the given timestamp, otherwise returns false.

Example 1:

• Input:

["Logger", "shouldPrintMessage", "shouldPrintMessage", "shouldPrintMessage", "shouldPrintMessage", "shouldPrintMessage", "shouldPrintMessage"]
[[], [1, "foo"], [2, "bar"], [3, "foo"], [8, "bar"], [10, "foo"], [11, "foo"]]

• Output: [null, true, true, false, false, false, true]

• Explanation:

Logger logger = new Logger();
logger.shouldPrintMessage(1, "foo");  // return true, next allowed timestamp for "foo" is 1 + 10 = 11
logger.shouldPrintMessage(2, "bar");  // return true, next allowed timestamp for "bar" is 2 + 10 = 12
logger.shouldPrintMessage(3, "foo");  // 3 < 11, return false
logger.shouldPrintMessage(8, "bar");  // 8 < 12, return false
logger.shouldPrintMessage(10, "foo"); // 10 < 11, return false
logger.shouldPrintMessage(11, "foo"); // 11 >= 11, return true, next allowed timestamp for "foo" is 11 + 10 = 21

Constraints:

• 0 <= timestamp <= 10^9
• Every timestamp will be passed in non-decreasing order (chronological order).
• 1 <= message.length <= 30
• At most 10^4 calls will be made to shouldPrintMessage.

Solution

Way 1

class Logger {
private:
    unordered_map<string, int> msgToTime;
public:
    Logger() {

    }

    bool shouldPrintMessage(int timestamp, string message) {
        if (!msgToTime.count(message)) {
            msgToTime[message] = timestamp;
            return true;
        } 

        if (timestamp - msgToTime[message] >= 10) {
            msgToTime[message] = timestamp;
            return true;
        } else {
            return false;
        }
    }
};

/**
 * Your Logger object will be instantiated and called as such:
 * Logger* obj = new Logger();
 * bool param_1 = obj->shouldPrintMessage(timestamp,message);
 */

• Time complexity: $O(1)$;
• Space complexity: $O(M)$, where $M$ is is the size of all incoming messages. Over the time, the hash table would have an entry for each unique message that has appeared.

Another way (reduce space complexity, but add time complexity):

class Logger {
private:
    unordered_map<string, int> msgToTime;
public:
    Logger() {

    }

    bool shouldPrintMessage(int timestamp, string message) {
        auto it = msgToTime.begin();
        while (it != msgToTime.end()) {
            if (timestamp - it->second >= 10) {
                it = msgToTime.erase(it);
            } else {
                ++it;
            }
        }

        if (!msgToTime.count(message)) {
            msgToTime[message] = timestamp;
            return true;
        }

        if (timestamp - msgToTime[message] >= 10) {
            msgToTime[message] = timestamp;
            return true;
        } else {
            return false;
        }
    }
};