ts_token_bucket_rate_limiter.cpp

#include <chrono>
#include <mutex>
#include <thread>

using namespace std;

/*
Problem:
Implement a thread-safe token bucket rate limiter.

Requirements:
1. Support multiple threads calling tryAcquire().
2. Rate = tokens added per second; capacity = max burst size.
3. Token refill should be based on elapsed time, not a background thread.
4. Keep the implementation and demo in a single file.

Follow-ups to practice:
- Add a blocking acquire(timeout).
- Add per-client buckets on top of a global bucket.
- Add metrics for granted/denied requests.
*/

class TokenBucketRateLimiter {
    double tokens = 0.0;
    double capacity = 0.0;
    double refill_rate_per_sec = 0.0;
    chrono::steady_clock::time_point last_refill = chrono::steady_clock::now();
    mutex mtx;

    void refillLocked() {
        // TODO:
        // 1. Compute elapsed time since last_refill.
        // 2. Add elapsed * refill_rate_per_sec to tokens.
        // 3. Cap at capacity.
        // 4. Update last_refill.
    }

public:
    TokenBucketRateLimiter(double rate_per_sec, double cap)
        : tokens(cap), capacity(cap), refill_rate_per_sec(rate_per_sec) {}

    bool tryAcquire(double cost = 1.0) {
        (void)cost;
        // TODO:
        // 1. Lock.
        // 2. Refill tokens.
        // 3. If enough tokens, deduct and return true.
        // 4. Otherwise return false.
        return false;
    }

    double approxTokens() {
        // TODO:
        // Optionally lock + refill, then return current token count.
        return 0.0;
    }
};

void client(TokenBucketRateLimiter& limiter, int id, int requests, int delay_ms) {
    (void)limiter;
    (void)id;
    (void)requests;
    (void)delay_ms;
    // TODO:
    // Simulate a caller issuing requests and printing ALLOWED/THROTTLED.
}

int main() {
    // TODO:
    // 1. Create limiter (rate + capacity).
    // 2. Spawn multiple client threads.
    // 3. Join threads and print final state.
    return 0;
}