Thread-safe Interval Average Calculator, C++ (clang)

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//clang 3.7.0 
// deepaknettem@gmail.com,
// code released in public domain 
// a simple interval based thread-safe avg calculator - for learning purposes. 
// in a concurrent (transactional) system, something like this is useful in tracking latencies, and pin-pointing
// the 'epoch' / 'time' at which latencies started going up or down.
#include <iostream>
#include <cstdint>
#include <algorithm>
#include <vector>
#include <functional>
#include <numeric>
#include <chrono>
#include <thread>
#include <memory>
#include <mutex>
#include <random>
#include <cassert>
#include <future>
#include <queue>

// A simple thread-safe class to calculate interval-based moving averages
template <typename T>
class IntervalAvg
{
public:
    IntervalAvg(
        const int64_t& intervalInSeconds) 
        : m_intervalInSeconds(intervalInSeconds),
        m_currentSecond(0),
        m_samples(std::make_shared<std::vector<T>>())
    {}
    
    void AddSample(const int64_t& sample)
    {
        // get current timestamp (outside lock)
        auto time_since_epoch =
            std::chrono::duration_cast<std::chrono::seconds>(std::chrono::system_clock::now().time_since_epoch());
        
        auto currentSecond = time_since_epoch.count();
        
        std::shared_ptr<std::vector<T>> prevSamples(nullptr);
        auto prevSecond = 0;
        
        // acquire mutex (lock) 
        {
            std::lock_guard<std::mutex> lck(m_mtx);
                
            if (m_currentSecond < currentSecond)
            {
                // capture older timestamp (for logging purposes), and update timestamp
                prevSecond = m_currentSecond;
                m_currentSecond = currentSecond;
                
                // copy shared_ptr within lock, so we hold a ref count to it
                prevSamples = m_samples;
                
                // update samples pointer
                m_samples = std::make_shared<std::vector<T>>();
            }
            else if (m_currentSecond > currentSecond)
            {
                std::cout << "Unexpected" << std::endl; 
            }   
            else            
            {
                m_samples->push_back(sample);
            }
         }
               
        // inside lock, only 1 thread would have been able to update the samples store
        // and hence has access to prevSamples
        // outside lock, calculate average in previous interval
        if (prevSamples != nullptr)
        {
            // sanity check
            assert(prevSamples.use_count() == 1);
            double sum = 0.0;
            if (prevSamples->size() > 0)
            {        
                for (const auto& elem : *prevSamples)
                {
                    sum+= elem;
                }
                double avg = sum / prevSamples->size();
                std::cout << " Time: " << prevSecond << " Average: " << avg << "  Sample Count " << prevSamples->size() << std::endl;
            }
        }    
    }

private:    
    
    int64_t m_intervalInSeconds;
    int64_t m_currentSecond;
    std::shared_ptr<std::vector<T>> m_samples;   
    std::mutex m_mtx;
};
 
void test_sequential()
{
    IntervalAvg<int64_t> avg(1);
    
    auto threadWork = [&avg](int64_t sample, int64_t sleepDuration = 50){
        std::this_thread::sleep_for(std::chrono::milliseconds(sleepDuration));
        avg.AddSample(sample);
    };

std::cout << "-------------------Test_Sequential-------------------------------" << std::endl;
    for (int i = 0; i < 40; i++)
    {
        threadWork(i);
    }
}

/* Using C++11 thread-pool implementation from https://github.com/galek/ThreadPool/blob/master/ThreadPool.h */
class ThreadPool {
public:
    explicit ThreadPool(std::size_t threads
        = (std::max)(2u, std::thread::hardware_concurrency() * 2));
    template<class F, class... Args>
    auto enqueue(F&& f, Args&&... args)
        -> std::future<typename std::result_of<F(Args...)>::type>;
    void wait_until_empty();
    void wait_until_nothing_in_flight();
    void set_queue_size_limit(std::size_t limit);
    ~ThreadPool();

private:
    // need to keep track of threads so we can join them
    std::vector< std::thread > workers;
    // the task queue
    std::queue< std::function<void()> > tasks;
    // queue length limit
    std::size_t max_queue_size = 100000;
    // stop signal
    bool stop = false;

// synchronization
    std::mutex queue_mutex;
    std::condition_variable condition_producers;
    std::condition_variable condition_consumers;

std::mutex in_flight_mutex;
    std::condition_variable in_flight_condition;
    std::atomic<std::size_t> in_flight;

struct handle_in_flight_decrement
    {
        ThreadPool & tp;

handle_in_flight_decrement(ThreadPool & tp_)
            : tp(tp_)
        { }

~handle_in_flight_decrement()
        {
            std::size_t prev
                = std::atomic_fetch_sub_explicit(&tp.in_flight,
                    std::size_t(1),
                    std::memory_order_acq_rel);
            if (prev == 1)
            {
                std::unique_lock<std::mutex> guard(tp.in_flight_mutex);
                tp.in_flight_condition.notify_all();
            }
        }
    };
};

// the constructor just launches some amount of workers
inline ThreadPool::ThreadPool(std::size_t threads)
    : in_flight(0)
{
    for(size_t i = 0;i<threads;++i)
        workers.emplace_back(
            [this]
            {
                for(;;)
                {
                    std::function<void()> task;
                    bool notify;

{
                        std::unique_lock<std::mutex> lock(this->queue_mutex);
                        this->condition_consumers.wait(lock,
                            [this]{ return this->stop || !this->tasks.empty(); });
                        if(this->stop && this->tasks.empty())
                            return;
                        task = std::move(this->tasks.front());
                        this->tasks.pop();
                        notify = this->tasks.size() + 1 ==  max_queue_size
                            || this->tasks.empty();
                    }

handle_in_flight_decrement guard(*this);

if (notify)
                    {
                        std::unique_lock<std::mutex> lock(this->queue_mutex);
                        condition_producers.notify_all();
                    }

task();
                }
            }
        );
}

// add new work item to the pool
template<class F, class... Args>
auto ThreadPool::enqueue(F&& f, Args&&... args)
    -> std::future<typename std::result_of<F(Args...)>::type>
{
    using return_type = typename std::result_of<F(Args...)>::type;

auto task = std::make_shared< std::packaged_task<return_type()> >(
            std::bind(std::forward<F>(f), std::forward<Args>(args)...)
        );

std::future<return_type> res = task->get_future();

std::unique_lock<std::mutex> lock(queue_mutex);
    if (tasks.size () >= max_queue_size)
        // wait for the queue to empty or be stopped
        condition_producers.wait(lock,
            [this]
            {
                return tasks.size () < max_queue_size
                    || stop;
            });

// don't allow enqueueing after stopping the pool
    if (stop)
        throw std::runtime_error("enqueue on stopped ThreadPool");

tasks.emplace([task](){ (*task)(); });
    std::atomic_fetch_add_explicit(&in_flight,
        std::size_t(1),
        std::memory_order_relaxed);
    condition_consumers.notify_one();

return res;
}

// the destructor joins all threads
inline ThreadPool::~ThreadPool()
{
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        stop = true;
        condition_consumers.notify_all();
        condition_producers.notify_all();
    }

for(std::thread &worker: workers)
        worker.join();

assert(in_flight == 0);
}

inline void ThreadPool::wait_until_empty()
{
    std::unique_lock<std::mutex> lock(this->queue_mutex);
    this->condition_producers.wait(lock,
        [this]{ return this->tasks.empty(); });
}

inline void ThreadPool::wait_until_nothing_in_flight()
{
    std::unique_lock<std::mutex> lock(this->in_flight_mutex);
    this->in_flight_condition.wait(lock,
        [this]{ return this->in_flight == 0; });
}

inline void ThreadPool::set_queue_size_limit(std::size_t limit)
{
    std::unique_lock<std::mutex> lock(this->queue_mutex);
    std::size_t const old_limit = max_queue_size;
    max_queue_size = (std::max)(limit, std::size_t(1));
    if (old_limit < max_queue_size)
        condition_producers.notify_all();
}

void test_concurrent()
{
    ThreadPool pool(8);
       
    IntervalAvg<int64_t> avg(5);
    
    auto threadWork = [&avg](int64_t sample, int64_t sleepDuration = 50){
        std::this_thread::sleep_for(std::chrono::milliseconds(sleepDuration));
        avg.AddSample(sample);
        return 0;
    };

std::cout << "-------------------Test_Concurrent-------------------------------" << std::endl;
    std::random_device rd;
    std::mt19937 mt(rd());
    std::uniform_int_distribution<int> dist(500, 4000);
    
    int numThreads = 50;
    std::vector<std::future<int>> results;
    for (int i = 0; i < numThreads; i++)
    {
        results.emplace_back(pool.enqueue(threadWork, i, dist(mt)));
    }
    
    pool.wait_until_empty();
    pool.wait_until_nothing_in_flight();
    
    for (auto && result : results)
        result.get();
}

int main()
{
    test_sequential();   
    test_concurrent();        
}

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Compilation time: 1.42 sec, absolute running time: 10.04 sec, cpu time: 0 sec, memory peak: 0 Mb, absolute service time: 11,48 sec