min_max_avg.hpp

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#pragma once
 
/// @file userver/utils/statistics/min_max_avg.hpp
/// @brief @copybrief utils::statistics::MinMaxAvg
 
#include <atomic>
#include <chrono>
#include <cstddef>
#include <type_traits>
 
#include <userver/formats/json/inline.hpp>
#include <userver/formats/serialize/to.hpp>
#include <userver/utils/assert.hpp>
#include <userver/utils/statistics/writer.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils::statistics {
 
/// @brief Class for concurrent safe calculation of minimum, maximum and
/// average over series of values.
template <typename ValueType, typename AverageType = ValueType>
class MinMaxAvg final {
    static_assert(
        std::is_integral_v<ValueType> && !std::is_same_v<ValueType, bool>,
        "only integral value types are supported in MinMaxAvg"
    );
    static_assert(
        std::is_same_v<AverageType, ValueType> || std::is_floating_point_v<AverageType>,
        "MinMaxAvg average type must either be equal to value type or "
        "be a floating point type"
    );
    static_assert(
        std::atomic<ValueType>::is_always_lock_free && std::atomic<size_t>::is_always_lock_free,
        "refusing to use locking atomics"
    );
 
public:
    struct Current {
        ValueType minimum;
        ValueType maximum;
        AverageType average;
    };
 
    constexpr MinMaxAvg() noexcept : minimum_(0), maximum_(0), sum_(0), count_(0) {}
 
    // NOLINTNEXTLINE(cppcoreguidelines-pro-type-member-init)
    MinMaxAvg(const MinMaxAvg& other) noexcept { *this = other; }
 
    MinMaxAvg& operator=(const MinMaxAvg& rhs) noexcept {
        if (this == &rhs) {
            return *this;
        }
 
        const auto count = rhs.count_.load(std::memory_order_acquire);
        minimum_ = rhs.minimum_.load(std::memory_order_relaxed);
        maximum_ = rhs.maximum_.load(std::memory_order_relaxed);
        sum_ = rhs.sum_.load(std::memory_order_relaxed);
        count_ = count;
        return *this;
    }
 
    Current GetCurrent() const {
        // NOLINTNEXTLINE(cppcoreguidelines-pro-type-member-init)
        Current current;
        const auto count = count_.load(std::memory_order_acquire);
        UASSERT(count >= 0);
        current.minimum = minimum_.load(std::memory_order_relaxed);
        current.maximum = maximum_.load(std::memory_order_relaxed);
        current.average =
            count ? static_cast<AverageType>(sum_.load(std::memory_order_relaxed)) / static_cast<AverageType>(count)
                  : AverageType{0};
        return current;
    }
 
    void Account(ValueType value) {
        ValueType current_minimum = minimum_.load(std::memory_order_relaxed);
        while (current_minimum > value || !count_.load(std::memory_order_relaxed)) {
            if (minimum_.compare_exchange_weak(current_minimum, value, std::memory_order_relaxed)) {
                break;
            }
        }
        ValueType current_maximum = maximum_.load(std::memory_order_relaxed);
        while (current_maximum < value || !count_.load(std::memory_order_relaxed)) {
            if (maximum_.compare_exchange_weak(current_maximum, value, std::memory_order_relaxed)) {
                break;
            }
        }
        sum_.fetch_add(value, std::memory_order_relaxed);
        count_.fetch_add(1, std::memory_order_release);
    }
 
    template <class Duration = std::chrono::seconds>
    void Add(
        const MinMaxAvg& other,
        [[maybe_unused]] Duration this_epoch_duration = Duration(),
        [[maybe_unused]] Duration before_this_epoch_duration = Duration()
    ) {
        ValueType current_minimum = minimum_.load(std::memory_order_relaxed);
        while (current_minimum > other.minimum_.load(std::memory_order_relaxed) ||
               !count_.load(std::memory_order_relaxed))
        {
            if (minimum_.compare_exchange_weak(
                    current_minimum,
                    other.minimum_.load(std::memory_order_relaxed),
                    std::memory_order_relaxed
                ))
            {
                break;
            }
        }
        ValueType current_maximum = maximum_.load(std::memory_order_relaxed);
        while (current_maximum < other.maximum_.load(std::memory_order_relaxed) ||
               !count_.load(std::memory_order_relaxed))
        {
            if (maximum_.compare_exchange_weak(
                    current_maximum,
                    other.maximum_.load(std::memory_order_relaxed),
                    std::memory_order_relaxed
                ))
            {
                break;
            }
        }
        sum_.fetch_add(other.sum_.load(std::memory_order_relaxed), std::memory_order_relaxed);
        count_.fetch_add(other.count_.load(std::memory_order_acquire), std::memory_order_release);
    }
 
    void Reset() {
        minimum_.store(0, std::memory_order_relaxed);
        maximum_.store(0, std::memory_order_relaxed);
        sum_.store(0, std::memory_order_relaxed);
        count_ = 0;
    }
 
    bool IsEmpty() const noexcept { return count_.load() == 0; }
 
private:
    std::atomic<ValueType> minimum_;
    std::atomic<ValueType> maximum_;
    std::atomic<ValueType> sum_;
    std::atomic<ssize_t> count_;
};
 
template <typename ValueType, typename AverageType>
auto Serialize(const MinMaxAvg<ValueType, AverageType>& mma, formats::serialize::To<formats::json::Value>) {
    const auto current = mma.GetCurrent();
    return formats::json::MakeObject("min", current.minimum, "max", current.maximum, "avg", current.average);
}
 
template <typename ValueType, typename AverageType>
void DumpMetric(Writer& writer, const MinMaxAvg<ValueType, AverageType>& value) {
    const auto current = value.GetCurrent();
    writer["min"] = current.minimum;
    writer["max"] = current.maximum;
    writer["avg"] = current.average;
}
 
}  // namespace utils::statistics
 
USERVER_NAMESPACE_END