ClickHouse/libs/libcommon/include/common/DateLUT.h

#pragma once

#include <common/DateLUTImpl.h>
#include <common/singleton.h>
#include <Core/Defines.h>

#include <unordered_map>
#include <atomic>
#include <mutex>
#include <memory>


/// This class provides lazy initialization and lookup of singleton DateLUTImpl objects for a given timezone.
class DateLUT : public Singleton<DateLUT>
{
    friend class Singleton<DateLUT>;

public:
    DateLUT(const DateLUT &) = delete;
    DateLUT & operator=(const DateLUT &) = delete;

    /// Return singleton DateLUTImpl instance for the default time zone.
    static ALWAYS_INLINE const DateLUTImpl & instance()
    {
        const auto & date_lut = Singleton<DateLUT>::instance();
        return *date_lut.default_impl.load(std::memory_order_acquire);
    }

    /// Return singleton DateLUTImpl instance for a given time zone.
    static ALWAYS_INLINE const DateLUTImpl & instance(const std::string & time_zone)
    {
        const auto & date_lut = Singleton<DateLUT>::instance();
        if (time_zone.empty())
            return *date_lut.default_impl.load(std::memory_order_acquire);

        return date_lut.getImplementation(time_zone);
    }

    static void setDefaultTimezone(const std::string & time_zone)
    {
        auto & date_lut = Singleton<DateLUT>::instance();
        const auto & impl = date_lut.getImplementation(time_zone);
        date_lut.default_impl.store(&impl, std::memory_order_release);
    }

protected:
    DateLUT();

private:
    const DateLUTImpl & getImplementation(const std::string & time_zone) const;

    using DateLUTImplPtr = std::unique_ptr<DateLUTImpl>;

    /// Time zone name -> implementation.
    mutable std::unordered_map<std::string, DateLUTImplPtr> impls;
    mutable std::mutex mutex;

    std::atomic<const DateLUTImpl *> default_impl;
};