ClickHouse/src/Functions/TransformDateTime64.h

#pragma once

#include <Core/Types.h>
#include <Core/DecimalFunctions.h>

namespace DB
{
/** Tansform-type wrapper for DateTime64, simplifies DateTime64 support for given Transform.
 *
 * Depending on what overloads of Transform::execute() are available, when called with DateTime64 value,
 * invokes Transform::execute() with either:
 * * whole part of DateTime64 value, discarding fractional part (1)
 * * DateTime64 value and scale factor (2)
 * * DateTime64 broken down to components, result of execute is then re-assembled back into DateTime64 value (3)
 *
 * Suitable Transform-types are commonly used in Date/DateTime manipulation functions,
 * and should implement static (or const) function with following signatures:
 * 1:
 *     R execute(Int64 whole_value, ... )
 * 2:
 *     R execute(DateTime64 value, Int64 scale_multiplier, ... )
 * 3:
 *     R execute(DecimalUtils::DecimalComponents<DateTime64> components, ... )
 *
 * Where R could be of arbitrary type, in case of (3) if R is DecimalUtils::DecimalComponents<DateTime64>, result is re-assembed back into DateTime64.
*/
template <typename Transform>
class TransformDateTime64
{
private:
    // Detect if Transform::execute is const or static method
    // with signature defined by template args (ignoring result type).
    template<typename = void, typename... Args>
    struct TransformHasExecuteOverload : std::false_type {};

    template<typename... Args>
    struct TransformHasExecuteOverload<std::void_t<decltype(std::declval<Transform>().execute(std::declval<Args>()...))>, Args...>
        : std::true_type {};

    template<typename... Args>
    static constexpr bool TransformHasExecuteOverload_v = TransformHasExecuteOverload<void, Args...>::value;

public:
    static constexpr auto name = Transform::name;

    // non-explicit constructor to allow creating from scale value (or with no scale at all), indispensable in some contexts.
    TransformDateTime64(UInt32 scale_ = 0) /// NOLINT
        : scale_multiplier(DecimalUtils::scaleMultiplier<DateTime64::NativeType>(scale_))
    {}

    template <typename ... Args>
    inline auto NO_SANITIZE_UNDEFINED execute(const DateTime64 & t, Args && ... args) const
    {
        /// Type conversion from float to integer may be required.
        /// We are Ok with implementation specific result for out of range and denormals conversion.

        if constexpr (TransformHasExecuteOverload_v<DateTime64, decltype(scale_multiplier), Args...>)
        {
            return wrapped_transform.execute(t, scale_multiplier, std::forward<Args>(args)...);
        }
        else if constexpr (TransformHasExecuteOverload_v<DecimalUtils::DecimalComponents<DateTime64>, Args...>)
        {
            auto components = DecimalUtils::splitWithScaleMultiplier(t, scale_multiplier);

            const auto result = wrapped_transform.execute(components, std::forward<Args>(args)...);
            using ResultType = std::decay_t<decltype(result)>;

            if constexpr (std::is_same_v<DecimalUtils::DecimalComponents<DateTime64>, ResultType>)
            {
                return DecimalUtils::decimalFromComponentsWithMultiplier<DateTime64>(result, scale_multiplier);
            }
            else
            {
                return result;
            }
        }
        else
        {
            const auto components = DecimalUtils::splitWithScaleMultiplier(t, scale_multiplier);
            return wrapped_transform.execute(static_cast<Int64>(components.whole), std::forward<Args>(args)...);
        }
    }

    template <typename T, typename ... Args, typename = std::enable_if_t<!std::is_same_v<T, DateTime64>>>
    inline auto execute(const T & t, Args && ... args) const
    {
        return wrapped_transform.execute(t, std::forward<Args>(args)...);
    }

private:
    DateTime64::NativeType scale_multiplier = 1;
    Transform wrapped_transform = {};
};

}