Source/WTF/wtf/GenericTimeMixin.h

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 * Copyright (C) 2021 Apple Inc. All rights reserved.
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#pragma once

#include <wtf/Seconds.h>

namespace WTF {

template<typename DerivedTime>
class GenericTimeMixin {
    WTF_DEPRECATED_MAKE_FAST_ALLOCATED(GenericTimeMixin);
public:
    // Call this if you know for sure that the double represents the time according to the
    // same time source as DerivedTime. It must be in seconds.
    static constexpr DerivedTime fromRawSeconds(double value)
    {
        return DerivedTime(value);
    }

    static constexpr DerivedTime infinity() { return fromRawSeconds(std::numeric_limits<double>::infinity()); }
    static constexpr DerivedTime nan() { return fromRawSeconds(std::numeric_limits<double>::quiet_NaN()); }

    bool isNaN() const { return std::isnan(m_value); }
    bool isInfinity() const { return std::isinf(m_value); }
    bool isFinite() const { return std::isfinite(m_value); }

    constexpr Seconds secondsSinceEpoch() const { return Seconds(m_value); }

    // Newer versions of Swift's C++ interop importer fail to synthesize a `bool`
    // conversion for an `operator bool` inherited from a dependent template base,
    // so each `DerivedTime` subclass re-declares this with `using` (rdar://181622867).
    explicit constexpr operator bool() const { return !!m_value; }

    constexpr DerivedTime operator+(Seconds other) const
    {
        return fromRawSeconds(m_value + other.value());
    }

    constexpr DerivedTime operator-(Seconds other) const
    {
        return fromRawSeconds(m_value - other.value());
    }

    Seconds operator%(Seconds other) const
    {
        return Seconds { fmod(m_value, other.value()) };
    }

    // Time is a scalar and scalars can be negated as this could arise from algebraic
    // transformations. So, we allow it.
    constexpr DerivedTime operator-() const
    {
        return fromRawSeconds(-m_value);
    }

    DerivedTime operator+=(Seconds other)
    {
        return *static_cast<DerivedTime*>(this) = *static_cast<DerivedTime*>(this) + other;
    }

    DerivedTime operator-=(Seconds other)
    {
        return *static_cast<DerivedTime*>(this) = *static_cast<DerivedTime*>(this) - other;
    }

    constexpr Seconds operator-(DerivedTime other) const
    {
        return Seconds(m_value - other.m_value);
    }

    friend constexpr auto operator<=>(GenericTimeMixin, GenericTimeMixin) = default;

    DerivedTime isolatedCopy() const
    {
        return *static_cast<const DerivedTime*>(this);
    }

    static constexpr DerivedTime timePointFromNow(Seconds relativeTimeFromNow)
    {
        if (relativeTimeFromNow.isInfinity())
            return DerivedTime::fromRawSeconds(relativeTimeFromNow.value());
        return DerivedTime::now() + relativeTimeFromNow;
    }

    template<typename TargetTime>
        requires (std::is_same_v<TargetTime, DerivedTime>)
    inline DerivedTime approximate() const
    {
        return *reinterpret_cast<const DerivedTime*>(this);
    }

    template<typename TargetTime>
        requires (std::derived_from<TargetTime, GenericTimeMixin<TargetTime>> && !std::is_same_v<TargetTime, DerivedTime>)
    TargetTime approximate() const
    {
        if (isInfinity())
            return TargetTime::fromRawSeconds(m_value);
        return *reinterpret_cast<const DerivedTime*>(this) - DerivedTime::now() + TargetTime::now();
    }

protected:
    // This is the epoch. So, x.secondsSinceEpoch() should be the same as x - DerivedTime().
    constexpr GenericTimeMixin() = default;

    constexpr GenericTimeMixin(double rawValue)
        : m_value(rawValue)
    {
    }

    double m_value { 0 };
};

} // namespace WTF