MonotonicClock.h

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#pragma once
 
#include <chrono>
#include <cstdint>
#include <optional>
 
namespace MonotonicClock {
inline uint64_t nowUs()
{
    return static_cast<uint64_t>(
        std::chrono::duration_cast<std::chrono::microseconds>(std::chrono::steady_clock::now().time_since_epoch())
            .count());
}
 
template <typename Rep, typename Period>
constexpr std::optional<std::chrono::duration<Rep, Period>> age(std::chrono::duration<Rep, Period> timestamp,
                                                                std::chrono::duration<Rep, Period> now)
{
    if (timestamp <= timestamp.zero() || timestamp > now) {
        return std::nullopt;
    }
    return now - timestamp;
}
 
constexpr std::optional<std::chrono::microseconds> age(uint64_t timestampUs, uint64_t nowUs)
{
    return age(std::chrono::microseconds(timestampUs), std::chrono::microseconds(nowUs));
}
 
constexpr std::chrono::microseconds remaining(uint64_t timestampUs, uint64_t nowUs, std::chrono::microseconds lifetime)
{
    const auto receiptAge = age(timestampUs, nowUs);
    if (!receiptAge || *receiptAge >= lifetime) {
        return std::chrono::microseconds::zero();
    }
    return lifetime - *receiptAge;
}
 
constexpr bool fresh(uint64_t timestampUs, uint64_t nowUs, std::chrono::microseconds lifetime)
{
    return remaining(timestampUs, nowUs, lifetime) > std::chrono::microseconds::zero();
}
 
constexpr bool withinAge(uint64_t timestampUs, uint64_t nowUs, std::chrono::microseconds maximumAge)
{
    const auto receiptAge = age(timestampUs, nowUs);
    return receiptAge && *receiptAge <= maximumAge;
}
}  // namespace MonotonicClock