/** * @file chrono.h * @author Nat Goodspeed * @date 2021-10-05 * @brief supplement <chrono> with utility functions * * $LicenseInfo:firstyear=2021&license=viewerlgpl$ * Copyright (c) 2021, Linden Research, Inc. * $/LicenseInfo$ */ #if ! defined(LL_CHRONO_H) #define LL_CHRONO_H #include <chrono> #include <type_traits> // std::enable_if namespace LL { // time_point_cast() is derived from https://stackoverflow.com/a/35293183 // without the iteration: we think errors in the ~1 microsecond range are // probably acceptable. // This variant is for the optimal case when the source and dest use the same // clock: that case is handled by std::chrono. template <typename DestTimePoint, typename SrcTimePoint, typename std::enable_if<std::is_same<typename DestTimePoint::clock, typename SrcTimePoint::clock>::value, bool>::type = true> DestTimePoint time_point_cast(const SrcTimePoint& time) { return std::chrono::time_point_cast<typename DestTimePoint::duration>(time); } // This variant is for when the source and dest use different clocks -- see // the linked StackOverflow answer, also Howard Hinnant's, for more context. template <typename DestTimePoint, typename SrcTimePoint, typename std::enable_if<! std::is_same<typename DestTimePoint::clock, typename SrcTimePoint::clock>::value, bool>::type = true> DestTimePoint time_point_cast(const SrcTimePoint& time) { // The basic idea is that we must adjust the passed time_point by the // difference between the clocks' epochs. But since time_point doesn't // expose its epoch, we fall back on what each of them thinks is now(). // However, since we necessarily make sequential calls to those now() // functions, the answers differ not only by the cycles spent executing // those calls, but by potential OS interruptions between them. Try to // reduce that error by capturing the source clock time both before and // after the dest clock, and splitting the difference. Of course an // interruption between two of these now() calls without a comparable // interruption between the other two will skew the result, but better is // more expensive. const auto src_before = typename SrcTimePoint::clock::now(); const auto dest_now = typename DestTimePoint::clock::now(); const auto src_after = typename SrcTimePoint::clock::now(); const auto src_diff = src_after - src_before; const auto src_now = src_before + src_diff / 2; return dest_now + (time - src_now); } } // namespace LL #endif /* ! defined(LL_CHRONO_H) */