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/**
* @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) */
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