aca829de13
Implements parts of: - P0355 Extending <chrono> to Calendars and Time Zones - P1361 Integration of chrono with text formatting - LWG3359 <chrono> leap second support should allow for negative leap seconds
109 lines
4.2 KiB
C++
109 lines
4.2 KiB
C++
// -*- C++ -*-
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___CHRONO_TAI_CLOCK_H
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#define _LIBCPP___CHRONO_TAI_CLOCK_H
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#include <version>
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// Enable the contents of the header only when libc++ was built with experimental features enabled.
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#if _LIBCPP_HAS_EXPERIMENTAL_TZDB
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# include <__assert>
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# include <__chrono/duration.h>
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# include <__chrono/time_point.h>
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# include <__chrono/utc_clock.h>
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# include <__config>
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# include <__type_traits/common_type.h>
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# if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
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# endif
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_LIBCPP_PUSH_MACROS
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# include <__undef_macros>
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_LIBCPP_BEGIN_NAMESPACE_STD
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# if _LIBCPP_STD_VER >= 20 && _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM && _LIBCPP_HAS_LOCALIZATION
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namespace chrono {
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class tai_clock;
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template <class _Duration>
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using tai_time = time_point<tai_clock, _Duration>;
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using tai_seconds = tai_time<seconds>;
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// [time.clock.tai.overview]/1
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// The clock tai_clock measures seconds since 1958-01-01 00:00:00 and is
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// offset 10s ahead of UTC at this date. That is, 1958-01-01 00:00:00 TAI is
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// equivalent to 1957-12-31 23:59:50 UTC. Leap seconds are not inserted into
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// TAI. Therefore every time a leap second is inserted into UTC, UTC shifts
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// another second with respect to TAI. For example by 2000-01-01 there had
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// been 22 positive and 0 negative leap seconds inserted so 2000-01-01
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// 00:00:00 UTC is equivalent to 2000-01-01 00:00:32 TAI (22s plus the
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// initial 10s offset).
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//
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// Note this does not specify what the UTC offset before 1958-01-01 00:00:00
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// TAI is, nor does it follow the "real" TAI clock between 1958-01-01 and the
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// start of the UTC epoch. So while the member functions are fully specified in
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// the standard, they do not technically follow the "real-world" TAI clock with
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// 100% accuracy.
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//
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// https://koka-lang.github.io/koka/doc/std_time_utc.html contains more
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// information and references.
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class tai_clock {
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public:
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using rep = utc_clock::rep;
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using period = utc_clock::period;
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using duration = chrono::duration<rep, period>;
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using time_point = chrono::time_point<tai_clock>;
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static constexpr bool is_steady = false; // The utc_clock is not steady.
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// The static difference between UTC and TAI time.
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static constexpr chrono::seconds __offset{378691210};
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[[nodiscard]] _LIBCPP_HIDE_FROM_ABI static time_point now() { return from_utc(utc_clock::now()); }
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template <class _Duration>
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[[nodiscard]] _LIBCPP_HIDE_FROM_ABI static utc_time<common_type_t<_Duration, seconds>>
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to_utc(const tai_time<_Duration>& __time) noexcept {
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using _Rp = common_type_t<_Duration, seconds>;
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_Duration __time_since_epoch = __time.time_since_epoch();
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_LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__time_since_epoch >= utc_time<_Rp>::min().time_since_epoch() + __offset,
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"the TAI to UTC conversion would underflow");
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return utc_time<_Rp>{__time_since_epoch - __offset};
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}
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template <class _Duration>
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[[nodiscard]] _LIBCPP_HIDE_FROM_ABI static tai_time<common_type_t<_Duration, seconds>>
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from_utc(const utc_time<_Duration>& __time) noexcept {
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using _Rp = common_type_t<_Duration, seconds>;
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_Duration __time_since_epoch = __time.time_since_epoch();
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_LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__time_since_epoch <= utc_time<_Rp>::max().time_since_epoch() - __offset,
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"the UTC to TAI conversion would overflow");
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return tai_time<_Rp>{__time_since_epoch + __offset};
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}
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};
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} // namespace chrono
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# endif // _LIBCPP_STD_VER >= 20 && _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM &&
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// _LIBCPP_HAS_LOCALIZATION
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_LIBCPP_END_NAMESPACE_STD
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_LIBCPP_POP_MACROS
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#endif // _LIBCPP_HAS_EXPERIMENTAL_TZDB
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#endif // _LIBCPP___CHRONO_TAI_CLOCK_H
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