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1// -*- C++ -*-
2//===----------------------------------------------------------------------===//
3//
4// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5// See https://llvm.org/LICENSE.txt for license information.
6// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7//
8//===----------------------------------------------------------------------===//
9
10#ifndef _LIBCPP___CHRONO_TAI_CLOCK_H
11#define _LIBCPP___CHRONO_TAI_CLOCK_H
12
13#include <version>
14// Enable the contents of the header only when libc++ was built with experimental features enabled.
15#if _LIBCPP_HAS_EXPERIMENTAL_TZDB
16
17# include <__assert>
18# include <__chrono/duration.h>
19# include <__chrono/time_point.h>
20# include <__chrono/utc_clock.h>
21# include <__config>
22# include <__type_traits/common_type.h>
23
24# if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
25# pragma GCC system_header
26# endif
27
28_LIBCPP_PUSH_MACROS
29# include <__undef_macros>
30
31_LIBCPP_BEGIN_NAMESPACE_STD
32
33# if _LIBCPP_STD_VER >= 20 && _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM && _LIBCPP_HAS_LOCALIZATION
34
35namespace chrono {
36
37class tai_clock;
38
39template <class _Duration>
40using tai_time = time_point<tai_clock, _Duration>;
41using tai_seconds = tai_time<seconds>;
42
43// [time.clock.tai.overview]/1
44// The clock tai_clock measures seconds since 1958-01-01 00:00:00 and is
45// offset 10s ahead of UTC at this date. That is, 1958-01-01 00:00:00 TAI is
46// equivalent to 1957-12-31 23:59:50 UTC. Leap seconds are not inserted into
47// TAI. Therefore every time a leap second is inserted into UTC, UTC shifts
48// another second with respect to TAI. For example by 2000-01-01 there had
49// been 22 positive and 0 negative leap seconds inserted so 2000-01-01
50// 00:00:00 UTC is equivalent to 2000-01-01 00:00:32 TAI (22s plus the
51// initial 10s offset).
52//
53// Note this does not specify what the UTC offset before 1958-01-01 00:00:00
54// TAI is, nor does it follow the "real" TAI clock between 1958-01-01 and the
55// start of the UTC epoch. So while the member functions are fully specified in
56// the standard, they do not technically follow the "real-world" TAI clock with
57// 100% accuracy.
58//
59// https://koka-lang.github.io/koka/doc/std_time_utc.html contains more
60// information and references.
61class tai_clock {
62public:
63 using rep = utc_clock::rep;
64 using period = utc_clock::period;
65 using duration = chrono::duration<rep, period>;
66 using time_point = chrono::time_point<tai_clock>;
67 static constexpr bool is_steady = false; // The utc_clock is not steady.
68
69 // The static difference between UTC and TAI time.
70 static constexpr chrono::seconds __offset{378691210};
71
72 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static time_point now() { return from_utc(utc_clock::now()); }
73
74 template <class _Duration>
75 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static utc_time<common_type_t<_Duration, seconds>>
76 to_utc(const tai_time<_Duration>& __time) noexcept {
77 using _Rp = common_type_t<_Duration, seconds>;
78 _Duration __time_since_epoch = __time.time_since_epoch();
79 _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__time_since_epoch >= utc_time<_Rp>::min().time_since_epoch() + __offset,
80 "the TAI to UTC conversion would underflow");
81
82 return utc_time<_Rp>{__time_since_epoch - __offset};
83 }
84
85 template <class _Duration>
86 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static tai_time<common_type_t<_Duration, seconds>>
87 from_utc(const utc_time<_Duration>& __time) noexcept {
88 using _Rp = common_type_t<_Duration, seconds>;
89 _Duration __time_since_epoch = __time.time_since_epoch();
90 _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__time_since_epoch <= utc_time<_Rp>::max().time_since_epoch() - __offset,
91 "the UTC to TAI conversion would overflow");
92
93 return tai_time<_Rp>{__time_since_epoch + __offset};
94 }
95};
96
97} // namespace chrono
98
99# endif // _LIBCPP_STD_VER >= 20 && _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM &&
100 // _LIBCPP_HAS_LOCALIZATION
101
102_LIBCPP_END_NAMESPACE_STD
103
104_LIBCPP_POP_MACROS
105
106#endif // _LIBCPP_HAS_EXPERIMENTAL_TZDB
107
108#endif // _LIBCPP___CHRONO_TAI_CLOCK_H