[libcxx-commits] [libcxx] [libc++][chrono] implements TAI clock. (PR #125550)

[Louis Dionne via libcxx-commits]

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@@ -0,0 +1,99 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___CHRONO_TAI_CLOCK_H
+#define _LIBCPP___CHRONO_TAI_CLOCK_H
+
+#include <version>
+// Enable the contents of the header only when libc++ was built with experimental features enabled.
+#if _LIBCPP_HAS_EXPERIMENTAL_TZDB
+
+#  include <__assert>
+#  include <__chrono/duration.h>
+#  include <__chrono/time_point.h>
+#  include <__chrono/utc_clock.h>
+#  include <__config>
+#  include <__type_traits/common_type.h>
+
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#  if _LIBCPP_STD_VER >= 20 && _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM && _LIBCPP_HAS_LOCALIZATION
+
+namespace chrono {
+
+class tai_clock;
+
+template <class _Duration>
+using tai_time    = time_point<tai_clock, _Duration>;
+using tai_seconds = tai_time<seconds>;
+
+// [time.clock.tai.overview]/1
+//    The clock tai_clock measures seconds since 1958-01-01 00:00:00 and is
+//    offset 10s ahead of UTC at this date. That is, 1958-01-01 00:00:00 TAI is
+//    equivalent to 1957-12-31 23:59:50 UTC. Leap seconds are not inserted into
+//    TAI. Therefore every time a leap second is inserted into UTC, UTC shifts
+//    another second with respect to TAI. For example by 2000-01-01 there had
+//    been 22 positive and 0 negative leap seconds inserted so 2000-01-01
+//    00:00:00 UTC is equivalent to 2000-01-01 00:00:32 TAI (22s plus the
+//    initial 10s offset).
+//
+// Note this does not specify what the UTC offset before 1958-01-01 00:00:00
+// TAI is. However the member functions are fully specified in the standard.
+// https://koka-lang.github.io/koka/doc/std_time_utc.html contains more
+// information and references.
----------------
ldionne wrote:

```suggestion
// Note this does not specify what the UTC offset before 1958-01-01 00:00:00
// TAI is, nor does it follow the "real" TAI clock between 1958-01-01 and the
// start of the UTC epoch. So while the member functions are fully specified in
// the standard, they do not technically follow the "real-world" TAI clock with
// 100% accuracy.
//
// https://koka-lang.github.io/koka/doc/std_time_utc.html contains more
// information and references.
```

https://github.com/llvm/llvm-project/pull/125550