================
@@ -0,0 +1,155 @@
+// -*- 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_UTC_CLOCK_H
+#define _LIBCPP___CHRONO_UTC_CLOCK_H
+
+#include <version>
+// Enable the contents of the header only when libc++ was built with
experimental features enabled.
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_TZDB)
+
+# include <__chrono/duration.h>
+# include <__chrono/system_clock.h>
+# include <__chrono/time_point.h>
+# include <__chrono/tzdb.h>
+# include <__chrono/tzdb_list.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 && !defined(_LIBCPP_HAS_NO_TIME_ZONE_DATABASE) &&
!defined(_LIBCPP_HAS_NO_FILESYSTEM) && \
+ !defined(_LIBCPP_HAS_NO_LOCALIZATION)
+
+namespace chrono {
+
+class utc_clock;
+
+template <class _Duration>
+using utc_time = time_point<utc_clock, _Duration>;
+using utc_seconds = utc_time<seconds>;
+
+class utc_clock {
+public:
+ using rep = system_clock::rep;
+ using period = system_clock::period;
+ using duration = chrono::duration<rep, period>;
+ using time_point = chrono::time_point<utc_clock>;
+ static constexpr bool is_steady = false; // The system_clock is not steady.
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static time_point now() { return
from_sys(system_clock::now()); }
+
+ template <class _Duration>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static sys_time<common_type_t<_Duration,
seconds>>
+ to_sys(const utc_time<_Duration>& __time);
+
+ template <class _Duration>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static utc_time<common_type_t<_Duration,
seconds>>
+ from_sys(const sys_time<_Duration>& __time) {
+ using _Rp = utc_time<common_type_t<_Duration, seconds>>;
+ // TODO TZDB investigate optimizations.
+ //
+ // The leap second database stores all transitions, this mean to calculate
+ // the current number of leap seconds the code needs to iterate over all
+ // leap seconds to accumulate the sum. Then the sum can be used to
determine
+ // the sys_time. Accessing the database involves acquiring a mutex.
+ //
+ // The historic entries in the database are immutable. Hard-coding these
+ // values in a table would allow:
+ // - To store the sum, allowing a binary search on the data.
+ // - Avoid acquiring a mutex.
+ // The disadvantage are:
+ // - A slightly larger code size.
+ //
+ // There are two optimization directions
+ // - hard-code the database and do a linear search for future entries. This
+ // search can start at the back, and should probably contain very few
+ // entries. (Adding leap seconds is quite rare and new release of libc++
+ // can add the new entries; they are announced half a year before they
are
+ // added.)
+ // - During parsing the leap seconds store an additional database in the
+ // dylib with the list of the sum of the leap seconds. In that case there
+ // can be a private function __get_utc_to_sys_table that returns the
+ // table.
+ //
+ // Note for to_sys there are no optimizations to be done; it uses
+ // get_leap_second_info. The function get_leap_second_info could benefit
+ // from optimizations as described above; again both options apply.
+
+ // Both UTC and the system clock use the same epoch. The Standard
+ // specifies from 1970-01-01 even when UTC starts at
+ // 1972-01-01 00:00:10 TAI. So when the sys_time is before epoch we can be
+ // sure there both clocks return the same value.
+
+ const tzdb& __tzdb = chrono::get_tzdb();
+ _Rp __result{__time.time_since_epoch()};
+ for (const auto& __leap_second : __tzdb.leap_seconds) {
+ if (__time < __leap_second)
+ return __result;
+
+ __result += __leap_second.value();
+ }
+ return __result;
+ }
+};
+
+struct leap_second_info {
+ bool is_leap_second;
+ seconds elapsed;
+};
+
+template <class _Duration>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI leap_second_info
get_leap_second_info(const utc_time<_Duration>& __time) {
+ const tzdb& __tzdb = chrono::get_tzdb();
+ if (__tzdb.leap_seconds.empty())
+ return {false, chrono::seconds{0}};
+
+ sys_seconds __sys{chrono::floor<seconds>(__time).time_since_epoch()};
+ seconds __elapsed{0};
+ for (const auto& __leap_second : __tzdb.leap_seconds) {
+ if (__sys == __leap_second.date() + __elapsed)
+ return {__leap_second.value() > 0s, // only positive leap seconds are
considered leap seconds
+ __elapsed + __leap_second.value()};
+
+ if (__sys < __leap_second.date() + __elapsed)
+ return {false, __elapsed};
+
+ __elapsed += __leap_second.value();
+ }
+
+ return {false, __elapsed};
+}
+
+template <class _Duration>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI sys_time<common_type_t<_Duration, seconds>>
+utc_clock::to_sys(const utc_time<_Duration>& __time) {
+ using _Dp = common_type_t<_Duration, seconds>;
+ leap_second_info __info = get_leap_second_info(__time);
+
+ sys_time<common_type_t<_Duration, seconds>>
__result{__time.time_since_epoch() - __info.elapsed};
+ if (__info.is_leap_second)
+ return chrono::floor<seconds>(__result) + chrono::seconds{1} - _Dp{1};
----------------
MattStephanson wrote:
I don't think this is right for floating point `duration` reps, where the ulp
can be more or less than `_Dp{1}`. I think `nextafter` is needed for reps where
`treat_as_floating_point_v` is `true`. (Assuming you have such an overload for
extended-precision floats. I'm also uncertain if
[[time.duration.general]/2](https://eel.is/c++draft/time#duration.general-2)
allows a user-defined type to "emulate" a floating point type by specializing
`treat_as_floating_point`.)
https://github.com/llvm/llvm-project/pull/90393
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