[libc++][chrono] Adds the sys_info class. (#85619)

Adds the sys_info class and time_zone::get_info(). The code still has a
few quirks and has not been optimized for performance yet.

The returned sys_info is compared against the output of the zdump tool
in the test giving confidence the implementation is correct.

Implements parts of:
- P0355 Extending <chrono> to Calendars and Time Zones

Implements:
- LWGXXXX The sys_info range should be affected by save

Author: mordante

libcxx/docs/Status/Cxx2cIssues.csv

@@ -63,4 +63,5 @@
 "`4054 <https://wg21.link/LWG4054>`__","Repeating a ``repeat_view`` should repeat the view","Tokyo March 2024","","","|ranges|"
 "","","","","",""
 "`3343 <https://wg21.link/LWG3343>`__","Ordering of calls to ``unlock()`` and ``notify_all()`` in Effects element of ``notify_all_at_thread_exit()`` should be reversed","Not Yet Adopted","|Complete|","16.0",""
+"XXXX","","The sys_info range should be affected by save","Not Yet Adopted","|Complete|","19.0"
 "","","","","",""

libcxx/include/CMakeLists.txt

@@ -291,6 +291,7 @@ set(files
   __chrono/parser_std_format_spec.h
   __chrono/statically_widen.h
   __chrono/steady_clock.h
+  __chrono/sys_info.h
   __chrono/system_clock.h
   __chrono/time_point.h
   __chrono/time_zone.h

libcxx/include/__chrono/sys_info.h

@@ -0,0 +1,51 @@
+// -*- 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
+//
+//===----------------------------------------------------------------------===//
+
+// For information see https://libcxx.llvm.org/DesignDocs/TimeZone.html
+
+#ifndef _LIBCPP___CHRONO_SYS_INFO_H
+#define _LIBCPP___CHRONO_SYS_INFO_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 <__config>
+#  include <string>
+
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#  if _LIBCPP_STD_VER >= 20
+
+namespace chrono {
+
+struct sys_info {
+  sys_seconds begin;
+  sys_seconds end;
+  seconds offset;
+  minutes save;
+  string abbrev;
+};
+
+} // namespace chrono
+
+#  endif //_LIBCPP_STD_VER >= 20
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_TZDB)
+
+#endif // _LIBCPP___CHRONO_SYS_INFO_H

libcxx/include/__chrono/time_zone.h

@@ -16,6 +16,9 @@
 // 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/sys_info.h>
+#  include <__chrono/system_clock.h>
 #  include <__compare/strong_order.h>
 #  include <__config>
 #  include <__memory/unique_ptr.h>
@@ -55,10 +58,18 @@ class _LIBCPP_AVAILABILITY_TZDB time_zone {
 
   _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI string_view name() const noexcept { return __name(); }
 
+  template <class _Duration>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI sys_info get_info(const sys_time<_Duration>& __time) const {
+    return __get_info(chrono::time_point_cast<seconds>(__time));
+  }
+
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI const __impl& __implementation() const noexcept { return *__impl_; }
 
 private:
   [[nodiscard]] _LIBCPP_EXPORTED_FROM_ABI string_view __name() const noexcept;
+
+  [[nodiscard]] _LIBCPP_AVAILABILITY_TZDB _LIBCPP_EXPORTED_FROM_ABI sys_info __get_info(sys_seconds __time) const;
+
   unique_ptr<__impl> __impl_;
 };
 

libcxx/include/chrono

@@ -724,6 +724,15 @@ const time_zone* current_zone()
 const tzdb& reload_tzdb();                                                       // C++20
 string remote_version();                                                         // C++20
 
+// [time.zone.info], information classes
+struct sys_info {                                                                // C++20
+  sys_seconds   begin;
+  sys_seconds   end;
+  seconds       offset;
+  minutes       save;
+  string        abbrev;
+};
+
 // 25.10.5, class time_zone                                                      // C++20
 enum class choose {earliest, latest};
 class time_zone {
@@ -733,6 +742,9 @@ class time_zone {
   // unspecified additional constructors
 
   string_view name() const noexcept;
+
+  template<class Duration>
+  sys_info get_info(const sys_time<Duration>& st) const;
 };
 bool operator==(const time_zone& x, const time_zone& y) noexcept;                // C++20
 strong_ordering operator<=>(const time_zone& x, const time_zone& y) noexcept;    // C++20
@@ -881,6 +893,7 @@ constexpr chrono::year                                  operator ""y(unsigned lo
 #include <__chrono/month_weekday.h>
 #include <__chrono/monthday.h>
 #include <__chrono/steady_clock.h>
+#include <__chrono/sys_info.h>
 #include <__chrono/system_clock.h>
 #include <__chrono/time_point.h>
 #include <__chrono/weekday.h>

libcxx/include/libcxx.imp

@@ -288,6 +288,7 @@
   { include: [ "<__chrono/parser_std_format_spec.h>", "private", "<chrono>", "public" ] },
   { include: [ "<__chrono/statically_widen.h>", "private", "<chrono>", "public" ] },
   { include: [ "<__chrono/steady_clock.h>", "private", "<chrono>", "public" ] },
+  { include: [ "<__chrono/sys_info.h>", "private", "<chrono>", "public" ] },
   { include: [ "<__chrono/system_clock.h>", "private", "<chrono>", "public" ] },
   { include: [ "<__chrono/time_point.h>", "private", "<chrono>", "public" ] },
   { include: [ "<__chrono/time_zone.h>", "private", "<chrono>", "public" ] },

libcxx/include/module.modulemap

@@ -1167,6 +1167,9 @@ module std_private_chrono_time_zone              [system] {
 module std_private_chrono_time_zone_link         [system] {
   header "__chrono/time_zone_link.h"
 }
+module std_private_chrono_sys_info               [system] {
+  header "__chrono/sys_info.h"
+}
 module std_private_chrono_system_clock           [system] {
   header "__chrono/system_clock.h"
   export std_private_chrono_time_point

libcxx/modules/std/chrono.inc

@@ -212,10 +212,12 @@ export namespace std {
     // [time.zone.exception], exception classes
     using std::chrono::ambiguous_local_time;
     using std::chrono::nonexistent_local_time;
+#    endif // if 0
 
     // [time.zone.info], information classes
     using std::chrono::sys_info;
 
+#    if 0
     // [time.zone.timezone], class time_zone
     using std::chrono::choose;
 #    endif // if 0

libcxx/src/include/tzdb/time_zone_private.h

@@ -24,7 +24,8 @@ namespace chrono {
 
 class time_zone::__impl {
 public:
-  explicit _LIBCPP_HIDE_FROM_ABI __impl(string&& __name) : __name_(std::move(__name)) {}
+  explicit _LIBCPP_HIDE_FROM_ABI __impl(string&& __name, const __tz::__rules_storage_type& __rules_db)
+      : __name_(std::move(__name)), __rules_db_(__rules_db) {}
 
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI string_view __name() const noexcept { return __name_; }
 
@@ -33,12 +34,20 @@ class time_zone::__impl {
     return __continuations_;
   }
 
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI const __tz::__rules_storage_type& __rules_db() const { return __rules_db_; }
+
 private:
   string __name_;
   // Note the first line has a name + __continuation, the other lines
   // are just __continuations. So there is always at least one item in
   // the vector.
   vector<__tz::__continuation> __continuations_;
+
+  // Continuations often depend on a set of rules. The rules are stored in
+  // parallel data structurs in tzdb_list. From the time_zone it's not possible
+  // to find its associated tzdb entry and thus not possible to find its
+  // associated rules. Therefore a link to the rules in stored in this class.
+  const __tz::__rules_storage_type& __rules_db_;
 };
 
 } // namespace chrono

libcxx/src/include/tzdb/types_private.h

@@ -33,7 +33,17 @@ namespace chrono::__tz {
 // Sun>=8   first Sunday on or after the eighth
 // Sun<=25  last Sunday on or before the 25th
 struct __constrained_weekday {
-  /*  year_month_day operator()(year __year, month __month);*/ // needed but not implemented
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI year_month_day operator()(year __year, month __month) const {
+    auto __result = static_cast<sys_days>(year_month_day{__year, __month, __day});
+    weekday __wd{static_cast<sys_days>(__result)};
+
+    if (__comparison == __le)
+      __result -= __wd - __weekday;
+    else
+      __result += __weekday - __wd;
+
+    return __result;
+  }
 
   weekday __weekday;
   enum __comparison_t { __le, __ge } __comparison;
@@ -85,7 +95,8 @@ struct __continuation {
   // used.
   // If this field contains - then standard time always
   // applies. This is indicated by the monostate.
-  using __rules_t = variant<monostate, __tz::__save, string, size_t>;
+  // TODO TZDB Investigate implantation the size_t based caching.
+  using __rules_t = variant<monostate, __tz::__save, string /*, size_t*/>;
 
   __rules_t __rules;