[libcxx-commits] [libcxx] [libc++][chrono] Adds the sys_info class. (PR #85619)

[Mark de Wever via libcxx-commits]

================
@@ -8,14 +8,709 @@
 
 // For information see https://libcxx.llvm.org/DesignDocs/TimeZone.html
 
+// TODO TZDB look at optimizations
+//
+// The current algorithm is correct but not efficient. For example, in a named
+// rule based continuation finding the next rule does quite a bit of work,
+// returns the next rule and "forgets" its state. This could be better.
+//
+// It would be possible to cache lookups. If a time for a zone is calculated its
+// sys_info could be kept and the next lookup could test whether the time is in
+// a "known" sys_info. The wording in the Standard hints at this slowness by
+// "suggesting" this could be implemented at the user's side.
+
+// TODO TZDB look at removing quirks
+//
+// The code has some special rules to adjust the timing at the continuation
+// switches. This works correctly, but some of the places feel odd. It would be
+// good to investigate this further and see whether all quirks are needed or
+// that there are better fixes.
+//
+// These quirks often use a 12h interval; this is the scan interval of zdump,
+// which implies there are no sys_info objects with a duration of less than 12h.
+
+#include <algorithm>
 #include <chrono>
+#include <expected>
+#include <map>
+#include <ranges>
 
 #include "include/tzdb/time_zone_private.h"
+#include "include/tzdb/tzdb_list_private.h"
+
+// TODO TZDB remove debug printing
+#ifdef PRINT
+#  include <print>
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#ifdef PRINT
+template <>
+struct formatter<chrono::sys_info, char> {
+  template <class ParseContext>
+  constexpr typename ParseContext::iterator parse(ParseContext& ctx) {
+    return ctx.begin();
+  }
+
+  template <class FormatContext>
+  typename FormatContext::iterator format(const chrono::sys_info& info, FormatContext& ctx) const {
+    return std::format_to(
+        ctx.out(), "[{}, {}) {:%Q%q} {:%Q%q} {}", info.begin, info.end, info.offset, info.save, info.abbrev);
+  }
+};
+#endif
+
 namespace chrono {
 
+//===----------------------------------------------------------------------===//
+//                           Details
+//===----------------------------------------------------------------------===//
+
+struct __sys_info {
+  sys_info __info;
+  bool __can_merge; // Can the returned sys_info object be merged with
+};
+
+// Return type for helper function to get a sys_info.
+// - The expected result returns the "best" sys_info object. This object can be
+//   before the requested time. Sometimes sys_info objects from different
+//   continuations share their offset, save, and abbrev and these objects are
+//   merged to one sys_info object. The __can_merge flag determines whether the
+//   current result can be merged with the next result.
+// - The unexpected result means no sys_info object was found and the time is
+//   the time to be used for the next search iteration.
+using __sys_info_result = expected<__sys_info, sys_seconds>;
+
+template <ranges::forward_range _Range,
+          class _Type,
+          class _Proj                                                                                  = identity,
+          indirect_strict_weak_order<const _Type*, projected<ranges::iterator_t<_Range>, _Proj>> _Comp = ranges::less>
+[[nodiscard]] static ranges::borrowed_iterator_t<_Range>
+__binary_find(_Range&& __r, const _Type& __value, _Comp __comp = {}, _Proj __proj = {}) {
+  auto __end = ranges::end(__r);
+  auto __ret = ranges::lower_bound(ranges::begin(__r), __end, __value, __comp, __proj);
+  if (__ret == __end)
+    return __end;
+
+  // When the value does not match the predicate it's equal and a valid result
+  // was found.
+  return !std::invoke(__comp, __value, std::invoke(__proj, *__ret)) ? __ret : __end;
+}
+
+// Format based on https://data.iana.org/time-zones/tz-how-to.html
+//
+// 1  a time zone abbreviation that is a string of three or more characters that
+//    are either ASCII alphanumerics, "+", or "-"
+// 2  the string "%z", in which case the "%z" will be replaced by a numeric time
+//    zone abbreviation
+// 3  a pair of time zone abbreviations separated by a slash ('/'), in which
+//    case the first string is the abbreviation for the standard time name and
+//    the second string is the abbreviation for the daylight saving time name
+// 4  a string containing "%s", in which case the "%s" will be replaced by the
+//    text in the appropriate Rule's LETTER column, and the resulting string
+//    should be a time zone abbreviation
+//
+// Accepting invalid formats that can be processed in a sensible way would better
----------------
mordante wrote:

Actually I'll be less strict again... America/Barbados has a two letter abbreviation of AT. I'll validate the abbreviation is not empty instead.

https://github.com/llvm/llvm-project/pull/85619