On Tue, 19 May 2026 at 09:02, Tomasz Kamiński <[email protected]> wrote:
>
> Below including a quick draft of alternative solution where the merging
> is integragted into pre-existing loop. We use merge_window (non-zero
> only after DST chgange), and skip adding sys_info related to them.
>
> Let me know what you think about these approach. I certainly like it
> more.
> ---
> libstdc++-v3/src/c++20/tzdb.cc | 58 ++++++++-----
> .../std/time/time_zone/zone_merge.cc | 84 +++++++++++++++++++
> 2 files changed, 122 insertions(+), 20 deletions(-)
> create mode 100644 libstdc++-v3/testsuite/std/time/time_zone/zone_merge.cc
>
> diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
> index 183a0c61a61..e9a348cc3c8 100644
> --- a/libstdc++-v3/src/c++20/tzdb.cc
> +++ b/libstdc++-v3/src/c++20/tzdb.cc
> @@ -28,6 +28,8 @@
> // In the usual dual-abi build, std::chrono::tzdb is only defined for cxx11.
> #define _GLIBCXX_USE_CXX11_ABI 1
>
> +#define _GLIBCXX_ASSERTIONS
> +
> #include <chrono>
> #include <fstream> // ifstream
> #include <sstream> // istringstream
> @@ -78,6 +80,8 @@
> # endif
> #endif
>
> +#include <iostream>
> +
> namespace __gnu_cxx
> {
> #ifdef _AIX
> @@ -504,6 +508,9 @@ namespace std::chrono
> seconds
> offset() const noexcept { return m_offset; }
>
> + minutes
> + save() const noexcept { return m_save; }
> +
> // RULES: The name of the rules that apply for this period.
> string_view
> rules() const noexcept
> @@ -867,8 +874,15 @@ namespace std::chrono
> info.abbrev = ri.format();
>
> string_view letters;
> - if (i != infos.begin() && i[-1].expanded())
> - letters = i[-1].next_letters();
> + seconds prev_offset = ri.offset();
> + if (i == infos.begin())
> + prev_offset = ri.offset();
> + else if (const ZoneInfo& prev = i[-1]; !prev.expanded())
> + prev_offset = prev.offset();
> + else if (letters = prev.next_letters(); letters.empty())
> + prev_offset = prev.offset() + prev.save();
> + else
> + prev_offset = ri.offset();
>
> if (letters.empty())
> {
> @@ -940,9 +954,10 @@ namespace std::chrono
> }
>
> const Rule* curr_rule = nullptr;
> -
> + seconds merge_window = (prev_offset > info.offset) ? (prev_offset -
> info.offset) : seconds(0);
> while (info.begin < info.end && num_after > 0)
> {
> +
> sys_seconds t = info.begin;
> const year_month_day date(chrono::floor<days>(t));
> const Rule* next_rule = nullptr;
> @@ -984,9 +999,6 @@ namespace std::chrono
>
> if (t < rule_start && rule_start < info.end)
> {
> - if (rule_start - t < days(1)) // XXX shouldn't be needed!
> - continue;
> -
> // Found a closer transition than the previous info.end.
> info.end = rule_start;
> next_rule = &rule;
> @@ -995,7 +1007,6 @@ namespace std::chrono
>
> format_abbrev_str(info, letters);
>
> - bool merged = false;
> #if 0
This #if 0 group was an earlier prototype for merging, so should be removed.
This draft approach certainly seems easier to reason about, and closer
to how I imagined merging would work.
> if (!new_infos.empty())
> {
> @@ -1015,21 +1026,28 @@ namespace std::chrono
> else
> letters = {};
>
> - if (!merged)
> + // Merge the rule with next one (full description goes here)
> + if (bool merged = (info.end - t <= merge_window))
> + info.begin = t;
> + else
> + {
> new_infos.emplace_back(info, letters);
> + if (info.begin <= tp && tp < info.end) // Found the result.
> + result_index = new_infos.size() - 1;
> + else if (result_index >= 0)
> + {
> + // Finish on a DST sys_info if possible, so that if we resume
> + // generating sys_info objects after this time point, save=0
> + // should be correct for the next sys_info.
> + if (num_after > 1 || info.save != 0min)
> + --num_after;
> + }
> + info.begin = info.end;
> + }
>
> - if (info.begin <= tp && tp < info.end) // Found the result.
> - result_index = new_infos.size() - 1;
> - else if (result_index >= 0 && !merged)
> - {
> - // Finish on a DST sys_info if possible, so that if we resume
> - // generating sys_info objects after this time point, save=0
> - // should be correct for the next sys_info.
> - if (num_after > 1 || info.save != 0min)
> - --num_after;
> - }
> -
> - info.begin = info.end;
> + // Merging applies only when switching between rules with different
> + // offset, for any later applications it should be zero.
> + merge_window = seconds(0);
> if (next_rule)
> {
> info.end = ri.until();
> diff --git a/libstdc++-v3/testsuite/std/time/time_zone/zone_merge.cc
> b/libstdc++-v3/testsuite/std/time/time_zone/zone_merge.cc
> new file mode 100644
> index 00000000000..75942f976b6
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/std/time/time_zone/zone_merge.cc
> @@ -0,0 +1,84 @@
> +// { dg-do run { target c++20 } }
> +// { dg-require-effective-target tzdb }
> +// { dg-require-effective-target cxx11_abi }
> +// { dg-xfail-run-if "no weak override on AIX" { powerpc-ibm-aix* } }
> +
> +// When two adjacent Zone lines differ in total offset and the new line's
> +// rule set has a rule firing within |jump| of the boundary (where jump
> +// is a backward local-time jump), zic.c's writezone folds that rule
> +// into the boundary, so the new line begins with the post-rule save.
> +//
> +// Mirrors America/Argentina/Buenos_Aires around 1999-10-03.
> +
> +#include <chrono>
> +#include <fstream>
> +#include <testsuite_hooks.h>
> +
> +static bool override_used = false;
> +
> +namespace __gnu_cxx
> +{
> + const char* zoneinfo_dir_override() {
> + override_used = true;
> + return "./";
> + }
> +}
> +
> +int
> +main()
> +{
> + using namespace std::chrono;
> +
> + // stdoff jumps from -3 to -4 at the same instant a save=1 rule fires.
> + // In the new (-4) frame the rule fires 1 hour after the boundary at
> + // UT 03:00, so the merge folds the rule into the boundary and the
> + // new line begins at offset=-3, save=1 (abbrev "-03").
> + std::ofstream("tzdata.zi") << R"(# version test_zone_merge
> +R T 1999 o - O 3 0 1 -
> +R T 2000 o - Mar 3 0 0 -
> +Z Test/BA -3 - %z 1999 O 3
> + -4 T %z 2000 Mar 3
> + -3 - %z
> +)";
> +
> + const auto& db = reload_tzdb();
> + VERIFY( override_used );
> + VERIFY( db.version == "test_zone_merge" );
> +
> + auto* tz = locate_zone("Test/BA");
> +
> + // The boundary is the wall UNTIL "1999 O 3" (default time 00:00)
> + // interpreted in the prior (-3) frame, i.e. UT 03:00 1999-10-03.
> + sys_seconds boundary{sys_days(1999y/October/3) + 3h};
> +
> + auto before = tz->get_info(boundary - 1s);
> + VERIFY( before.offset == -3h );
> + VERIFY( before.save == 0min );
> + VERIFY( before.abbrev == "-03" );
> +
> + // The new line's first sys_info already has save=1 from the merge,
> + // total offset -3h, abbrev "-03".
> + auto at_boundary = tz->get_info(boundary);
> + VERIFY( at_boundary.offset == -3h );
> + VERIFY( at_boundary.save == 60min );
> + VERIFY( at_boundary.abbrev == "-03" );
> +
> + auto plus_30min = tz->get_info(boundary + 30min);
> + VERIFY( plus_30min.offset == -3h );
> + VERIFY( plus_30min.save == 60min );
> + VERIFY( plus_30min.abbrev == "-03" );
> +
> + // Sanity: well after the boundary, still in the merged sys_info
> + // until the Mar 3 2000 transition.
> + auto winter = tz->get_info(sys_days(2000y/January/15));
> + VERIFY( winter.offset == -3h );
> + VERIFY( winter.save == 60min );
> + VERIFY( winter.abbrev == "-03" );
> +
> + // After Mar 3 2000: line 2 ends, line 3 begins. No DST rule fires
> + // at this boundary, so total offset reverts to -3h with save=0.
> + auto spring = tz->get_info(sys_days(2000y/April/15));
> + VERIFY( spring.offset == -3h );
> + VERIFY( spring.save == 0min );
> + VERIFY( spring.abbrev == "-03" );
> +}
> --
> 2.54.0
>
