From 93e340a081bcf32ba775baf11a2aaa468af99239 Mon Sep 17 00:00:00 2001
From: Aleksei Bavshin <alebastr89@gmail.com>
Date: Mon, 16 Jan 2023 17:30:06 -0800
Subject: [PATCH] feat(clock): support chrono Time Zone extensions.

Use chrono Calendars and Time Zones (P0355R7, P1466R3) when available
instead of the `date` library.
Verified with a patched build of a recent GCC 13 snapshot.
---
 include/factory.hpp       |  2 +-
 include/modules/clock.hpp |  3 +--
 include/util/date.hpp     | 28 +++++++++++++++++++++++++++-
 meson.build               | 22 ++++++++++++++++------
 src/modules/clock.cpp     |  1 -
 5 files changed, 45 insertions(+), 11 deletions(-)

diff --git a/include/factory.hpp b/include/factory.hpp
index 21dc647b..558a8d4e 100644
--- a/include/factory.hpp
+++ b/include/factory.hpp
@@ -1,7 +1,7 @@
 #pragma once
 
 #include <json/json.h>
-#ifdef HAVE_LIBDATE
+#if defined(HAVE_CHRONO_TIMEZONES) || defined(HAVE_LIBDATE)
 #include "modules/clock.hpp"
 #else
 #include "modules/simpleclock.hpp"
diff --git a/include/modules/clock.hpp b/include/modules/clock.hpp
index c97565de..a7290f6a 100644
--- a/include/modules/clock.hpp
+++ b/include/modules/clock.hpp
@@ -1,8 +1,7 @@
 #pragma once
 
-#include <date/tz.h>
-
 #include "ALabel.hpp"
+#include "util/date.hpp"
 #include "util/sleeper_thread.hpp"
 
 namespace waybar::modules {
diff --git a/include/util/date.hpp b/include/util/date.hpp
index ec948bd5..380bb6e7 100644
--- a/include/util/date.hpp
+++ b/include/util/date.hpp
@@ -1,8 +1,34 @@
 #pragma once
 
-#include <date/tz.h>
 #include <fmt/format.h>
 
+#if HAVE_CHRONO_TIMEZONES
+#include <chrono>
+#include <format>
+
+/* Compatibility layer for <date/tz.h> on top of C++20 <chrono> */
+namespace date {
+
+using namespace std::chrono;
+
+namespace literals {
+using std::chrono::last;
+}
+
+inline auto format(const std::string& spec, const auto& ztime) {
+  return spec.empty() ? "" : std::vformat("{:L" + spec + "}", std::make_format_args(ztime));
+}
+
+inline auto format(const std::locale& loc, const std::string& spec, const auto& ztime) {
+  return spec.empty() ? "" : std::vformat(loc, "{:L" + spec + "}", std::make_format_args(ztime));
+}
+
+}  // namespace date
+
+#else
+#include <date/tz.h>
+#endif
+
 template <typename Duration, typename TimeZonePtr>
 struct fmt::formatter<date::zoned_time<Duration, TimeZonePtr>> {
   std::string_view specs;
diff --git a/meson.build b/meson.build
index 7b631816..32bfd2bb 100644
--- a/meson.build
+++ b/meson.build
@@ -123,11 +123,18 @@ gtk_layer_shell = dependency('gtk-layer-shell-0',
         required: get_option('gtk-layer-shell'),
         fallback : ['gtk-layer-shell', 'gtk_layer_shell_dep'])
 systemd = dependency('systemd', required: get_option('systemd'))
-tz_dep = dependency('date',
-    required: false,
-    default_options : [ 'use_system_tzdb=true' ],
-    modules : [ 'date::date', 'date::date-tz' ],
-    fallback: [ 'date', 'tz_dep' ])
+
+cpp_lib_chrono = compiler.compute_int('__cpp_lib_chrono', prefix : '#include <chrono>')
+have_chrono_timezones = cpp_lib_chrono >= 201907
+if have_chrono_timezones
+  tz_dep = declare_dependency()
+else
+  tz_dep = dependency('date',
+      required: false,
+      default_options : [ 'use_system_tzdb=true' ],
+      modules : [ 'date::date', 'date::date-tz' ],
+      fallback: [ 'date', 'tz_dep' ])
+endif
 
 prefix = get_option('prefix')
 sysconfdir = get_option('sysconfdir')
@@ -312,7 +319,10 @@ if get_option('rfkill').enabled() and is_linux
     )
 endif
 
-if tz_dep.found()
+if have_chrono_timezones
+    add_project_arguments('-DHAVE_CHRONO_TIMEZONES', language: 'cpp')
+    src_files += 'src/modules/clock.cpp'
+elif tz_dep.found()
     add_project_arguments('-DHAVE_LIBDATE', language: 'cpp')
     src_files += 'src/modules/clock.cpp'
 else
diff --git a/src/modules/clock.cpp b/src/modules/clock.cpp
index 0dbd255d..76ec73ce 100644
--- a/src/modules/clock.cpp
+++ b/src/modules/clock.cpp
@@ -9,7 +9,6 @@
 #include <sstream>
 #include <type_traits>
 
-#include "util/date.hpp"
 #include "util/ustring_clen.hpp"
 #ifdef HAVE_LANGINFO_1STDAY
 #include <langinfo.h>