Detect presence, call nl_langinfo() to get first day of week.

meson.build

@@ -42,6 +42,18 @@ if not compiler.has_header('filesystem')
     endif
 endif
 
+code = '''#include<langinfo.h>
+int main(int argc, char** argv) {
+    char* str;
+    str = nl_langinfo(_NL_TIME_WEEK_1STDAY);
+    str = nl_langinfo(_NL_TIME_FIRST_WEEKDAY);
+    return 0;
+}
+'''
+if compiler.links(code, name : 'nl_langinfo with _NL_TIME_WEEK_1STDAY, _NL_TIME_FIRST_WEEKDAY')
+    add_project_arguments('-DHAVE_LANGINFO_1STDAY', language: 'cpp')
+endif
+
 add_global_arguments(cpp_args, language : 'cpp')
 add_global_link_arguments(cpp_link_args, language : 'cpp')
 

src/modules/clock.cpp

@@ -1,5 +1,8 @@
 #include "modules/clock.hpp"
 #include <sstream>
+#ifdef HAVE_LANGINFO_1STDAY
+#include <langinfo.h>
+#endif
 
 using zoned_time = date::zoned_time<std::chrono::system_clock::duration>;
 
@@ -10,6 +13,17 @@ struct waybar_time {
 
 namespace {
 
+#ifdef HAVE_LANGINFO_1STDAY
+// Computations done similarly to Linux cal utility.
+date::weekday first_day_of_week() {
+  const int i = (std::intptr_t) nl_langinfo(_NL_TIME_WEEK_1STDAY);
+  auto ymd = date::year(i / 10000)/(i / 100 % 100)/(i % 100);
+  auto wd = date::weekday(ymd);
+  uint8_t j = *nl_langinfo(_NL_TIME_FIRST_WEEKDAY);
+  return wd + date::days(j - 1);
+}
+#endif
+
 void weekdays_header(const std::locale& locale, const date::weekday& first_dow, std::ostream& os) {
   auto wd = first_dow;
   do {
@@ -38,7 +52,7 @@ struct CachedCalendar {
 
 CachedCalendar cached_calendar;
 
-std::string calendar_text(const waybar_time& wtime, const date::weekday& first_dow) {
+std::string calendar_text(const waybar_time& wtime) {
   const auto daypoint = date::floor<date::days>(wtime.ztime.get_local_time());
   const auto ymd = date::year_month_day(daypoint);
   if (cached_calendar.ymd == ymd) {
@@ -49,6 +63,11 @@ std::string calendar_text(const waybar_time& wtime, const date::weekday& first_d
   const auto curr_day = ymd.day();
 
   std::stringstream os;
+#ifdef HAVE_LANGINFO_1STDAY
+  const auto first_dow = first_day_of_week();
+#else
+  const auto first_dow = date::Sunday;
+#endif
   weekdays_header(wtime.locale, first_dow, os);
 
   // First week prefixed with spaces if needed.
@@ -115,7 +134,7 @@ auto waybar::modules::Clock::update() -> void {
 
   if (tooltipEnabled()) {
     if (config_["tooltip-format"].isString()) {
-      const auto calendar = calendar_text(wtime, date::Sunday);
+      const auto calendar = calendar_text(wtime);
       auto tooltip_format = config_["tooltip-format"].asString();
       auto tooltip_text = fmt::format(tooltip_format, wtime, fmt::arg("calendar", calendar));
       label_.set_tooltip_markup(tooltip_text);