feat: add support for configurable first day of the week in calendar

man/waybar-clock.5.scd

@@ -138,6 +138,11 @@ View all valid format options in *strftime(3)* or have a look https://en.cpprefe
 :[ When enabled, the calendar follows the ISO 8601 standard: weeks begin on
    Monday, and the first week of the year is numbered 1. The default week format is
    '{:%V}'.
+|[ *first-day-of-week*
+:[ integer
+:[
+:[ The first day of the week, where 0 is Sunday and 6 is Saturday.
+   When not set, the first day of the week is determined by the locale settings.
 
 3. Addressed by *clock: calendar: format*
 [- *Option*
@@ -207,11 +212,12 @@ View all valid format options in *strftime(3)* or have a look https://en.cpprefe
 	"format-alt": "{:%A, %B %d, %Y (%R)}  ",
 	"tooltip-format": "<tt><small>{calendar}</small></tt>",
 	"calendar": {
-		"mode"          : "year",
-		"mode-mon-col"  : 3,
-		"weeks-pos"     : "right",
-		"on-scroll"     : 1,
-		"on-click-right": "mode",
+		"mode"             : "year",
+		"mode-mon-col"     : 3,
+		"weeks-pos"        : "right",
+		"first-day-of-week": 1,
+		"on-scroll"        : 1,
+		"on-click-right"   : "mode",
 		"format": {
 			"months":     "<span color='#ffead3'><b>{}</b></span>",
 			"days":       "<span color='#ecc6d9'><b>{}</b></span>",

src/modules/clock.cpp

@@ -509,6 +509,18 @@ using deleting_unique_ptr = std::unique_ptr<T, deleter_from_fn<fn>>;
 
 // Computations done similarly to Linux cal utility.
 auto waybar::modules::Clock::first_day_of_week() -> weekday {
+  const auto firstdow = config_[kCldPlaceholder]["first-day-of-week"];
+  if (firstdow.isInt()) {
+    const int firstDay = firstdow.asInt();
+    if (!(firstDay >= 0 && firstDay <= 6)) {
+      spdlog::warn(
+          "Clock calender configuration first-day-of-week = {0} must be in range [0, 6]. Default "
+          "value is used instead",
+          firstDay);
+    } else {
+      return weekday{static_cast<unsigned>(firstDay)};
+    }
+  }
   if (iso8601Calendar_) {
     return Monday;
   }