utils: Add macro for computing days after first weekday

Currently, `gcal_date_time_get_start_of_week()` seems to be
the only user of this macro function. In the next commit, the
month view will use it to traverse weeks while maintaining
the weekday.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/839>

src/utils/gcal-date-time-utils.c

@@ -90,7 +90,7 @@ gcal_date_time_get_start_of_week (GDateTime *date)
 
   first_weekday = gcal_context_get_week_start_day (context);
   weekday = g_date_time_get_day_of_week (date) % GCAL_N_WEEKDAYS;
-  n_days_after_week_start = (GCAL_N_WEEKDAYS + weekday - first_weekday) % GCAL_N_WEEKDAYS;
+  n_days_after_week_start = GCAL_NTH_WEEKDAY (weekday, first_weekday);
 
   start_of_week = g_date_time_add_days (date, -n_days_after_week_start);
 

src/utils/gcal-utils.h

@@ -32,6 +32,15 @@
 #define GCAL_MINUTES_PER_DAY 1440
 #define GCAL_MAX_MINUTES     (GCAL_N_WEEKDAYS * GCAL_MINUTES_PER_DAY)
 
+/**
+ * GCAL_NTH_WEEKDAY:
+ *
+ * Computes days after the first weekday for use in conjunction
+ * with [struct@GLib.DateTime].
+ */
+#define GCAL_NTH_WEEKDAY(weekday, first_weekday) \
+  (weekday - first_weekday + GCAL_N_WEEKDAYS) % GCAL_N_WEEKDAYS
+
 #define GCAL_DEFAULT_APPLICATION GCAL_APPLICATION (g_application_get_default ())
 
 #define gcal_clear_timeout(pp) { if (pp && *pp) { g_source_remove (*pp); *pp = 0; } }