utils: Use ISO weekday numbering for 'get_first_weekday' function

Change the 'get_first_weekday' function to number the weekdays
Monday(1)-Sunday(7) like ISO does. Change the function name to reflect
that. Add tests for different locales, although the tests depend on
system-installed locales.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/822>

src/core/gcal-context.c

@@ -116,7 +116,7 @@ set_week_start_day_from_variant (GcalContext *self,
 
   /* find the actual 'default' from locale */
   if (weekday->value == G_DESKTOP_WEEKDAY_DEFAULT)
-    weekday = g_enum_get_value (g_type_class_get (G_DESKTOP_TYPE_WEEKDAY), gcal_util_get_first_weekday ());
+    weekday = g_enum_get_value (g_type_class_get (G_DESKTOP_TYPE_WEEKDAY), gcal_util_get_first_weekday_iso ());
 
   if (self->week_start_day == weekday->value)
     return;
@@ -391,7 +391,7 @@ gcal_context_class_init (GcalContextClass *klass)
 
   properties[PROP_WEEK_START_DAY] =
     g_param_spec_enum ("week-start-day", NULL, NULL,
-                       G_DESKTOP_TYPE_WEEKDAY, gcal_util_get_first_weekday (),
+                       G_DESKTOP_TYPE_WEEKDAY, gcal_util_get_first_weekday_iso (),
                        G_PARAM_READABLE | G_PARAM_EXPLICIT_NOTIFY | G_PARAM_STATIC_STRINGS);
 
   g_object_class_install_properties (object_class, N_PROPS, properties);
@@ -535,7 +535,7 @@ gcal_context_get_time_format (GcalContext *self)
 GDesktopWeekday
 gcal_context_get_week_start_day (GcalContext *self)
 {
-  g_return_val_if_fail (GCAL_IS_CONTEXT (self), gcal_util_get_first_weekday ());
+  g_return_val_if_fail (GCAL_IS_CONTEXT (self), gcal_util_get_first_weekday_iso ());
 
   return self->week_start_day;
 }

src/gui/common/gcal-date-chooser.c

@@ -1083,7 +1083,7 @@ gcal_date_chooser_init (GcalDateChooser *self)
   self->date = g_date_time_new_now_local ();
   g_date_time_get_ymd (self->date, &self->this_year, NULL, NULL);
 
-  self->week_start = gcal_util_get_first_weekday ();
+  self->week_start = gcal_util_get_first_weekday_iso ();
 
   gtk_widget_init_template (GTK_WIDGET (self));
 

src/gui/views/gcal-month-view.c

@@ -194,7 +194,7 @@ allocate_overflow_popover (GcalMonthView *self,
 static inline void
 update_weekday_labels (GcalMonthView *self)
 {
-  const gint first_weekday = gcal_util_get_first_weekday ();
+  const gint first_weekday = gcal_util_get_first_weekday_iso ();
 
   for (gint i = 0; i < GCAL_N_WEEKDAYS; i++)
     {

src/gui/views/gcal-week-header.c

@@ -100,8 +100,6 @@ struct _GcalWeekHeader
   GtkWidget          *overflow_label[GCAL_N_WEEKDAYS];
   WeekdayHeader       weekday_header[GCAL_N_WEEKDAYS];
 
-  gint                first_weekday;
-
   /*
    * Used for checking if the header is in collapsed state or expand state
    * false is collapse state true is expand state
@@ -1762,8 +1760,6 @@ gcal_week_header_init (GcalWeekHeader *self)
   gtk_widget_set_visible (self->dnd.widget, FALSE);
   gtk_widget_set_parent (self->dnd.widget, GTK_WIDGET (self));
 
-  self->first_weekday = gcal_util_get_first_weekday ();
-
   gtk_widget_init_template (GTK_WIDGET (self));
 
   for (i = 0; i < GCAL_N_WEEKDAYS; i++)

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

@@ -83,7 +83,7 @@ gcal_date_time_get_start_of_week (GDateTime *date)
 
   g_assert (date != NULL);
 
-  first_weekday = gcal_util_get_first_weekday ();
+  first_weekday = gcal_util_get_first_weekday_iso ();
   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;
 

src/utils/gcal-utils.c

@@ -213,36 +213,46 @@ gcal_util_get_desc_from_component (ECalComponent *component,
 /**
  * gcal_util_get_first_weekday:
  *
- * Copied from Clocks, which by itself is
+ * Finds the locale default first day of week
- * copied from GtkCalendar.
+ * respecting the ISO day counting (Monday: 1 - Sunday: 7).
  *
- * Returns: the first weekday, from 0 to 6
+ * Prefer using `GcalContext:week-start-day` property or
+ * the `GcalContext:gcal_context_get_week_start_day()` method
+ * to get the current setting from gsettings-desktop-schemas.
+ * Use this utility method only as a fallback.
+ *
+ * Returns: the first weekday, from 1 to 7
  */
 gint
-gcal_util_get_first_weekday (void)
+gcal_util_get_first_weekday_iso (void)
 {
   gint week_start;
 
 #ifdef HAVE__NL_TIME_FIRST_WEEKDAY
 
-  union { unsigned int word; char *string; } langinfo;
+  union { unsigned int baseline; char *offset; } langinfo;
-  gint week_1stday = 0;
+  gint iso_week_base = 7; /* default to Sunday (19971130) */
   gint first_weekday = 1;
   guint week_origin;
 
-  langinfo.string = nl_langinfo (_NL_TIME_FIRST_WEEKDAY);
+  /* first day of week index */
-  first_weekday = langinfo.string[0];
+  langinfo.offset = nl_langinfo (_NL_TIME_FIRST_WEEKDAY);
-  langinfo.string = nl_langinfo (_NL_TIME_WEEK_1STDAY);
+  first_weekday = langinfo.offset[0];
-  week_origin = langinfo.word;
+
+  /* find the baseline week origin */
+  langinfo.offset = nl_langinfo (_NL_TIME_WEEK_1STDAY);
+  week_origin = langinfo.baseline;
+
   if (week_origin == 19971130) /* Sunday */
-    week_1stday = 0;
+    iso_week_base = 7;
   else if (week_origin == 19971201) /* Monday */
-    week_1stday = 1;
+    iso_week_base = 1;
+  else if (week_origin == 19971129) /* unusual: Saturday */
+    iso_week_base = 6;
   else
     g_warning ("Unknown value of _NL_TIME_WEEK_1STDAY.\n");
 
-  week_start = (week_1stday + first_weekday - 1) % GCAL_N_WEEKDAYS;
+  week_start = ((iso_week_base - 1 + (first_weekday - 1)) % GCAL_N_WEEKDAYS) + 1;
-
 #else
 
   gchar *gtk_week_start;

src/utils/gcal-utils.h

@@ -63,7 +63,7 @@ GdkPaintable*        gcal_util_get_circle_paintable_from_color   (const GdkRGBA
 gchar*               gcal_util_get_desc_from_component           (ECalComponent      *component,
                                                                   const gchar        *joint_char);
 
-gint                 gcal_util_get_first_weekday                 (void);
+gint                 gcal_util_get_first_weekday_iso             (void);
 
 ECalComponent*       gcal_util_build_component_from_details      (const gchar        *summary,
                                                                   GDateTime          *initial_date,

tests/test-utils.c

@@ -17,6 +17,7 @@
  */
 
 #include <glib.h>
+#include <locale.h>
 
 #include "gcal-utils.h"
 
@@ -191,6 +192,47 @@ extract_meeting_url (void)
 
 /*********************************************************************************************************************/
 
+typedef struct {
+  const char *locale_name;
+  int         week_start_index;
+} LocaleIsoWeekDay;
+
+static void
+test_first_weekday (void)
+{
+  LocaleIsoWeekDay test_cases[] = {
+    { "C",           7 }, // Baseline standard (Sunday)
+    { "en_US.UTF-8", 7 }, // US Standard (Sunday)
+    { "de_DE.UTF-8", 1 }, // Euro Standard (Monday)
+    { "en_GB.UTF-8", 1 }, // UK Standard (Monday)
+    { "fa_IR.UTF-8", 6 }  // Iran, Somalia, Afghanistan (Saturday)
+  };
+
+  const char* initial_locale = setlocale (LC_ALL, NULL);
+
+  for (size_t i = 0; i < G_N_ELEMENTS (test_cases); ++i)
+  {
+    LocaleIsoWeekDay testcase = test_cases[i];
+    char *current_locale = setlocale (LC_ALL, testcase.locale_name);
+
+    if (!current_locale)
+      {
+        g_message ("Locale %s not installed. Skipping", testcase.locale_name);
+        continue;
+      }
+    else
+      {
+        g_message ("Testing locale %s", testcase.locale_name);
+        g_assert_cmpint (testcase.week_start_index, ==, gcal_util_get_first_weekday_iso ());
+      }
+  }
+
+  /* restore locale */
+  setlocale (LC_ALL, initial_locale);
+}
+
+/*********************************************************************************************************************/
+
 gint
 main (gint   argc,
       gchar *argv[])
@@ -202,6 +244,7 @@ main (gint   argc,
 
   g_test_add_func ("/utils/date-time/date_time_from_icaltime", date_time_from_icaltime);
   g_test_add_func ("/utils/misc/extract_meeting_url", extract_meeting_url);
+  g_test_add_func ("/utils/misc/get_first_weekday", test_first_weekday);
 
   return g_test_run ();
 }