[goffice] datetime: fix problems with week number calculations.

[Morten Welinder <mortenw src gnome org>]

commit 03bbb6d7c63131ac21fc6efe2999a54185878084
Author: Morten Welinder <terra gnome org>
Date:   Thu Apr 22 14:22:19 2010 -0400

    datetime: fix problems with week number calculations.

 ChangeLog                |    5 +++++
 goffice/utils/datetime.c |   42 ++++++++++++++++++++++++++++++------------
 2 files changed, 35 insertions(+), 12 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index 4fdbf24..beeb5c6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2010-04-21  Morten Welinder  <terra gnome org>
+
+	* goffice/utils/datetime.c (go_date_weeknum): Return the Excel
+	compatible week numbers as intended, not the glib ones.
+
 2010-04-16  Valek Filippov  <frob gnome org>
 
 	* tests/shapes-demo.c: fix warnings.
diff --git a/goffice/utils/datetime.c b/goffice/utils/datetime.c
index 0580cb2..28941c7 100644
--- a/goffice/utils/datetime.c
+++ b/goffice/utils/datetime.c
@@ -202,15 +202,13 @@ go_date_g_years_between (GDate const *date1, GDate const *date2)
  * @method:    week numbering method
  *
  * Returns: week number according to the given method.
- * 1:   Week starts on Sunday. Days before first Sunday are in week 0.
- * 2:   Week starts on Monday. Days before first Monday are in week 0.
- * 150: ISO 8601 week number. See datetime_isoweeknum.
+ * 1:   Week starts on Sunday.  January 1 is in week 1.
+ * 2:   Week starts on Monday.  January 1 is in week 1.
+ * 150: ISO 8601 week number.
  */
 int
 go_date_weeknum (GDate const *date, int method)
 {
-	int res = -1;
-
 	g_return_val_if_fail (g_date_valid (date), -1);
 	g_return_val_if_fail (method == GO_WEEKNUM_METHOD_SUNDAY ||
 			      method == GO_WEEKNUM_METHOD_MONDAY ||
@@ -218,15 +216,35 @@ go_date_weeknum (GDate const *date, int method)
 			      -1);
 
 	switch (method) {
-	case GO_WEEKNUM_METHOD_SUNDAY:
-		res = g_date_get_sunday_week_of_year (date); break;
-	case GO_WEEKNUM_METHOD_MONDAY:
-		res = g_date_get_monday_week_of_year (date); break;
-	case GO_WEEKNUM_METHOD_ISO:
-		res = g_date_get_iso8601_week_of_year (date); break;
+	case GO_WEEKNUM_METHOD_SUNDAY: {
+		GDate jan1;
+		GDateWeekday wd;
+		int doy;
+
+		g_date_clear (&jan1, 1);
+		g_date_set_dmy (&jan1, 1, 1, g_date_get_year (date));
+		wd = g_date_get_weekday (&jan1);
+		doy = g_date_get_day_of_year (date);
+
+		return (doy + (int)wd + 6 - (wd == G_DATE_SUNDAY ? 7 : 0)) / 7;
 	}
+	case GO_WEEKNUM_METHOD_MONDAY: {
+		GDate jan1;
+		GDateWeekday wd;
+		int doy;
+
+		g_date_clear (&jan1, 1);
+		g_date_set_dmy (&jan1, 1, 1, g_date_get_year (date));
+		wd = g_date_get_weekday (&jan1);
+		doy = g_date_get_day_of_year (date);
 
-	return res;
+		return (doy + (int)wd + 5) / 7;
+	}
+	case GO_WEEKNUM_METHOD_ISO:
+		return g_date_get_iso8601_week_of_year (date);
+	default:
+		return -1;
+	}
 }
 
 /* ------------------------------------------------------------------------- */