Add a category for week of year.

git-svn-id: svn+ssh://svn.gna.org/svn/gnustep/libs/base/trunk@15570 72102866-910b-0410-8b05-ffd578937521

Source/Additions/GSCategories.m

@@ -0,0 +1,84 @@
+/* Implementation of extension methods to standard classes
+
+   Copyright (C) 2003 Free Software Foundation, Inc.
+
+   Written by:  Richard Frith-Macdonald <rfm@gnu.org>
+
+   This file is part of the GNUstep Base Library.
+
+   This library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Library General Public
+   License as published by the Free Software Foundation; either
+   version 2 of the License, or (at your option) any later version.
+   
+   This library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Library General Public License for more details.
+
+   You should have received a copy of the GNU Library General Public
+   License along with this library; if not, write to the Free
+   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111 USA.
+
+*/
+#include	<Foundation/Foundation.h>
+
+@implementation NSCalendarDate (GSCategories)
+
+/**
+ * The ISO standard week of the year is based on the first week of the
+ * year being that week (starting on monday) for which the thursday
+ * is on or after the first of january.<br />
+ * This has the effect that, if january first is a friday, saturday or
+ * sunday, the days of that week (up to and including the sunday) are
+ * considered to be in week 53 of the preceeding year. Similarly if the
+ * last day of the year is a monday tuesday or wednesday, these days are
+ * part of week 1 of the next year.
+ */
+- (int) weekOfYear
+{
+  int	dayOfWeek = [self dayOfWeek];
+  int	dayOfYear;
+
+  /*
+   * Whether a week is considered to be in a year or not depends on its
+   * thursday ... so find thursday for the receivers week.
+   * NB. this may result in a date which is not in the same year as the
+   * receiver.
+   */
+  if (dayOfWeek != 4)
+    {
+      CREATE_AUTORELEASE_POOL(arp);
+      NSCalendarDate	*thursday;
+
+      /*
+       * A week starts on monday ... so adjust from 0 to 7 so that a
+       * sunday is counted as the last day of the week.
+       */
+      if (dayOfWeek == 0)
+	{
+	  dayOfWeek = 7;
+	}
+      thursday = [self dateByAddingYears: 0
+				  months: 0
+				    days: 4 - dayOfWeek
+				   hours: 0
+				 minutes: 0
+				 seconds: 0];
+      dayOfYear = [thursday dayOfYear];
+      RELEASE(arp);
+    }
+  else
+    {
+      dayOfYear = [self dayOfYear];
+    }
+  
+  /*
+   * Round up to a week boundary, so that when we divide by seven we
+   * get a result in the range 1 to 53 as mandated by the ISO standard.
+   */
+  dayOfYear += (7 - dayOfYear % 7);
+  return dayOfYear / 7;
+}
+
+@end