- Plugged in a different algorithm for calculating the day of the week in

strftime() and asctime_r(). This one isn't sensitive to the effects of
  adding/subtracting the local time zone.


git-svn-id: file:///Users/olsen/Code/migration-svn-zu-git/logical-line-staging/clib2/trunk@14813 87f5fb63-7c3d-0410-a384-fd976d0f7a62

library/GNUmakefile.68k

@@ -1,5 +1,5 @@
 #
-# $Id: GNUmakefile.68k,v 1.21 2005-01-24 10:25:46 obarthel Exp $
+# $Id: GNUmakefile.68k,v 1.22 2005-01-26 18:41:38 obarthel Exp $
 #
 # :ts=8
 #
@@ -376,6 +376,7 @@ C_LIB = \
 	time_numbertostring.o \
 	time_strftime.o \
 	time_time.o \
+	time_weekday.o \
 	unistd_access.o \
 	unistd_chdir.o \
 	unistd_chdir_exit.o \

library/GNUmakefile.os4

@@ -1,5 +1,5 @@
 #
-# $Id: GNUmakefile.os4,v 1.21 2005-01-24 10:25:46 obarthel Exp $
+# $Id: GNUmakefile.os4,v 1.22 2005-01-26 18:41:39 obarthel Exp $
 #
 # :ts=8
 #
@@ -378,6 +378,7 @@ C_LIB = \
 	time_numbertostring.o \
 	time_strftime.o \
 	time_time.o \
+	time_weekday.o \
 	unistd_access.o \
 	unistd_chdir.o \
 	unistd_chdir_exit.o \

library/changes

@@ -14,6 +14,10 @@
   UTC. This didn't really work up until now since the time value returned
   was given as local time.
 
+- Plugged in a different algorithm for calculating the day of the week in
+  strftime() and asctime_r(). This one isn't sensitive to the effects of
+  adding/subtracting the local time zone.
+
 
 c.lib 1.186 (14.1.2005)
 

library/smakefile

@@ -1,5 +1,5 @@
 #
-# $Id: smakefile,v 1.16 2005-01-24 10:25:46 obarthel Exp $
+# $Id: smakefile,v 1.17 2005-01-26 18:41:39 obarthel Exp $
 #
 # :ts=8
 #
@@ -484,7 +484,8 @@ TIME_OBJ = \
 	time_mktime.o \
 	time_numbertostring.o \
 	time_strftime.o \
-	time_time.o
+	time_time.o \
+	time_weekday.o
 
 UNISTD_OBJ = \
 	unistd_access.o \

library/time_asctime_r.c

@@ -1,5 +1,5 @@
 /*
- * $Id: time_asctime_r.c,v 1.3 2005-01-02 09:07:19 obarthel Exp $
+ * $Id: time_asctime_r.c,v 1.4 2005-01-26 18:41:39 obarthel Exp $
  *
  * :ts=4
  *
@@ -101,21 +101,13 @@ __asctime_r(const struct tm *tm,char * buffer,size_t buffer_size)
 	/* Fill in the week day if it's not in proper range. */
 	if(tm->tm_wday < 0 || tm->tm_wday > 6)
 	{
-		struct tm other_tm;
-		time_t seconds;
+		/* We use a peculiar algorithm rather than falling back onto
+		   mktime() here in order to avoid trouble with skewed results
+		   owing to time zone influence. */
+		copy_tm = (*tm);
+		copy_tm.tm_wday = __calculate_weekday(tm->tm_year+1900,tm->tm_mon+1,tm->tm_mday);
 
-		other_tm = (*tm);
-
-		seconds = mktime(&other_tm);
-		if(seconds != (time_t)-1)
-		{
-			__convert_time(seconds,0,&other_tm);
-
-			copy_tm = (*tm);
-			copy_tm.tm_wday = other_tm.tm_wday;
-
-			tm = &copy_tm;
-		}
+		tm = &copy_tm;
 	}
 
 	if(0 <= tm->tm_wday && tm->tm_wday <= 6)

library/time_headers.h

@@ -1,5 +1,5 @@
 /*
- * $Id: time_headers.h,v 1.6 2005-01-24 10:25:46 obarthel Exp $
+ * $Id: time_headers.h,v 1.7 2005-01-26 18:41:39 obarthel Exp $
  *
  * :ts=4
  *
@@ -95,6 +95,7 @@ extern char * __asctime_r(const struct tm *tm,char * buffer,size_t buffer_size);
 extern char * __number_to_string(unsigned int number,char * string,size_t max_len,size_t min_len);
 extern struct tm * __convert_time(ULONG seconds, LONG gmt_offset, struct tm * tm);
 extern time_t __convert_datestamp_to_time(const struct DateStamp * ds);
+extern int __calculate_weekday(int year,int month,int day);
 
 /****************************************************************************/
 

library/time_strftime.c

@@ -1,5 +1,5 @@
 /*
- * $Id: time_strftime.c,v 1.3 2005-01-24 10:25:46 obarthel Exp $
+ * $Id: time_strftime.c,v 1.4 2005-01-26 18:41:39 obarthel Exp $
  *
  * :ts=4
  *
@@ -118,20 +118,13 @@ format_date(const char *format,const struct tm *tm,time_t time_value,struct Hook
 	/* Fill in the week day if it's not in proper range. */
 	if(tm->tm_wday < 0 || tm->tm_wday > 6)
 	{
-		time_t seconds;
+		/* We use a peculiar algorithm rather than falling back onto
+		   mktime() here in order to avoid trouble with skewed results
+		   owing to time zone influence. */
+		copy_tm = (*tm);
+		copy_tm.tm_wday = __calculate_weekday(tm->tm_year+1900,tm->tm_mon+1,tm->tm_mday);
 
-		other_tm = (*tm);
-
-		seconds = mktime(&other_tm);
-		if(seconds != (time_t)-1)
-		{
-			__convert_time(seconds,0,&other_tm);
-
-			copy_tm = (*tm);
-			copy_tm.tm_wday = other_tm.tm_wday;
-
-			tm = &copy_tm;
-		}
+		tm = &copy_tm;
 	}
 
 	while((c = (*format++)) != '\0')

library/time_weekday.c

@@ -0,0 +1,86 @@
+/*
+ * $Id: time_weekday.c,v 1.1 2005-01-26 18:41:39 obarthel Exp $
+ *
+ * :ts=4
+ *
+ * Portable ISO 'C' (1994) runtime library for the Amiga computer
+ * Copyright (c) 2002-2005 by Olaf Barthel <olsen@sourcery.han.de>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ *   - Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ *
+ *   - Neither the name of Olaf Barthel nor the names of contributors
+ *     may be used to endorse or promote products derived from this
+ *     software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/****************************************************************************/
+
+#ifndef _TIME_HEADERS_H
+#include "time_headers.h"
+#endif /* _TIME_HEADERS_H */
+
+/****************************************************************************/
+
+int
+__calculate_weekday(int year,int month,int day)
+{
+	/* This algorithm comes from <http://klausler.com/new-dayofweek.html>. */
+	static const char month_skew[12] =
+	{
+		0,3,3,6,
+		1,4,6,2,
+		5,0,3,5
+	};
+
+	int sum;
+
+	/* Subtract 1900 from the year. */
+	sum = year - 1900;
+
+	/* To that number, add one fourth of itself, discarding
+	   any remainder. This sum is the year's skew value. */
+	sum += sum / 4;
+
+	/* If the month in question is January or February in a leap
+	   year, subtract 1 from the sum. */
+	if(month == 1 || month == 2)
+	{
+		int leap_year_adjust;
+
+		/* Figure out if the year is a leap year. */
+		if((year % 4) != 0)
+			leap_year_adjust = 0;
+		else if ((year % 400) == 0)
+			leap_year_adjust = 1;
+		else if ((year % 100) == 0)
+			leap_year_adjust = 0;
+		else
+			leap_year_adjust = 1;
+
+		sum -= leap_year_adjust;
+	}
+
+	/* Add the month's skew value from the table. */
+	sum += day + month_skew[month-1];
+
+	/* The sum is the number of days after Sunday on which the date falls. */
+	return(sum % 7);
+}