strptime: Fix day-of-week calculation.

The day-of-week calculation used the raw year value without adjusting
for TM_YEAR_BASE, so it was off by one for 300 years out of every 400;
it just happened to be correct for 1901 through 2000.  It also used a
loop where a simple addition would have sufficed.

While here, simplify our version of Gauss's algorithm, and document
that we assume the Gregorian calendar.

MFC after:	1 week
PR:		282916
Reviewed by:	imp, allanjude, philip
Differential Revision:	https://reviews.freebsd.org/D47977

(cherry picked from commit 4285e024baa80f81d13cdcc016fdf0721fe57862)

etc/mtree/BSD.tests.dist

@@ -364,6 +364,8 @@
             ..
             stdlib
             ..
+            stdtime
+            ..
             string
             ..
             sys

lib/libc/stdtime/strptime.3

@@ -23,7 +23,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\" "
-.Dd October 2, 2014
+.Dd December 9, 2024
 .Dt STRPTIME 3
 .Os
 .Sh NAME
@@ -135,6 +135,11 @@ function has been contributed by Powerdog Industries.
 .Pp
 This man page was written by
 .An J\(:org Wunsch .
+.Sh CAVEATS
+The
+.Fn strptime
+function assumes the Gregorian calendar and will produce incorrect
+results for dates prior to its introduction.
 .Sh BUGS
 Both the
 .Fa %e

lib/libc/stdtime/strptime.c

@@ -70,17 +70,16 @@ static char * _strptime(const char *, const char *, struct tm *, int *, locale_t
 #define	FLAG_WDAY	(1 << 5)
 
 /*
- * Calculate the week day of the first day of a year. Valid for
- * the Gregorian calendar, which began Sept 14, 1752 in the UK
- * and its colonies. Ref:
- * http://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week
+ * Gauss's algorithm for the day of the week of the first day of any year
+ * in the Gregorian calendar.
  */
-
 static int
 first_wday_of(int year)
 {
-	return (((2 * (3 - (year / 100) % 4)) + (year % 100) +
-		((year % 100) / 4) + (isleap(year) ? 6 : 0) + 1) % 7);
+	return ((1 +
+	    5 * ((year - 1) % 4) +
+	    4 * ((year - 1) % 100) +
+	    6 * ((year - 1) % 400)) % 7);
 }
 
 static char *
@@ -682,13 +681,8 @@ label:
 			flags |= FLAG_MDAY;
 		}
 		if (!(flags & FLAG_WDAY)) {
-			i = 0;
-			wday_offset = first_wday_of(tm->tm_year);
-			while (i++ <= tm->tm_yday) {
-				if (wday_offset++ >= 6)
-					wday_offset = 0;
-			}
-			tm->tm_wday = wday_offset;
+			wday_offset = first_wday_of(tm->tm_year + TM_YEAR_BASE);
+			tm->tm_wday = (wday_offset + tm->tm_yday) % 7;
 			flags |= FLAG_WDAY;
 		}
 	}

lib/libc/tests/Makefile

@@ -16,6 +16,7 @@ TESTS_SUBDIRS+=	rpc
 TESTS_SUBDIRS+=	setjmp
 TESTS_SUBDIRS+=	stdio
 TESTS_SUBDIRS+=	stdlib
+TESTS_SUBDIRS+=	stdtime
 TESTS_SUBDIRS+=	string
 TESTS_SUBDIRS+=	sys
 TESTS_SUBDIRS+=	termios

lib/libc/tests/stdtime/Makefile

@@ -0,0 +1,7 @@
+.include <bsd.own.mk>
+
+ATF_TESTS_C+=		strptime_test
+
+TESTSDIR:=	${TESTSBASE}/${RELDIR:C/libc\/tests/libc/}
+
+.include <bsd.test.mk>

lib/libc/tests/stdtime/strptime_test.c

@@ -0,0 +1,50 @@
+/*-
+ * Copyright (c) 2024 Dag-Erling Smørgrav
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <time.h>
+
+#include <atf-c.h>
+
+ATF_TC_WITHOUT_HEAD(dayofweek);
+ATF_TC_BODY(dayofweek, tc)
+{
+	static const struct {
+		const char *str;
+		int wday;
+	} cases[] = {
+		{ "1582-12-20", 1 },
+		{ "1700-03-01", 1 },
+		{ "1752-09-14", 4 },
+		{ "1800-12-31", 3 },
+		{ "1801-01-01", 4 },
+		{ "1900-12-31", 1 },
+		{ "1901-01-01", 2 },
+		{ "2000-12-31", 0 },
+		{ "2001-01-01", 1 },
+		{ "2100-12-31", 5 },
+		{ "2101-01-01", 6 },
+		{ "2200-12-31", 3 },
+		{ "2201-01-01", 4 },
+		{ },
+	};
+	struct tm tm;
+
+	for (unsigned int i = 0; cases[i].str != NULL; i++) {
+		if (strptime(cases[i].str, "%Y-%m-%d", &tm) == NULL) {
+			atf_tc_fail_nonfatal("failed to parse %s",
+			    cases[i].str);
+		} else if (tm.tm_wday != cases[i].wday) {
+			atf_tc_fail_nonfatal("expected %d for %s, got %d",
+			    cases[i].wday, cases[i].str, tm.tm_wday);
+		}
+	}
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+	ATF_TP_ADD_TC(tp, dayofweek);
+	return (atf_no_error());
+}