[src/trunk]: src/sys/dev Counting leap years was fine while we had 32bit time...

[martin <martin%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/c44a72eb885d
branches:  trunk
changeset: 332091:c44a72eb885d
user:      martin <martin%NetBSD.org@localhost>
date:      Sat Sep 06 18:04:28 2014 +0000

description:
Counting leap years was fine while we had 32bit time_t - but now it
is not a good idea for dates far away in the future.
For dates in the year 2000 or later, use arithmetic instead (since the
repeating periods are well aligned). Should fix PR 49144.

diffstat:

 sys/dev/clock_subr.c |  97 +++++++++++++++++++++++++++++++++++++++------------
 1 files changed, 74 insertions(+), 23 deletions(-)

diffs (151 lines):

diff -r 92617a38573b -r c44a72eb885d sys/dev/clock_subr.c
--- a/sys/dev/clock_subr.c      Sat Sep 06 17:10:17 2014 +0000
+++ b/sys/dev/clock_subr.c      Sat Sep 06 18:04:28 2014 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: clock_subr.c,v 1.20 2014/09/01 12:25:52 apb Exp $      */
+/*     $NetBSD: clock_subr.c,v 1.21 2014/09/06 18:04:28 martin Exp $   */
 
 /*
  * Copyright (c) 1988 University of Utah.
@@ -50,7 +50,7 @@
 
 #ifdef _KERNEL
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: clock_subr.c,v 1.20 2014/09/01 12:25:52 apb Exp $");
+__KERNEL_RCSID(0, "$NetBSD: clock_subr.c,v 1.21 2014/09/06 18:04:28 martin Exp $");
 
 #include <sys/param.h>
 #include <sys/systm.h>
@@ -66,6 +66,19 @@
 #define        days_in_year(a)         (leapyear(a) ? 366 : 365)
 #define        days_in_month(a)        (month_days[(a) - 1])
 
+/* for easier alignment:
+ * time from the epoch to 2000 (there were 7 leap years): */
+#define        DAYSTO2000      (365*30+7)
+
+/* 4 year intervals include 1 leap year */
+#define        DAYS4YEARS      (365*4+1)
+
+/* 100 year intervals include 24 leap years */
+#define        DAYS100YEARS    (365*100+24)
+
+/* 400 year intervals include 97 leap years */
+#define        DAYS400YEARS    (365*400+97)
+
 static const int month_days[12] = {
        31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
 };
@@ -109,11 +122,35 @@
        if (year < POSIX_BASE_YEAR)
                return -1;
        days = 0;
-       for (i = POSIX_BASE_YEAR; i < year; i++)
-               days += days_in_year(i);
        if (leapyear(year) && dt->dt_mon > FEBRUARY)
                days++;
 
+       if (year < 2000) {
+               /* simple way for early years */
+               for (i = POSIX_BASE_YEAR; i < year; i++)
+                       days += days_in_year(i);
+       } else {
+               /* years are properly aligned */
+               days += DAYSTO2000;
+               year -= 2000;
+
+               i = year / 400;
+               days += i * DAYS400YEARS;
+               year -= i * 400;
+
+               i = year / 100;
+               days += i * DAYS100YEARS;
+               year -= i * 100;
+
+               i = year / 4;
+               days += i * DAYS4YEARS;
+               year -= i * 4;
+
+               for (i = dt->dt_year-year; i < dt->dt_year; i++)
+                       days += days_in_year(i);
+       }
+
+
        /* Months */
        for (i = 1; i < dt->dt_mon; i++)
                days += days_in_month(i);
@@ -133,35 +170,50 @@
 void
 clock_secs_to_ymdhms(time_t secs, struct clock_ymdhms *dt)
 {
-       int mthdays[12];
-       int i;
+       int i, leap;
        time_t days;
        time_t rsec;    /* remainder seconds */
 
-       /*
-        * This function uses a local copy of month_days[]
-        * so the copy can be modified (and thread-safe).
-        * See the definition of days_in_month() above.
-        */
-       memcpy(mthdays, month_days, sizeof(mthdays));
-#define month_days mthdays
-
        days = secs / SECDAY;
        rsec = secs % SECDAY;
 
        /* Day of week (Note: 1/1/1970 was a Thursday) */
        dt->dt_wday = (days + 4) % 7;
 
-       /* Subtract out whole years, counting them in i. */
-       for (i = POSIX_BASE_YEAR; days >= days_in_year(i); i++)
-               days -= days_in_year(i);
-       dt->dt_year = i;
+       if (days >= DAYSTO2000) {
+               days -= DAYSTO2000;
+               dt->dt_year = 2000;
+
+               i = days / DAYS400YEARS;
+               days -= i*DAYS400YEARS;
+               dt->dt_year += i*400;
+
+               i = days / DAYS100YEARS;
+               days -= i*DAYS100YEARS;
+               dt->dt_year += i*100;
+
+               i = days / DAYS4YEARS;
+               days -= i*DAYS4YEARS;
+               dt->dt_year += i*4;
+
+               for (i = dt->dt_year; days >= days_in_year(i); i++)
+                       days -= days_in_year(i);
+               dt->dt_year = i;
+       } else {
+               /* Subtract out whole years, counting them in i. */
+               for (i = POSIX_BASE_YEAR; days >= days_in_year(i); i++)
+                       days -= days_in_year(i);
+               dt->dt_year = i;
+       }
 
        /* Subtract out whole months, counting them in i. */
-       if (leapyear(i))
-               days_in_month(FEBRUARY) = 29;
-       for (i = 1; days >= days_in_month(i); i++)
-               days -= days_in_month(i);
+       for (leap = 0, i = 1; days >= days_in_month(i)+leap; i++) {
+               days -= days_in_month(i)+leap;
+               if (i == 1 && leapyear(dt->dt_year))
+                       leap = 1;
+               else
+                       leap = 0;
+       }
        dt->dt_mon = i;
 
        /* Days are what is left over (+1) from all that. */
@@ -173,5 +225,4 @@
        dt->dt_min  = rsec / 60;
        rsec = rsec % 60;
        dt->dt_sec  = rsec;
-#undef month_days
 }