path: root/libc/misc/time/mktime.c

/* Convert a `struct tm' to a time_t value.
   Copyright (C) 1993, 94, 95, 96, 97, 98, 99 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Paul Eggert (eggert@twinsun.com).

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C 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
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

/* Define this to have a standalone program to test this implementation of
   mktime.  */

#include <features.h>
/* Assume that leap seconds are not possible */
#undef LEAP_SECONDS_POSSIBLE 
#include <sys/types.h>		/* Some systems define `time_t' here.  */
#include <time.h>
#include <limits.h>

#if 0
#ifndef CHAR_BIT
# define CHAR_BIT 8
#endif

/* The extra casts work around common compiler bugs.  */
#define TYPE_SIGNED(t) (! ((t) 0 < (t) -1))
/* The outer cast is needed to work around a bug in Cray C 5.0.3.0.
   It is necessary at least when t == time_t.  */
#define TYPE_MINIMUM(t) ((t) (TYPE_SIGNED (t) \
			      ? ~ (t) 0 << (sizeof (t) * CHAR_BIT - 1) : (t) 0))
#define TYPE_MAXIMUM(t) ((t) (~ (t) 0 - TYPE_MINIMUM (t)))

#ifndef INT_MIN
# define INT_MIN TYPE_MINIMUM (int)
#endif
#ifndef INT_MAX
# define INT_MAX TYPE_MAXIMUM (int)
#endif

#ifndef TIME_T_MIN
# define TIME_T_MIN TYPE_MINIMUM (time_t)
#endif
#ifndef TIME_T_MAX
# define TIME_T_MAX TYPE_MAXIMUM (time_t)
#endif

#define TM_YEAR_BASE 1900
#define EPOCH_YEAR 1970


/* How many days come before each month (0-12).  */
extern const unsigned short int __mon_yday[2][13];



/* Yield the difference between (YEAR-YDAY HOUR:MIN:SEC) and (*TP),
   measured in seconds, ignoring leap seconds.
   YEAR uses the same numbering as TM->tm_year.
   All values are in range, except possibly YEAR.
   If TP is null, return a nonzero value.
   If overflow occurs, yield the low order bits of the correct answer.  */
static time_t
__ydhms_tm_diff (int year, int yday, int hour, int min, int sec,
	       const struct tm *tp)
{
    if (!tp)
	return 1;
    else
    {
	/* Compute intervening leap days correctly even if year is negative.
	   Take care to avoid int overflow.  time_t overflow is OK, since
	   only the low order bits of the correct time_t answer are needed.
	   Don't convert to time_t until after all divisions are done, since
	   time_t might be unsigned.  */
	int a4 = (year >> 2) + (TM_YEAR_BASE >> 2) - ! (year & 3);
	int b4 = (tp->tm_year >> 2) + (TM_YEAR_BASE >> 2) - ! (tp->tm_year & 3);
	int a100 = a4 / 25 - (a4 % 25 < 0);
	int b100 = b4 / 25 - (b4 % 25 < 0);
	int a400 = a100 >> 2;
	int b400 = b100 >> 2;
	int intervening_leap_days = (a4 - b4) - (a100 - b100) + (a400 - b400);
	time_t years = year - (time_t) tp->tm_year;
	time_t days = (365 * years + intervening_leap_days
		+ (yday - tp->tm_yday));
	return (60 * (60 * (24 * days + (hour - tp->tm_hour))
		    + (min - tp->tm_min))
		+ (sec - tp->tm_sec));
    }
}

/* Use CONVERT to convert *T to a broken down time in *TP.
   If *T is out of range for conversion, adjust it so that
   it is the nearest in-range value and then convert that.  */
static struct tm *
__ranged_convert (struct tm *(*convert) (const time_t *, struct tm *),
		time_t *t, struct tm *tp)
{
    struct tm *r;

    if (! (r = (*convert) (t, tp)) && *t)
    {
	time_t bad = *t;
	time_t ok = 0;
	struct tm tm;

	/* BAD is a known unconvertible time_t, and OK is a known good one.
	   Use binary search to narrow the range between BAD and OK until
	   they differ by 1.  */
	while (bad != ok + (bad < 0 ? -1 : 1))
	{
	    time_t mid = *t = (bad < 0
		    ? bad + ((ok - bad) >> 1)
		    : ok + ((bad - ok) >> 1));
	    if ((r = (*convert) (t, tp)))
	    {
		tm = *r;
		ok = mid;
	    }
	    else
		bad = mid;
	}

	if (!r && ok)
	{
	    /* The last conversion attempt failed;
	       revert to the most recent successful attempt.  */
	    *t = ok;
	    *tp = tm;
	    r = tp;
	}
    }

    return r;
}


/* Convert *TP to a time_t value, inverting
   the monotonic and mostly-unit-linear conversion function CONVERT.
   Use *OFFSET to keep track of a guess at the offset of the result,
   compared to what the result would be for UTC without leap seconds.
   If *OFFSET's guess is correct, only one CONVERT call is needed.  */
time_t __mktime_internal (struct tm *tp, 
	struct tm *(*convert) (const time_t *, struct tm *), time_t *offset)
{
    time_t t, dt, t0, t1, t2;
    struct tm tm;

    /* The maximum number of probes (calls to CONVERT) should be enough
       to handle any combinations of time zone rule changes, solar time,
       leap seconds, and oscillations around a spring-forward gap.
       POSIX.1 prohibits leap seconds, but some hosts have them anyway.  */
    int remaining_probes = 6;

    /* Time requested.  Copy it in case CONVERT modifies *TP; this can
       occur if TP is localtime's returned value and CONVERT is localtime.  */
    int sec = tp->tm_sec;
    int min = tp->tm_min;
    int hour = tp->tm_hour;
    int mday = tp->tm_mday;
    int mon = tp->tm_mon;
    int year_requested = tp->tm_year;
    int isdst = tp->tm_isdst;

    /* Ensure that mon is in range, and set year accordingly.  */
    int mon_remainder = mon % 12;
    int negative_mon_remainder = mon_remainder < 0;
    int mon_years = mon / 12 - negative_mon_remainder;
    int year = year_requested + mon_years;

    /* The other values need not be in range:
       the remaining code handles minor overflows correctly,
       assuming int and time_t arithmetic wraps around.
       Major overflows are caught at the end.  */

    /* Calculate day of year from year, month, and day of month.
       The result need not be in range.  */
    int yday = ((__mon_yday[__isleap (year + TM_YEAR_BASE)]
		[mon_remainder + 12 * negative_mon_remainder])
	    + mday - 1);

    int sec_requested = sec;
#if LEAP_SECONDS_POSSIBLE
    /* Handle out-of-range seconds specially,
       since __ydhms_tm_diff assumes every minute has 60 seconds.  */
    if (sec < 0)
	sec = 0;
    if (59 < sec)
	sec = 59;
#endif

    /* Invert CONVERT by probing.  First assume the same offset as last time.
       Then repeatedly use the error to improve the guess.  */

    tm.tm_year = EPOCH_YEAR - TM_YEAR_BASE;
    tm.tm_yday = tm.tm_hour = tm.tm_min = tm.tm_sec = 0;
    t0 = __ydhms_tm_diff (year, yday, hour, min, sec, &tm);

    for (t = t1 = t2 = t0 + *offset;
	    (dt = __ydhms_tm_diff (year, yday, hour, min, sec,
				 __ranged_convert (convert, &t, &tm)));
	    t1 = t2, t2 = t, t += dt)
	if (t == t1 && t != t2
		&& (isdst < 0 || tm.tm_isdst < 0
		    || (isdst != 0) != (tm.tm_isdst != 0)))
	    /* We can't possibly find a match, as we are oscillating
	       between two values.  The requested time probably falls
	       within a spring-forward gap of size DT.  Follow the common
	       practice in this case, which is to return a time that is DT
	       away from the requested time, preferring a time whose
	       tm_isdst differs from the requested value.  In practice,
	       this is more useful than returning -1.  */
	    break;
	else if (--remaining_probes == 0)
	    return -1;

    /* If we have a match, check whether tm.tm_isdst has the requested
       value, if any.  */
    if (dt == 0 && isdst != tm.tm_isdst && 0 <= isdst && 0 <= tm.tm_isdst)
    {
	/* tm.tm_isdst has the wrong value.  Look for a neighboring
	   time with the right value, and use its UTC offset.
Heuristic: probe the previous three calendar quarters (approximately),
looking for the desired isdst.  This isn't perfect,
but it's good enough in practice.  */
	int quarter = 7889238; /* seconds per average 1/4 Gregorian year */
	int i;

	/* If we're too close to the time_t limit, look in future quarters.  */
	if (t < TIME_T_MIN + 3 * quarter)
	    quarter = -quarter;

	for (i = 1; i <= 3; i++)
	{
	    time_t ot = t - i * quarter;
	    struct tm otm;
	    __ranged_convert (convert, &ot, &otm);
	    if (otm.tm_isdst == isdst)
	    {
		/* We found the desired tm_isdst.
		   Extrapolate back to the desired time.  */
		t = ot + __ydhms_tm_diff (year, yday, hour, min, sec, &otm);
		__ranged_convert (convert, &t, &tm);
		break;
	    }
	}
    }

    *offset = t - t0;

#if LEAP_SECONDS_POSSIBLE
    if (sec_requested != tm.tm_sec)
    {
	/* Adjust time to reflect the tm_sec requested, not the normalized value.
	   Also, repair any damage from a false match due to a leap second.  */
	t += sec_requested - sec + (sec == 0 && tm.tm_sec == 60);
	if (! (*convert) (&t, &tm))
	    return -1;
    }
#endif

    if (TIME_T_MAX / INT_MAX / 366 / 24 / 60 / 60 < 3)
    {
	/* time_t isn't large enough to rule out overflows in __ydhms_tm_diff,
	   so check for major overflows.  A gross check suffices,
	   since if t has overflowed, it is off by a multiple of
	   TIME_T_MAX - TIME_T_MIN + 1.  So ignore any component of
	   the difference that is bounded by a small value.  */

	double dyear = (double) year_requested + mon_years - tm.tm_year;
	double dday = 366 * dyear + mday;
	double dsec = 60 * (60 * (24 * dday + hour) + min) + sec_requested;

	/* On Irix4.0.5 cc, dividing TIME_T_MIN by 3 does not produce
	   correct results, ie., it erroneously gives a positive value
	   of 715827882.  Setting a variable first then doing math on it
	   seems to work.  (ghazi@caip.rutgers.edu) */

	const time_t time_t_max = TIME_T_MAX;
	const time_t time_t_min = TIME_T_MIN;

	if (time_t_max / 3 - time_t_min / 3 < (dsec < 0 ? - dsec : dsec))
	    return -1;
    }

    *tp = tm;
    return t;
}



/* Convert *TP to a time_t value.  */
time_t mktime (struct tm *tp)
{
    static time_t localtime_offset;
    /* POSIX.1 8.1.1 requires that whenever mktime() is called, the
       time zone names contained in the external variable `tzname' shall
       be set as if the tzset() function had been called.  */
    tzset ();

    return __mktime_internal (tp, localtime_r, &localtime_offset);
}
#else

/* Convert *TP to a time_t value.  */
time_t mktime (struct tm *tp)
{
    time_t m_secs=tp->tm_min*60;
    time_t h_secs=tp->tm_hour*3600;
    time_t d_secs=tp->tm_yday*86400;
    time_t y_secs=(tp->tm_year-70)*31536000;
    time_t l_secs1=((tp->tm_year-69)/4)*86400;
    time_t l_secs2=((tp->tm_year-1)/100)*86400;
    time_t l_secs3=((tp->tm_year+299)/400)*86400;
    return m_secs+h_secs+d_secs+y_secs+l_secs1-l_secs2+l_secs3+tp->tm_gmtoff;
}
#endif