time/mk_gmtime.c

   1 /*
   2  * (C)2012 Michael Duane Rice All rights reserved.
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions are
   6  * met:
   7  *
   8  * Redistributions of source code must retain the above copyright notice, this
   9  * list of conditions and the following disclaimer. Redistributions in binary
  10  * form must reproduce the above copyright notice, this list of conditions
  11  * and the following disclaimer in the documentation and/or other materials
  12  * provided with the distribution. Neither the name of the copyright holders
  13  * nor the names of contributors may be used to endorse or promote products
  14  * derived from this software without specific prior written permission.
  15  *
  16  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  17  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  18  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  19  * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
  20  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  21  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  22  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  23  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  24  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  25  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  26  * POSSIBILITY OF SUCH DAMAGE.
  27  */
  28
  29 /* $Id: mk_gmtime.c 2369 2013-04-28 14:19:35Z swfltek $ */
  30
  31 /*
  32     'Break down' a y2k time stamp into the elements of struct tm.
  33     Unlike mktime(), this function does not 'normalize' the elements of timeptr.
  34
  35 */
  36
  37 #include <time.h>
  38
  39 time_t
  40 mk_gmtime(const struct tm * timeptr)
  41 {
  42
  43     time_t          ret;
  44     uint32_t        tmp;
  45     int             n, m, d, leaps;
  46
  47     /*
  48         Determine elapsed whole days since the epoch to the beginning of this year. Since our epoch is
  49         at a conjunction of the leap cycles, we can do this rather quickly.
  50         */
  51     n = timeptr->tm_year - 100;
  52     leaps = 0;
  53     if (n) {
  54         m = n - 1;
  55         leaps = m / 4;
  56         leaps -= m / 100;
  57         leaps++;
  58     }
  59     tmp = 365UL * n + leaps;
  60
  61     /*
  62                 Derive the day of year from month and day of month. We use the pattern of 31 day months
  63                 followed by 30 day months to our advantage, but we must 'special case' Jan/Feb, and
  64                 account for a 'phase change' between July and August (153 days after March 1).
  65             */
  66     d = timeptr->tm_mday - 1;   /* tm_mday is one based */
  67
  68     /* handle Jan/Feb as a special case */
  69     if (timeptr->tm_mon < 2) {
  70         if (timeptr->tm_mon)
  71             d += 31;
  72
  73     } else {
  74         n = 59 + is_leap_year(timeptr->tm_year + 1900);
  75         d += n;
  76         n = timeptr->tm_mon - MARCH;
  77
  78         /* account for phase change */
  79         if (n > (JULY - MARCH))
  80             d += 153;
  81         n %= 5;
  82
  83         /*
  84          * n is now an index into a group of alternating 31 and 30
  85          * day months... 61 day pairs.
  86          */
  87         m = n / 2;
  88         m *= 61;
  89         d += m;
  90
  91         /*
  92          * if n is odd, we are in the second half of the
  93          * month pair
  94          */
  95         if (n & 1)
  96             d += 31;
  97     }
  98
  99     /* Add day of year to elapsed days, and convert to seconds */
 100     tmp += d;
 101     tmp *= ONE_DAY;
 102     ret = tmp;
 103
 104     /* compute 'fractional' day */
 105     tmp = timeptr->tm_hour;
 106     tmp *= ONE_HOUR;
 107     tmp += timeptr->tm_min * 60UL;
 108     tmp += timeptr->tm_sec;
 109
 110     ret += tmp;
 111
 112     return ret;
 113 }