src/core/time.c

   1 /*
   2  * Copyright (C) 2012 Michael Brown <mbrown@fensystems.co.uk>.
   3  *
   4  * This program is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU General Public License as
   6  * published by the Free Software Foundation; either version 2 of the
   7  * License, or any later version.
   8  *
   9  * This program is distributed in the hope that it will be useful, but
  10  * WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12  * General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU General Public License
  15  * along with this program; if not, write to the Free Software
  16  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
  17  * 02110-1301, USA.
  18  *
  19  * You can also choose to distribute this program under the terms of
  20  * the Unmodified Binary Distribution Licence (as given in the file
  21  * COPYING.UBDL), provided that you have satisfied its requirements.
  22  */
  23
  24 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  25
  26 #include <time.h>
  27
  28 /** @file
  29  *
  30  * Date and time
  31  *
  32  * POSIX:2008 section 4.15 defines "seconds since the Epoch" as an
  33  * abstract measure approximating the number of seconds that have
  34  * elapsed since the Epoch, excluding leap seconds.  The formula given
  35  * is
  36  *
  37  *    tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 +
  38  *    (tm_year-70)*31536000 + ((tm_year-69)/4)*86400 -
  39  *    ((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400
  40  *
  41  * This calculation assumes that leap years occur in each year that is
  42  * either divisible by 4 but not divisible by 100, or is divisible by
  43  * 400.
  44  */
  45
  46 /** Days of week (for debugging) */
  47 static const char *weekdays[] = {
  48         "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
  49 };
  50
  51 /**
  52  * Determine whether or not year is a leap year
  53  *
  54  * @v tm_year           Years since 1900
  55  * @v is_leap_year      Year is a leap year
  56  */
  57 static int is_leap_year ( int tm_year ) {
  58         int leap_year = 0;
  59
  60         if ( ( tm_year % 4 ) == 0 )
  61                 leap_year = 1;
  62         if ( ( tm_year % 100 ) == 0 )
  63                 leap_year = 0;
  64         if ( ( tm_year % 400 ) == 100 )
  65                 leap_year = 1;
  66
  67         return leap_year;
  68 }
  69
  70 /**
  71  * Calculate number of leap years since 1900
  72  *
  73  * @v tm_year           Years since 1900
  74  * @v num_leap_years    Number of leap years
  75  */
  76 static int leap_years_to_end ( int tm_year ) {
  77         int leap_years = 0;
  78
  79         leap_years += ( tm_year / 4 );
  80         leap_years -= ( tm_year / 100 );
  81         leap_years += ( ( tm_year + 300 ) / 400 );
  82
  83         return leap_years;
  84 }
  85
  86 /**
  87  * Calculate day of week
  88  *
  89  * @v tm_year           Years since 1900
  90  * @v tm_mon            Month of year [0,11]
  91  * @v tm_day            Day of month [1,31]
  92  */
  93 static int day_of_week ( int tm_year, int tm_mon, int tm_mday ) {
  94         static const uint8_t offset[12] =
  95                 { 1, 4, 3, 6, 1, 4, 6, 2, 5, 0, 3, 5 };
  96         int pseudo_year = tm_year;
  97
  98         if ( tm_mon < 2 )
  99                 pseudo_year--;
 100         return ( ( pseudo_year + leap_years_to_end ( pseudo_year ) +
 101                    offset[tm_mon] + tm_mday ) % 7 );
 102 }
 103
 104 /** Days from start of year until start of months (in non-leap years) */
 105 static const uint16_t days_to_month_start[] =
 106         { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };
 107
 108 /**
 109  * Calculate seconds since the Epoch
 110  *
 111  * @v tm                Broken-down time
 112  * @ret time            Seconds since the Epoch
 113  */
 114 time_t mktime ( struct tm *tm ) {
 115         int days_since_epoch;
 116         int seconds_since_day;
 117         time_t seconds;
 118
 119         /* Calculate day of year */
 120         tm->tm_yday = ( ( tm->tm_mday - 1 ) +
 121                         days_to_month_start[ tm->tm_mon ] );
 122         if ( ( tm->tm_mon >= 2 ) && is_leap_year ( tm->tm_year ) )
 123                 tm->tm_yday++;
 124
 125         /* Calculate day of week */
 126         tm->tm_wday = day_of_week ( tm->tm_year, tm->tm_mon, tm->tm_mday );
 127
 128         /* Calculate seconds since the Epoch */
 129         days_since_epoch = ( tm->tm_yday + ( 365 * tm->tm_year ) - 25567 +
 130                              leap_years_to_end ( tm->tm_year - 1 ) );
 131         seconds_since_day =
 132                 ( ( ( ( tm->tm_hour * 60 ) + tm->tm_min ) * 60 ) + tm->tm_sec );
 133         seconds = ( ( ( ( time_t ) days_since_epoch ) * ( ( time_t ) 86400 ) ) +
 134                     seconds_since_day );
 135
 136         DBGC ( &weekdays, "TIME %04d-%02d-%02d %02d:%02d:%02d => %lld (%s, "
 137                "day %d)\n", ( tm->tm_year + 1900 ), ( tm->tm_mon + 1 ),
 138                tm->tm_mday, tm->tm_hour, tm->tm_min, tm->tm_sec, seconds,
 139                weekdays[ tm->tm_wday ], tm->tm_yday );
 140
 141         return seconds;
 142 }