src/date.c

   1 /* date.c
   2  * Routines for date handling
   3  * Copyright (C) 2010,2015,2018 Olly Betts
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License
  16  * along with this program; if not, write to the Free Software
  17  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
  18  */
  19
  20 #ifdef HAVE_CONFIG_H
  21 # include <config.h>
  22 #endif
  23
  24 #include "date.h"
  25
  26 #include "debug.h"
  27
  28 int
  29 is_leap_year(int year)
  30 {
  31     return (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
  32 }
  33
  34 unsigned int
  35 last_day(int year, int month)
  36 {
  37     static const unsigned char lastday[13] = {
  38         0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
  39     };
  40     SVX_ASSERT(month >= 1 && month <= 12);
  41     return (month == 2 && is_leap_year(year)) ? 29 : lastday[month];
  42 }
  43
  44 int
  45 days_since_1900(int y, int m, int d)
  46 {
  47     static const int m_to_d[12] = {
  48         0 - (1900 * 365) - 461 + 365,
  49         31 - (1900 * 365) - 461 + 365,
  50         59 - (1900 * 365) - 461,
  51         90 - (1900 * 365) - 461,
  52         120 - (1900 * 365) - 461,
  53         151 - (1900 * 365) - 461,
  54         181 - (1900 * 365) - 461,
  55         212 - (1900 * 365) - 461,
  56         243 - (1900 * 365) - 461,
  57         273 - (1900 * 365) - 461,
  58         304 - (1900 * 365) - 461,
  59         334 - (1900 * 365) - 461
  60     };
  61     if (m < 3)
  62         --y;
  63     return d + (y * 365) + m_to_d[m - 1] + (y / 4) - (y / 100) + (y / 400);
  64 }
  65
  66 void
  67 ymd_from_days_since_1900(int days, int * py, int * pm, int * pd)
  68 {
  69     int g, dg, c, dc, b, db, a, da, y, m;
  70     days += 693901;
  71     g = days / 146097;
  72     dg = days % 146097;
  73     c = (dg / 36524 + 1) * 3 / 4;
  74     dc = dg - c * 36524;
  75     b = dc / 1461;
  76     db = dc % 1461;
  77     a = (db / 365 + 1) * 3 / 4;
  78     da = db - a * 365;
  79     y = g * 400 + c * 100 + b * 4 + a;
  80     m = (da * 5 + 308) / 153;
  81     *py = y + m / 12;
  82     *pm = m % 12 + 1;
  83     *pd = da - (m + 2) * 153 / 5 + 123;
  84 }
  85
  86 double
  87 julian_date_from_days_since_1900(int days)
  88 {
  89     int g, dg, c, dc, b, db, a, da, y, m;
  90     int days_in = days;
  91     int dys;
  92     double scale;
  93     days += 693901;
  94     g = days / 146097;
  95     dg = days % 146097;
  96     c = (dg / 36524 + 1) * 3 / 4;
  97     dc = dg - c * 36524;
  98     b = dc / 1461;
  99     db = dc % 1461;
 100     a = (db / 365 + 1) * 3 / 4;
 101     da = db - a * 365;
 102     y = g * 400 + c * 100 + b * 4 + a;
 103     m = (da * 5 + 308) / 153;
 104     y = y + m / 12;
 105     /* dys is days since 1900 for the start of the year y. */
 106     dys = (y - 1900) * 365 - 460;
 107     dys += ((y - 1) / 4) - ((y - 1) / 100) + ((y - 1) / 400);
 108     scale = (is_leap_year(y) ? 1 / 366.0 : 1 / 365.0);
 109     return y + scale * (days_in - dys);
 110 }