2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
4 * Copyright (c) 1997 Wolfgang Helbig
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
10 * 1. Redistributions of source code must retain the above copyright
11 * notice, this list of conditions and the following disclaimer.
12 * 2. Redistributions in binary form must reproduce the above copyright
13 * notice, this list of conditions and the following disclaimer in the
14 * documentation and/or other materials provided with the distribution.
16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
17 * 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 AUTHOR OR CONTRIBUTORS BE LIABLE
20 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
29 #include <sys/cdefs.h>
30 __FBSDID("$FreeBSD$");
39 * For each month tabulate the number of days elapsed in a year before the
40 * month. This assumes the internal date representation, where a year
41 * starts on March 1st. So we don't need a special table for leap years.
42 * But we do need a special table for the year 1582, since 10 days are
43 * deleted in October. This is month1s for the switch from Julian to
46 static int const month1[] =
47 {0, 31, 61, 92, 122, 153, 184, 214, 245, 275, 306, 337};
48 /* M A M J J A S O N D J */
49 static int const month1s[]=
50 {0, 31, 61, 92, 122, 153, 184, 214, 235, 265, 296, 327};
52 typedef struct date date;
54 /* The last day of Julian calendar, in internal and ndays representation */
55 static int nswitch; /* The last day of Julian calendar */
56 static date jiswitch = {1582, 7, 3};
58 static date *date2idt(date *idt, date *dt);
59 static date *idt2date(date *dt, date *idt);
60 static int ndaysji(date *idt);
61 static int ndaysgi(date *idt);
62 static int firstweek(int year);
65 * Compute the Julian date from the number of days elapsed since
66 * March 1st of year zero.
69 jdate(int ndays, date *dt)
71 date idt; /* Internal date representation */
72 int r; /* hold the rest of days */
75 * Compute the year by starting with an approximation not smaller
76 * than the answer and using linear search for the greatest
77 * year which does not begin after ndays.
82 while ((r = ndaysji(&idt)) > ndays)
86 * Set r to the days left in the year and compute the month by
87 * linear search as the largest month that does not begin after r
91 for (idt.m = 11; month1[idt.m] > r; idt.m--)
94 /* Compute the days left in the month */
95 idt.d = r - month1[idt.m];
97 /* return external representation of the date */
98 return (idt2date(dt, &idt));
102 * Return the number of days since March 1st of the year zero.
103 * The date is given according to Julian calendar.
108 date idt; /* Internal date representation */
110 if (date2idt(&idt, dt) == NULL)
113 return (ndaysji(&idt));
117 * Same as above, where the Julian date is given in internal notation.
118 * This formula shows the beauty of this notation.
124 return (idt->d + month1[idt->m] + idt->y * 365 + idt->y / 4);
128 * Compute the date according to the Gregorian calendar from the number of
129 * days since March 1st, year zero. The date computed will be Julian if it
130 * is older than 1582-10-05. This is the reverse of the function ndaysg().
133 gdate(int ndays, date *dt)
135 int const *montht; /* month-table */
136 date idt; /* for internal date representation */
137 int r; /* holds the rest of days */
140 * Compute the year by starting with an approximation not smaller
141 * than the answer and search linearly for the greatest year not
142 * starting after ndays.
147 while ((r = ndaysgi(&idt)) > ndays)
151 * Set ndays to the number of days left and compute by linear
152 * search the greatest month which does not start after ndays. We
153 * use the table month1 which provides for each month the number
154 * of days that elapsed in the year before that month. Here the
155 * year 1582 is special, as 10 days are left out in October to
156 * resynchronize the calendar with the earth's orbit. October 4th
157 * 1582 is followed by October 15th 1582. We use the "switch"
158 * table month1s for this year.
166 for (idt.m = 11; montht[idt.m] > ndays; idt.m--)
169 idt.d = ndays - montht[idt.m]; /* the rest is the day in month */
171 /* Advance ten days deleted from October if after switch in Oct 1582 */
172 if (idt.y == jiswitch.y && idt.m == jiswitch.m && jiswitch.d < idt.d)
175 /* return external representation of found date */
176 return (idt2date(dt, &idt));
180 * Return the number of days since March 1st of the year zero. The date is
181 * assumed Gregorian if younger than 1582-10-04 and Julian otherwise. This
182 * is the reverse of gdate.
187 date idt; /* Internal date representation */
189 if (date2idt(&idt, dt) == NULL)
191 return (ndaysgi(&idt));
195 * Same as above, but with the Gregorian date given in internal
201 int nd; /* Number of days--return value */
203 /* Cache nswitch if not already done */
205 nswitch = ndaysji(&jiswitch);
208 * Assume Julian calendar and adapt to Gregorian if necessary, i. e.
209 * younger than nswitch. Gregori deleted
210 * the ten days from Oct 5th to Oct 14th 1582.
211 * Thereafter years which are multiples of 100 and not multiples
212 * of 400 were not leap years anymore.
213 * This makes the average length of a year
214 * 365d +.25d - .01d + .0025d = 365.2425d. But the tropical
215 * year measures 365.2422d. So in 10000/3 years we are
216 * again one day ahead of the earth. Sigh :-)
217 * (d is the average length of a day and tropical year is the
218 * time from one spring point to the next.)
220 if ((nd = ndaysji(idt)) == -1)
223 nd = (nd - 10 - (idt->y - 1600) / 100 + (idt->y - 1600) / 400);
224 else if (nd > nswitch)
230 * Compute the week number from the number of days since March 1st year 0.
231 * The weeks are numbered per year starting with 1. If the first
232 * week of a year includes at least four days of that year it is week 1,
233 * otherwise it gets the number of the last week of the previous year.
234 * The variable y will be filled with the year that contains the greater
241 int fw; /* 1st day of week 1 of previous, this and
244 for (*y = dt.y + 1; nd < (fw = firstweek(*y)); (*y)--)
246 return ((nd - fw) / 7 + 1);
249 /* return the first day of week 1 of year y */
256 idt.y = y - 1; /* internal representation of y-1-1 */
262 * If more than 3 days of this week are in the preceding year, the
263 * next week is week 1 (and the next monday is the answer),
264 * otherwise this week is week 1 and the last monday is the
267 if ((wd = weekday(nd)) > 3)
268 return (nd - wd + 7);
273 /* return the weekday (Mo = 0 .. Su = 6) */
277 date dmondaygi = {1997, 8, 16}; /* Internal repr. of 1997-11-17 */
278 static int nmonday; /* ... which is a monday */
280 /* Cache the daynumber of one monday */
282 nmonday = ndaysgi(&dmondaygi);
284 /* return (nd - nmonday) modulo 7 which is the weekday */
285 nd = (nd - nmonday) % 7;
293 * Convert a date to internal date representation: The year starts on
294 * March 1st, month and day numbering start at zero. E. g. March 1st of
295 * year zero is written as y=0, m=0, d=0.
298 date2idt(date *idt, date *dt)
309 if (idt->m < 0 || idt->m > 11 || idt->y < 0)
315 /* Reverse of date2idt */
317 idt2date(date *dt, date *idt)