/*- * Copyright (c) 1997 Wolfgang Helbig * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF * SUCH DAMAGE. */ #include <sys/cdefs.h> __FBSDID("$FreeBSD: src/lib/libcalendar/calendar.c,v 1.4 2001/09/30 21:09:57 dillon Exp $"); #include "calendar.h" #ifndef NULL #define NULL 0 #endif /* * For each month tabulate the number of days elapsed in a year before the * month. This assumes the internal date representation, where a year * starts on March 1st. So we don't need a special table for leap years. * But we do need a special table for the year 1582, since 10 days are * deleted in October. This is month1s for the switch from Julian to * Gregorian calendar. */ static int const month1[] = {0, 31, 61, 92, 122, 153, 184, 214, 245, 275, 306, 337}; /* M A M J J A S O N D J */ static int const month1s[]= {0, 31, 61, 92, 122, 153, 184, 214, 235, 265, 296, 327}; typedef struct date date; /* The last day of Julian calendar, in internal and ndays representation */ static int nswitch; /* The last day of Julian calendar */ static date jiswitch = {1582, 7, 3}; static date *date2idt(date *idt, date *dt); static date *idt2date(date *dt, date *idt); static int ndaysji(date *idt); static int ndaysgi(date *idt); static int firstweek(int year); /* * Compute the Julian date from the number of days elapsed since * March 1st of year zero. */ date * jdate(int ndays, date *dt) { date idt; /* Internal date representation */ int r; /* hold the rest of days */ /* * Compute the year by starting with an approximation not smaller * than the answer and using linear search for the greatest * year which does not begin after ndays. */ idt.y = ndays / 365; idt.m = 0; idt.d = 0; while ((r = ndaysji(&idt)) > ndays) idt.y--; /* * Set r to the days left in the year and compute the month by * linear search as the largest month that does not begin after r * days. */ r = ndays - r; for (idt.m = 11; month1[idt.m] > r; idt.m--) ; /* Compute the days left in the month */ idt.d = r - month1[idt.m]; /* return external representation of the date */ return (idt2date(dt, &idt)); } /* * Return the number of days since March 1st of the year zero. * The date is given according to Julian calendar. */ int ndaysj(date *dt) { date idt; /* Internal date representation */ if (date2idt(&idt, dt) == NULL) return (-1); else return (ndaysji(&idt)); } /* * Same as above, where the Julian date is given in internal notation. * This formula shows the beauty of this notation. */ static int ndaysji(date * idt) { return (idt->d + month1[idt->m] + idt->y * 365 + idt->y / 4); } /* * Compute the date according to the Gregorian calendar from the number of * days since March 1st, year zero. The date computed will be Julian if it * is older than 1582-10-05. This is the reverse of the function ndaysg(). */ date * gdate(int ndays, date *dt) { int const *montht; /* month-table */ date idt; /* for internal date representation */ int r; /* holds the rest of days */ /* * Compute the year by starting with an approximation not smaller * than the answer and search linearly for the greatest year not * starting after ndays. */ idt.y = ndays / 365; idt.m = 0; idt.d = 0; while ((r = ndaysgi(&idt)) > ndays) idt.y--; /* * Set ndays to the number of days left and compute by linear * search the greatest month which does not start after ndays. We * use the table month1 which provides for each month the number * of days that elapsed in the year before that month. Here the * year 1582 is special, as 10 days are left out in October to * resynchronize the calendar with the earth's orbit. October 4th * 1582 is followed by October 15th 1582. We use the "switch" * table month1s for this year. */ ndays = ndays - r; if (idt.y == 1582) montht = month1s; else montht = month1; for (idt.m = 11; montht[idt.m] > ndays; idt.m--) ; idt.d = ndays - montht[idt.m]; /* the rest is the day in month */ /* Advance ten days deleted from October if after switch in Oct 1582 */ if (idt.y == jiswitch.y && idt.m == jiswitch.m && jiswitch.d < idt.d) idt.d += 10; /* return external representation of found date */ return (idt2date(dt, &idt)); } /* * Return the number of days since March 1st of the year zero. The date is * assumed Gregorian if younger than 1582-10-04 and Julian otherwise. This * is the reverse of gdate. */ int ndaysg(date *dt) { date idt; /* Internal date representation */ if (date2idt(&idt, dt) == NULL) return (-1); return (ndaysgi(&idt)); } /* * Same as above, but with the Gregorian date given in internal * representation. */ static int ndaysgi(date *idt) { int nd; /* Number of days--return value */ /* Cache nswitch if not already done */ if (nswitch == 0) nswitch = ndaysji(&jiswitch); /* * Assume Julian calendar and adapt to Gregorian if necessary, i. e. * younger than nswitch. Gregori deleted * the ten days from Oct 5th to Oct 14th 1582. * Thereafter years which are multiples of 100 and not multiples * of 400 were not leap years anymore. * This makes the average length of a year * 365d +.25d - .01d + .0025d = 365.2425d. But the tropical * year measures 365.2422d. So in 10000/3 years we are * again one day ahead of the earth. Sigh :-) * (d is the average length of a day and tropical year is the * time from one spring point to the next.) */ if ((nd = ndaysji(idt)) == -1) return (-1); if (idt->y >= 1600) nd = (nd - 10 - (idt->y - 1600) / 100 + (idt->y - 1600) / 400); else if (nd > nswitch) nd -= 10; return (nd); } /* * Compute the week number from the number of days since March 1st year 0. * The weeks are numbered per year starting with 1. If the first * week of a year includes at least four days of that year it is week 1, * otherwise it gets the number of the last week of the previous year. * The variable y will be filled with the year that contains the greater * part of the week. */ int week(int nd, int *y) { date dt; int fw; /* 1st day of week 1 of previous, this and * next year */ gdate(nd, &dt); for (*y = dt.y + 1; nd < (fw = firstweek(*y)); (*y)--) ; return ((nd - fw) / 7 + 1); } /* return the first day of week 1 of year y */ static int firstweek(int y) { date idt; int nd, wd; idt.y = y - 1; /* internal representation of y-1-1 */ idt.m = 10; idt.d = 0; nd = ndaysgi(&idt); /* * If more than 3 days of this week are in the preceding year, the * next week is week 1 (and the next monday is the answer), * otherwise this week is week 1 and the last monday is the * answer. */ if ((wd = weekday(nd)) > 3) return (nd - wd + 7); else return (nd - wd); } /* return the weekday (Mo = 0 .. Su = 6) */ int weekday(int nd) { date dmondaygi = {1997, 8, 16}; /* Internal repr. of 1997-11-17 */ static int nmonday; /* ... which is a monday */ /* Cache the daynumber of one monday */ if (nmonday == 0) nmonday = ndaysgi(&dmondaygi); /* return (nd - nmonday) modulo 7 which is the weekday */ nd = (nd - nmonday) % 7; if (nd < 0) return (nd + 7); else return (nd); } /* * Convert a date to internal date representation: The year starts on * March 1st, month and day numbering start at zero. E. g. March 1st of * year zero is written as y=0, m=0, d=0. */ static date * date2idt(date *idt, date *dt) { idt->d = dt->d - 1; if (dt->m > 2) { idt->m = dt->m - 3; idt->y = dt->y; } else { idt->m = dt->m + 9; idt->y = dt->y - 1; } if (idt->m < 0 || idt->m > 11 || idt->y < 0) return (NULL); else return idt; } /* Reverse of date2idt */ static date * idt2date(date *dt, date *idt) { dt->d = idt->d + 1; if (idt->m < 10) { dt->m = idt->m + 3; dt->y = idt->y; } else { dt->m = idt->m - 9; dt->y = idt->y + 1; } if (dt->m < 1) return (NULL); else return (dt); }