Annotation of embedaddon/php/ext/calendar/julian.c, revision 1.1
1.1 ! misho 1: /* $selId: julian.c,v 2.0 1995/10/24 01:13:06 lees Exp $
! 2: * Copyright 1993-1995, Scott E. Lee, all rights reserved.
! 3: * Permission granted to use, copy, modify, distribute and sell so long as
! 4: * the above copyright and this permission statement are retained in all
! 5: * copies. THERE IS NO WARRANTY - USE AT YOUR OWN RISK.
! 6: */
! 7:
! 8: /**************************************************************************
! 9: *
! 10: * These are the externally visible components of this file:
! 11: *
! 12: * void
! 13: * SdnToJulian(
! 14: * long int sdn,
! 15: * int *pYear,
! 16: * int *pMonth,
! 17: * int *pDay);
! 18: *
! 19: * Convert a SDN to a Julian calendar date. If the input SDN is less than
! 20: * 1, the three output values will all be set to zero, otherwise *pYear
! 21: * will be >= -4713 and != 0; *pMonth will be in the range 1 to 12
! 22: * inclusive; *pDay will be in the range 1 to 31 inclusive.
! 23: *
! 24: * long int
! 25: * JulianToSdn(
! 26: * int inputYear,
! 27: * int inputMonth,
! 28: * int inputDay);
! 29: *
! 30: * Convert a Julian calendar date to a SDN. Zero is returned when the
! 31: * input date is detected as invalid or out of the supported range. The
! 32: * return value will be > 0 for all valid, supported dates, but there are
! 33: * some invalid dates that will return a positive value. To verify that a
! 34: * date is valid, convert it to SDN and then back and compare with the
! 35: * original.
! 36: *
! 37: * VALID RANGE
! 38: *
! 39: * 4713 B.C. to at least 10000 A.D.
! 40: *
! 41: * Although this software can handle dates all the way back to 4713
! 42: * B.C., such use may not be meaningful. The calendar was created in
! 43: * 46 B.C., but the details did not stabilize until at least 8 A.D.,
! 44: * and perhaps as late at the 4th century. Also, the beginning of a
! 45: * year varied from one culture to another - not all accepted January
! 46: * as the first month.
! 47: *
! 48: * CALENDAR OVERVIEW
! 49: *
! 50: * Julias Ceasar created the calendar in 46 B.C. as a modified form of
! 51: * the old Roman republican calendar which was based on lunar cycles.
! 52: * The new Julian calendar set fixed lengths for the months, abandoning
! 53: * the lunar cycle. It also specified that there would be exactly 12
! 54: * months per year and 365.25 days per year with every 4th year being a
! 55: * leap year.
! 56: *
! 57: * Note that the current accepted value for the tropical year is
! 58: * 365.242199 days, not 365.25. This lead to an 11 day shift in the
! 59: * calendar with respect to the seasons by the 16th century when the
! 60: * Gregorian calendar was created to replace the Julian calendar.
! 61: *
! 62: * The difference between the Julian and today's Gregorian calendar is
! 63: * that the Gregorian does not make centennial years leap years unless
! 64: * they are a multiple of 400, which leads to a year of 365.2425 days.
! 65: * In other words, in the Gregorian calendar, 1700, 1800 and 1900 are
! 66: * not leap years, but 2000 is. All centennial years are leap years in
! 67: * the Julian calendar.
! 68: *
! 69: * The details are unknown, but the lengths of the months were adjusted
! 70: * until they finally stablized in 8 A.D. with their current lengths:
! 71: *
! 72: * January 31
! 73: * February 28/29
! 74: * March 31
! 75: * April 30
! 76: * May 31
! 77: * June 30
! 78: * Quintilis/July 31
! 79: * Sextilis/August 31
! 80: * September 30
! 81: * October 31
! 82: * November 30
! 83: * December 31
! 84: *
! 85: * In the early days of the calendar, the days of the month were not
! 86: * numbered as we do today. The numbers ran backwards (decreasing) and
! 87: * were counted from the Ides (15th of the month - which in the old
! 88: * Roman republican lunar calendar would have been the full moon) or
! 89: * from the Nonae (9th day before the Ides) or from the beginning of
! 90: * the next month.
! 91: *
! 92: * In the early years, the beginning of the year varied, sometimes
! 93: * based on the ascension of rulers. It was not always the first of
! 94: * January.
! 95: *
! 96: * Also, today's epoch, 1 A.D. or the birth of Jesus Christ, did not
! 97: * come into use until several centuries later when Christianity became
! 98: * a dominant religion.
! 99: *
! 100: * ALGORITHMS
! 101: *
! 102: * The calculations are based on two different cycles: a 4 year cycle
! 103: * of leap years and a 5 month cycle of month lengths.
! 104: *
! 105: * The 5 month cycle is used to account for the varying lengths of
! 106: * months. You will notice that the lengths alternate between 30 and
! 107: * 31 days, except for three anomalies: both July and August have 31
! 108: * days, both December and January have 31, and February is less than
! 109: * 30. Starting with March, the lengths are in a cycle of 5 months
! 110: * (31, 30, 31, 30, 31):
! 111: *
! 112: * Mar 31 days \
! 113: * Apr 30 days |
! 114: * May 31 days > First cycle
! 115: * Jun 30 days |
! 116: * Jul 31 days /
! 117: *
! 118: * Aug 31 days \
! 119: * Sep 30 days |
! 120: * Oct 31 days > Second cycle
! 121: * Nov 30 days |
! 122: * Dec 31 days /
! 123: *
! 124: * Jan 31 days \
! 125: * Feb 28/9 days |
! 126: * > Third cycle (incomplete)
! 127: *
! 128: * For this reason the calculations (internally) assume that the year
! 129: * starts with March 1.
! 130: *
! 131: * TESTING
! 132: *
! 133: * This algorithm has been tested from the year 4713 B.C. to 10000 A.D.
! 134: * The source code of the verification program is included in this
! 135: * package.
! 136: *
! 137: * REFERENCES
! 138: *
! 139: * Conversions Between Calendar Date and Julian Day Number by Robert J.
! 140: * Tantzen, Communications of the Association for Computing Machinery
! 141: * August 1963. (Also published in Collected Algorithms from CACM,
! 142: * algorithm number 199). [Note: the published algorithm is for the
! 143: * Gregorian calendar, but was adjusted to use the Julian calendar's
! 144: * simpler leap year rule.]
! 145: *
! 146: **************************************************************************/
! 147:
! 148: #include "sdncal.h"
! 149: #include <limits.h>
! 150:
! 151: #define JULIAN_SDN_OFFSET 32083
! 152: #define DAYS_PER_5_MONTHS 153
! 153: #define DAYS_PER_4_YEARS 1461
! 154:
! 155: void SdnToJulian(
! 156: long int sdn,
! 157: int *pYear,
! 158: int *pMonth,
! 159: int *pDay)
! 160: {
! 161: int year;
! 162: int month;
! 163: int day;
! 164: long int temp;
! 165: int dayOfYear;
! 166:
! 167: if (sdn <= 0) {
! 168: goto fail;
! 169: }
! 170: /* Check for overflow */
! 171: if (sdn > (LONG_MAX - JULIAN_SDN_OFFSET * 4 + 1) / 4 || sdn < LONG_MIN / 4) {
! 172: goto fail;
! 173: }
! 174: temp = sdn * 4 + (JULIAN_SDN_OFFSET * 4 - 1);
! 175:
! 176: /* Calculate the year and day of year (1 <= dayOfYear <= 366). */
! 177: {
! 178: long yearl = temp / DAYS_PER_4_YEARS;
! 179: if (yearl > INT_MAX || yearl < INT_MIN) {
! 180: goto fail;
! 181: }
! 182: year = (int) yearl;
! 183: }
! 184: dayOfYear = (temp % DAYS_PER_4_YEARS) / 4 + 1;
! 185:
! 186: /* Calculate the month and day of month. */
! 187: temp = dayOfYear * 5 - 3;
! 188: month = temp / DAYS_PER_5_MONTHS;
! 189: day = (temp % DAYS_PER_5_MONTHS) / 5 + 1;
! 190:
! 191: /* Convert to the normal beginning of the year. */
! 192: if (month < 10) {
! 193: month += 3;
! 194: } else {
! 195: year += 1;
! 196: month -= 9;
! 197: }
! 198:
! 199: /* Adjust to the B.C./A.D. type numbering. */
! 200: year -= 4800;
! 201: if (year <= 0)
! 202: year--;
! 203:
! 204: *pYear = year;
! 205: *pMonth = month;
! 206: *pDay = day;
! 207: return;
! 208:
! 209: fail:
! 210: *pYear = 0;
! 211: *pMonth = 0;
! 212: *pDay = 0;
! 213: }
! 214:
! 215: long int JulianToSdn(
! 216: int inputYear,
! 217: int inputMonth,
! 218: int inputDay)
! 219: {
! 220: int year;
! 221: int month;
! 222:
! 223: /* check for invalid dates */
! 224: if (inputYear == 0 || inputYear < -4713 ||
! 225: inputMonth <= 0 || inputMonth > 12 ||
! 226: inputDay <= 0 || inputDay > 31) {
! 227: return (0);
! 228: }
! 229: /* check for dates before SDN 1 (Jan 2, 4713 B.C.) */
! 230: if (inputYear == -4713) {
! 231: if (inputMonth == 1 && inputDay == 1) {
! 232: return (0);
! 233: }
! 234: }
! 235: /* Make year always a positive number. */
! 236: if (inputYear < 0) {
! 237: year = inputYear + 4801;
! 238: } else {
! 239: year = inputYear + 4800;
! 240: }
! 241:
! 242: /* Adjust the start of the year. */
! 243: if (inputMonth > 2) {
! 244: month = inputMonth - 3;
! 245: } else {
! 246: month = inputMonth + 9;
! 247: year--;
! 248: }
! 249:
! 250: return ((year * DAYS_PER_4_YEARS) / 4
! 251: + (month * DAYS_PER_5_MONTHS + 2) / 5
! 252: + inputDay
! 253: - JULIAN_SDN_OFFSET);
! 254: }
! 255:
! 256: /*
! 257: * Local variables:
! 258: * tab-width: 4
! 259: * c-basic-offset: 4
! 260: * End:
! 261: * vim600: sw=4 ts=4 fdm=marker
! 262: * vim<600: sw=4 ts=4
! 263: */
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