//----------------------------------------------------------------------------- // Copyright © 2004 - Philip Howard - All rights reserved // // This program is free software; you can redistribute it and/or // modify it under the terms of the GNU General Public License // as published by the Free Software Foundation; either version 2 // of the License, or (at your option) any later version. // // This program is distributed in the hope that it will be useful, // but WITHOUT ANY WARRANTY; without even the implied warranty of // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the // GNU General Public License for more details. // // You should have received a copy of the GNU General Public License // along with this program; if not, write to the Free Software // Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. //----------------------------------------------------------------------------- // package libh/time // homepage http://libh.slashusr.org/ //----------------------------------------------------------------------------- // author Philip Howard // email libh at ipal dot org // homepage http://phil.ipal.org/ //----------------------------------------------------------------------------- // This file is best viewed using a fixed spaced font such as Courier // and in a display at least 120 columns wide. //----------------------------------------------------------------------------- // reference Explanatory Supplement to the Astronomical Almanac, // P. Kenneth Seidelmann, editor [ISBN 0-935702-68-7] // http://shop.bn.com/bookSearch/isbnInquiry.asp?isbn=0935702687 //----------------------------------------------------------------------------- #include #include "time_lib.h" __PROTO_BEGIN__ //----------------------------------------------------------------------------- // function eday_to_ecal_gregorian // alias eday_to_ecal // // purpose Convert an Earth Day value to Gregorian calendar elements. // // arguments 1 (eday_t) Earth Day value // 2 (ecal_t *) where to store calendar elements. // // returns (void) //----------------------------------------------------------------------------- #define eday_to_ecal eday_to_ecal_gregorian void eday_to_ecal_gregorian ( eday_t arg_eday , ecal_t * arg_caltime ) __PROTO_END__ { long day ; long quadcent ; long cent ; long quadyear ; long year ; //------------------------------------------------------------------ // Adjust epoch from Earth Day to Gregorian era at year 0. //------------------------------------------------------------------ day = arg_eday - EDAY_1_JAN_0; //------------------------------------------------------------------ // Calculate the quad century number. //------------------------------------------------------------------ // Dates and times require a consistent periodic modulus even when // the dividend is negative. Standard C 1999 defines the / and % // operators to use "division by truncation toward zero" which is // not what produces a consistent periodic modulus. Instead, the // quo_ni() and mod_ni() functions are used to get the proper results. //------------------------------------------------------------------ quadcent = quo_ni( day, 146097 ); //------------------------------------------------------------------ // Normalize the day number to within one quad-century. //------------------------------------------------------------------ // This is easy because a quad-century (400 years) is always // exactly 146097 days in the Gregorian calendar. A negative // quad-century number will move the negative day number up to // a positive value. //------------------------------------------------------------------ day -= quadcent * 146097; //------------------------------------------------------------------ // Calculate the century within the quad-century. //------------------------------------------------------------------ // The first century has an extra leap year so it is longer by one // day. Shift the range of days down by this one so the centuries // line up with where the division gives different quotients. //------------------------------------------------------------------ -- day; cent = ( day < 0 ) ? 0 : day / 36524; //------------------------------------------------------------------ // Normalize the day number to within one century. //------------------------------------------------------------------ // Since the first century has an extra leap year, if this century // is after the first one, an extra day must be subtracted to // account for the extra leap year. Since one is already // subtracted, just add one back if this is the first century // (none preceeding). //------------------------------------------------------------------ day -= cent * 36524; day += ( cent == 0 ); //------------------------------------------------------------------ // Calculate quad-year within the century. //------------------------------------------------------------------ // Since the first quad-year in centuries other than the first // century has no leap year, if this is that first century, then // shift the range of days up by one so the quad-years line up // with where the division gives different quotients. //------------------------------------------------------------------ quadyear = ( day + ( cent > 0 ) ) / 1461; //------------------------------------------------------------------ // Normalize the day number to within one quad-year. //------------------------------------------------------------------ // Since the first quad-year in centuries other than the first // lacks a leap year where otherwise there is one, in the case of // the first quad-year being a preceeding one in a century other // than the first, add back the day that was overly subtracted. //------------------------------------------------------------------ day -= quadyear * 1461; day += ( quadyear > 0 && cent > 0 ); //------------------------------------------------------------------ // Calculate year within the quad-year. //------------------------------------------------------------------ // Since the first year in a quad-year is leap, but only when the // quad-year is other than first or when the century is the first, // shift the range of days down by one so the years line up with // where the division gives different quotients. //------------------------------------------------------------------ // Standard C 1999 guarantees division is "truncation to zero" // which means -1/365 = 0. If not Standard C 1999, then we have // to handle the first day to ensure the quotient is zero since // some division may result in "truncation down" to -1. Only when // day == 0 can this rare problem happen and only when division // is not conforming to Standard C 1999. //------------------------------------------------------------------ #if defined(__STDC_VERSION__) && __STDC_VERSION__ == 199901L year = ( day - ( quadyear > 0 || cent == 0 ) ) / 365; #else year = ( day == 0 ) ? 0 : ( day - ( quadyear > 0 || cent == 0 ) ) / 365; #endif //------------------------------------------------------------------ // Normalize the day number to within one year. //------------------------------------------------------------------ // Since the first year in a quad-year is leap, but only when the // quad-year is other than first or when the century is the first, // if the first year preceeds this year, and this is a quad-year // other than the first or is the first century, an extra day is // subtracted to account for the extra day in the leap year. //------------------------------------------------------------------ day -= year * 365; if ( year > 0 && ( quadyear > 0 || cent == 0 ) ) -- day; //------------------------------------------------------------------ // Reconstruct the actual Gregorian calendar elements. //------------------------------------------------------------------ arg_caltime->year = year + ( ( quadcent * 4 + cent ) * 25 + quadyear ) * 4; arg_caltime->isleap = year == 0 && ( quadyear > 0 || cent == 0 ); arg_caltime->month = doy_to_mon( day, arg_caltime->isleap ); arg_caltime->mday = doy_to_dom( day, arg_caltime->isleap ); arg_caltime->yday = day; //------------------------------------------------------------------ // Calculate the day of week. //------------------------------------------------------------------ // The eday epoch is 1 day offset from giving the day of week // directly so it is necessary to add 1 before calculating the // modulus. Because this would overflow on EDAY_MAX, subtract 6 // instead for values over 7. Then an additional 1 is added to // make the day of week be in the range of 1..7. //------------------------------------------------------------------ arg_caltime->wday = mod_ni( arg_eday + ( arg_eday > 7 ? -6 : 1 ), 7 ) + 1; //-------------------------------------------------------------- // The week number is 0 to 53. The first full week is number 1. //-------------------------------------------------------------- arg_caltime->week = ( day + 8 - arg_caltime->wday ) / 7; //-------------------------------------------------------- // Make it known this is a date in the Gregorian calendar. //-------------------------------------------------------- arg_caltime->type = ECAL_TYPE_GREGORIAN; //------------------------------------------------------------- // Set time elements to zero to represent the start of the day. //------------------------------------------------------------- arg_caltime->hour = 0; arg_caltime->min = 0; arg_caltime->sec = 0; arg_caltime->nsec = 0; return; }