hebcal-fixed/src/greg.js

An easy to use, richly-featured, perpetual Jewish Calendar API

/*
	Hebcal - A Jewish Calendar Generator
	Copyright (C) 1994-2004  Danny Sadinoff
	Portions Copyright (c) 2002 Michael J. Radwin. All Rights Reserved.

	https://github.com/hebcal/hebcal-js

	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 3 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, see <http://www.gnu.org/licenses/>.

	Danny Sadinoff can be reached at danny@sadinoff.com

	Michael Radwin has made significant contributions as a result of
	maintaining hebcal.com.

	The JavaScript code was completely rewritten in 2014 by Eyal Schachter.
 */
var floor = Math.floor,
	t0t1 = [30, 31],
	tMonthLengths = [0, 31, 28, 31].concat(t0t1, t0t1, 31, t0t1, t0t1),
	monthLengths = [
		tMonthLengths.slice()
	];
tMonthLengths[2]++;
monthLengths.push(tMonthLengths);

exports.daysInMonth = function(month, year) { // 1 based months
	return monthLengths[+LEAP(year)][month];
};

exports.monthNames = [
	'',
	'January',
	'February',
	'March',
	'April',
	'May',
	'June',
	'July',
	'August',
	'September',
	'October',
	'November',
	'December'
];

exports.lookupMonthNum = function(month) {
	return new Date(month + ' 1').getMonth() + 1;
};

function dayOfYear (date) {
	if (!date instanceof Date) {
		throw new TypeError('Argument to greg.dayOfYear not a Date');
	}
	var doy = date.getDate() + 31 * date.getMonth();
	if (date.getMonth() > 1) { // FEB
		doy -= floor((4 * (date.getMonth() + 1) + 23) / 10);
		if (LEAP(date.getFullYear())) {
			doy++;
		}
	}
	return doy;
}
exports.dayOfYear = dayOfYear;

function LEAP (year) {
	return !(year % 4) && ( !!(year % 100) || !(year % 400) );
}
exports.LEAP = LEAP;

exports.greg2abs = function(date) { // "absolute date"
	var year = date.getFullYear() - 1;
	return (dayOfYear(date) + // days this year
			365 * year + // + days in prior years
			( floor(year / 4) - // + Julian Leap years
			floor(year / 100) + // - century years
			floor(year / 400))); // + Gregorian leap years
};


/*
 * See the footnote on page 384 of ``Calendrical Calculations, Part II:
 * Three Historical Calendars'' by E. M. Reingold,  N. Dershowitz, and S. M.
 * Clamen, Software--Practice and Experience, Volume 23, Number 4
 * (April, 1993), pages 383-404 for an explanation.
 */
exports.abs2greg = function(theDate) {
// calculations copied from original JS code

	var d0 = theDate - 1;
	var n400 = floor(d0 / 146097);
	var d1 =  floor(d0 % 146097);
	var n100 =  floor(d1 / 36524);
	var d2 = d1 % 36524;
	var n4 =  floor(d2 / 1461);
	var d3 = d2 % 1461;
	var n1 =  floor(d3 / 365);

	var day = ((d3 % 365) + 1);
	var year = (400 * n400 + 100 * n100 + 4 * n4 + n1);

	if (4 == n100 || 4 == n1) {
		return new Date(year, 11, 31);
	}

	return new Date(new Date(++year, 0, day).setFullYear(year)); // new Date() is very smart
};