YomiCalculator.java

/*
 * Zmanim Java API
 * Copyright (C) 2011-2025 Eliyahu Hershfeld
 *
 * This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General
 * Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option)
 * any later version.
 *
 * This library 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 Lesser General Public License for more
 * details.
 * You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA,
 * or connect to: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html
 */
package com.kosherjava.zmanim.hebrewcalendar;

import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.TimeZone;

/**
 * This class calculates the Daf Yomi Bavli page (daf) for a given date. To calculate Daf Yomi Yerushalmi
 * use the {@link YerushalmiYomiCalculator}. The library may cover Mishna Yomi etc. at some point in the future.
 * 
 * @author © Bob Newell (original C code)
 * @author © Eliyahu Hershfeld 2011 - 2025
 */
public class YomiCalculator {

	/**
	 * The start date of the first Daf Yomi Bavli cycle of September 11, 1923 / Rosh Hashana 5684.
	 */
	private static final Calendar DAF_YOMI_START_DAY;
	static {
		DAF_YOMI_START_DAY = new GregorianCalendar(1923, Calendar.SEPTEMBER, 11,0,0,0);
		DAF_YOMI_START_DAY.setTimeZone(TimeZone.getTimeZone("UTC"));
	}
	/** The start date of the first Daf Yomi Bavli cycle in the Julian calendar. Used internally for calculations.*/
	private static final int DAF_YOMI_JULIAN_START_DAY = getJulianDay(DAF_YOMI_START_DAY);
	/**
	 * The date that the pagination for the Daf Yomi <em>Maseches Shekalim</em> changed to use the commonly used Vilna
	 * Shas pagination from the no longer commonly available Zhitomir / Slavuta Shas used by Rabbi Meir Shapiro. 
	 */
	private static final Calendar SHEKALIM_CHANGE_DAY;
	static {
		SHEKALIM_CHANGE_DAY = new GregorianCalendar(1975, Calendar.JUNE, 24,0,0,0);
		SHEKALIM_CHANGE_DAY.setTimeZone(TimeZone.getTimeZone("UTC"));
	}
	
	/** The Julian date that the cycle for Shekalim changed.
	 * @see #getDafYomiBavli(JewishCalendar) for details.
	 */
	private static final int SHEKALIM_JULIAN_CHANGE_DAY = getJulianDay(SHEKALIM_CHANGE_DAY);
	
	/**
	 * Default constructor.
	 */
	public YomiCalculator() {
		// nothing here
	}

	/**
	 * Returns the <a href="http://en.wikipedia.org/wiki/Daf_yomi">Daf Yomi</a> <a
	 * href="http://en.wikipedia.org/wiki/Talmud">Bavli</a> {@link Daf} for a given date. The first Daf Yomi cycle
	 * started on Rosh Hashana 5684 (September 11, 1923) and calculations prior to this date will result in an
	 * IllegalArgumentException thrown. For historical calculations (supported by this method), it is important to note
	 * that a change in length of the cycle was instituted starting in the eighth Daf Yomi cycle beginning on June 24,
	 * 1975. The Daf Yomi Bavli cycle has a single masechta of the Talmud Yerushalmi - Shekalim as part of the cycle.
	 * Unlike the Bavli where the number of daf per masechta was standardized since the original <a
	 * href="http://en.wikipedia.org/wiki/Daniel_Bomberg">Bomberg Edition</a> published from 1520 - 1523, there is no
	 * uniform page length in the Yerushalmi. The early cycles had the Yerushalmi Shekalim length of 13 days following the
	 * <a href=
	 * "https://he.wikipedia.org/wiki/%D7%93%D7%A4%D7%95%D7%A1_%D7%A1%D7%9C%D7%90%D7%95%D7%95%D7%99%D7%98%D7%90">Slavuta/Zhytomyr</a>
	 * Shas used by <a href="http://en.wikipedia.org/wiki/Meir_Shapiro">Rabbi Meir Shapiro</a>. With the start of the eighth Daf Yomi
	 * cycle beginning on June 24, 1975, the length of the Yerushalmi Shekalim was changed from 13 to 22 daf to follow
	 * the <a href="https://en.wikipedia.org/wiki/Vilna_Edition_Shas">Vilna Shas</a> that is in common use today.
	 * 
	 * @param jewishCalendar
	 *            The JewishCalendar date for calculation. TODO: this can be changed to use a regular GregorianCalendar since
	 *            there is nothing specific to the JewishCalendar in this class.
	 * @return the {@link Daf}.
	 * 
	 * @throws IllegalArgumentException
	 *             if the date is prior to the September 11, 1923, the start date of the first Daf Yomi cycle.
	 */
	public static Daf getDafYomiBavli(JewishCalendar jewishCalendar) {
		/*
		 * The number of daf per masechta. Since the number of blatt in Shekalim changed on the 8th Daf Yomi cycle
		 * beginning on June 24, 1975, from 13 to 22, the actual calculation for blattPerMasechta[4] will later be
		 * adjusted based on the cycle.
		 */
		int[] blattPerMasechta = { 64, 157, 105, 121, 22, 88, 56, 40, 35, 31, 32, 29, 27, 122, 112, 91, 66, 49, 90, 82,
				119, 119, 176, 113, 24, 49, 76, 14, 120, 110, 142, 61, 34, 34, 28, 22, 4, 9, 5, 73 };
		Calendar calendar = jewishCalendar.getGregorianCalendar();

		Daf dafYomi = null;
		int julianDay = getJulianDay(calendar);
		int cycleNo;
		int dafNo;
		if (calendar.before(DAF_YOMI_START_DAY)) {
			// TODO: should we return a null or throw an IllegalArgumentException?
			throw new IllegalArgumentException(calendar + " is prior to organized Daf Yomi Bavli cycles that started on "
					+ DAF_YOMI_START_DAY);
		}
		if (calendar.equals(SHEKALIM_CHANGE_DAY) || calendar.after(SHEKALIM_CHANGE_DAY)) {
			cycleNo = 8 + ((julianDay - SHEKALIM_JULIAN_CHANGE_DAY) / 2711);
			dafNo = ((julianDay - SHEKALIM_JULIAN_CHANGE_DAY) % 2711);
		} else {
			cycleNo = 1 + ((julianDay - DAF_YOMI_JULIAN_START_DAY) / 2702);
			dafNo = ((julianDay - DAF_YOMI_JULIAN_START_DAY) % 2702);
		}

		int total = 0;
		int masechta = -1;
		int blatt;

		// Fix Shekalim for old cycles.
		if (cycleNo <= 7) {
			blattPerMasechta[4] = 13;
		}
		
		// Finally find the daf.
		for (int i : blattPerMasechta) {
			masechta++;
			total = total + i - 1;
			if (dafNo < total) {
				blatt = 1 + i - (total - dafNo);
				// Fiddle with the weird ones near the end.
				if (masechta == 36) {
				    blatt += 21;
				} else if (masechta == 37) {
				    blatt += 24;
				} else if (masechta == 38) {
				    blatt += 32;
				}
				dafYomi = new Daf(masechta, blatt);
				break;
			}
		}

		return dafYomi;
	}

	/**
	 * Return the <a href="http://en.wikipedia.org/wiki/Julian_day">Julian day</a> from a Java Calendar.
	 * 
	 * @param calendar
	 *            The Java Calendar of the date to be calculated
	 * @return the Julian day number corresponding to the date
	 */
	private static int getJulianDay(Calendar calendar) {
		int year = calendar.get(Calendar.YEAR);
		int month = calendar.get(Calendar.MONTH) + 1;
		int day = calendar.get(Calendar.DAY_OF_MONTH);
		if (month <= 2) {
			year -= 1;
			month += 12;
		}
		int a = year / 100;
		int b = 2 - a + a / 4;
		return (int) (Math.floor(365.25 * (year + 4716)) + Math.floor(30.6001 * (month + 1)) + day + b - 1524.5);
	}
}