/*
 * [VernalEquinox.java]
 *
 * Summary: calculate when the Vernal (Spring) Equinox occurs.
 *
 * Copyright: (c) 1999-2017 Roedy Green, Canadian Mind Products, http://mindprod.com
 *
 * Licence: This software may be copied and used freely for any purpose but military.
 *          http://mindprod.com/contact/nonmil.html
 *
 * Requires: JDK 1.8+
 *
 * Created with: JetBrains IntelliJ IDEA IDE http://www.jetbrains.com/idea/
 *
 * Version History:
 *  4.2 2008-12-03 add World AIDS day
 */
package com.mindprod.holidays;

import com.mindprod.common18.BigDate;

import static java.lang.System.*;

/**
 * calculate when the Vernal (Spring) Equinox occurs.
 *
 * @author Roedy Green, Canadian Mind Products
 * @version 4.2 2008-12-03 add World AIDS day
 * @since 1999
 */
public final class VernalEquinox extends HolInfo
    {
    /**
     * Test driver
     *
     * @param args not used
     */
    public static void main( String[] args )
        {
        if ( DEBUGGING )
            {
            HolInfo h = new VernalEquinox();
            BigDate d = new BigDate();
            for ( int year = 1999; year <= 2012; year++ )
                {
                d.setOrdinal( h.when( year, false ) );
                out.println( d.getYYYY()
                             + "-"
                             + d.getMM()
                             + "-"
                             + d.getDD() );
                }
            }
        } // end main

    /**
     * @inheritDoc
     */
    public String getAuthority()
        {
        return "Jean Meeus\u2019s \"Astronomical Algorithms\"";
        }

    /**
     * @inheritDoc
     */
    public int getFirstYear( int base )
        {
        return -10000;
        }

    /**
     * @inheritDoc
     */
    public String getName()
        {
        return "Vernal Equinox";
        }

    /**
     * @inheritDoc
     */
    public String getRule()
        {
        return "March 19 to 21, based on earth\u2019s revolution around the sun.\n"
               + "Formulas we use are only valid 1000 .. 30000";
        }

    /**
     * @inheritDoc
     */
    public int when( int year, boolean shift, int base )
        {
        if ( !isYearValid( year, base ) )
            {
            return BigDate.NULL_ORDINAL;
            }
        if ( year < 1000 || year > 3000 )
            {
            // our formula is not currently accurate outside this range.
            return BigDate.NULL_ORDINAL;
            }
        // Roughly each equinox is 365.2423704 days after the previous,
        // with some second, third and fourth order tweaking.
        // The base equinox is 2000-03-20.
        double m = ( ( double ) year - 2000 ) / 1000;
        // result ve is astronomical Proleptic Julian day number
        double ve =
                2451623.80984 + 365242.37404 * m + 0.05169 * m * m
                - 0.00411 * m * m * m
                - 0.00057 * m * m * m * m;
        BigDate d = new BigDate( ve );
        return shiftSatToFriSunToMon( d.getOrdinal(), shift );
        } // end when
    }