/*
 *  Copyright 2001-2014 Stephen Colebourne
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */
package org.joda.time.chrono;

import java.util.concurrent.ConcurrentHashMap;

import org.joda.time.Chronology;
import org.joda.time.DateTimeConstants;
import org.joda.time.DateTimeZone;

Implements a pure proleptic Gregorian calendar system, which defines every
fourth year as leap, unless the year is divisible by 100 and not by 400.
This improves upon the Julian calendar leap year rule.

Although the Gregorian calendar did not exist before 1582 CE, this
chronology assumes it did, thus it is proleptic. This implementation also
fixes the start of the year at January 1, and defines the year zero.

GregorianChronology is thread-safe and immutable.
Author:
Guy Allard, Stephen Colebourne, Brian S O'Neill
See Also:
Wikipedia
JulianChronology
GJChronology
Since:
1.0/**
 * Implements a pure proleptic Gregorian calendar system, which defines every
 * fourth year as leap, unless the year is divisible by 100 and not by 400.
 * This improves upon the Julian calendar leap year rule.
 * <p>
 * Although the Gregorian calendar did not exist before 1582 CE, this
 * chronology assumes it did, thus it is proleptic. This implementation also
 * fixes the start of the year at January 1, and defines the year zero.
 * <p>
 * GregorianChronology is thread-safe and immutable.
 *
 * @see <a href="http://en.wikipedia.org/wiki/Gregorian_calendar">Wikipedia</a>
 * @see JulianChronology
 * @see GJChronology
 * 
 * @author Guy Allard
 * @author Stephen Colebourne
 * @author Brian S O'Neill
 * @since 1.0
 */
public final class GregorianChronology extends BasicGJChronology {

    
Serialization lock /** Serialization lock */
    private static final long serialVersionUID = -861407383323710522L;

    private static final long MILLIS_PER_YEAR =
        (long) (365.2425 * DateTimeConstants.MILLIS_PER_DAY);

    private static final long MILLIS_PER_MONTH =
        (long) (365.2425 * DateTimeConstants.MILLIS_PER_DAY / 12);

    private static final int DAYS_0000_TO_1970 = 719527;

    
The lowest year that can be fully supported. /** The lowest year that can be fully supported. */
    private static final int MIN_YEAR = -292275054;

    
The highest year that can be fully supported. /** The highest year that can be fully supported. */
    private static final int MAX_YEAR = 292278993;

    
Singleton instance of a UTC GregorianChronology /** Singleton instance of a UTC GregorianChronology */
    private static final GregorianChronology INSTANCE_UTC;

    
Cache of zone to chronology arrays /** Cache of zone to chronology arrays */
    private static final ConcurrentHashMap<DateTimeZone, GregorianChronology[]> cCache = new ConcurrentHashMap<DateTimeZone, GregorianChronology[]>();

    static {
        INSTANCE_UTC = getInstance(DateTimeZone.UTC);
    }

    
Gets an instance of the GregorianChronology.
The time zone of the returned instance is UTC.
Returns:
a singleton UTC instance of the chronology/**
     * Gets an instance of the GregorianChronology.
     * The time zone of the returned instance is UTC.
     * 
     * @return a singleton UTC instance of the chronology
     */
    public static GregorianChronology getInstanceUTC() {
        return INSTANCE_UTC;
    }

    
Gets an instance of the GregorianChronology in the default time zone.
Returns:
a chronology in the default time zone/**
     * Gets an instance of the GregorianChronology in the default time zone.
     * 
     * @return a chronology in the default time zone
     */
    public static GregorianChronology getInstance() {
        return getInstance(DateTimeZone.getDefault(), 4);
    }

    
Gets an instance of the GregorianChronology in the given time zone.
Params:
zone –  the time zone to get the chronology in, null is default
Returns:
a chronology in the specified time zone/**
     * Gets an instance of the GregorianChronology in the given time zone.
     * 
     * @param zone  the time zone to get the chronology in, null is default
     * @return a chronology in the specified time zone
     */
    public static GregorianChronology getInstance(DateTimeZone zone) {
        return getInstance(zone, 4);
    }

    
Gets an instance of the GregorianChronology in the given time zone.
Params:
zone –  the time zone to get the chronology in, null is default
minDaysInFirstWeek –  minimum number of days in first week of the year; default is 4
Returns:
a chronology in the specified time zone/**
     * Gets an instance of the GregorianChronology in the given time zone.
     * 
     * @param zone  the time zone to get the chronology in, null is default
     * @param minDaysInFirstWeek  minimum number of days in first week of the year; default is 4
     * @return a chronology in the specified time zone
     */
    public static GregorianChronology getInstance(DateTimeZone zone, int minDaysInFirstWeek) {
        if (zone == null) {
            zone = DateTimeZone.getDefault();
        }
        GregorianChronology chrono;
        GregorianChronology[] chronos = cCache.get(zone);
        if (chronos == null) {
            chronos = new GregorianChronology[7];
            GregorianChronology[] oldChronos = cCache.putIfAbsent(zone, chronos);
            if (oldChronos != null) {
                chronos = oldChronos;
            }
        }
        try {
            chrono = chronos[minDaysInFirstWeek - 1];
        } catch (ArrayIndexOutOfBoundsException e) {
            throw new IllegalArgumentException
                ("Invalid min days in first week: " + minDaysInFirstWeek);
        }
        if (chrono == null) {
            synchronized (chronos) {
                chrono = chronos[minDaysInFirstWeek - 1];
                if (chrono == null) {
                    if (zone == DateTimeZone.UTC) {
                        chrono = new GregorianChronology(null, null, minDaysInFirstWeek);
                    } else {
                        chrono = getInstance(DateTimeZone.UTC, minDaysInFirstWeek);
                        chrono = new GregorianChronology
                            (ZonedChronology.getInstance(chrono, zone), null, minDaysInFirstWeek);
                    }
                    chronos[minDaysInFirstWeek - 1] = chrono;
                }
            }
        }
        return chrono;
    }

    // Constructors and instance variables
    //-----------------------------------------------------------------------

    
Restricted constructor
/**
     * Restricted constructor
     */
    private GregorianChronology(Chronology base, Object param, int minDaysInFirstWeek) {
        super(base, param, minDaysInFirstWeek);
    }

    
Serialization singleton
/**
     * Serialization singleton
     */
    private Object readResolve() {
        Chronology base = getBase();
        int minDays = getMinimumDaysInFirstWeek();
        minDays = (minDays == 0 ? 4 : minDays);  // handle rename of BaseGJChronology
        return base == null ?
                getInstance(DateTimeZone.UTC, minDays) :
                    getInstance(base.getZone(), minDays);
    }

    // Conversion
    //-----------------------------------------------------------------------
    
Gets the Chronology in the UTC time zone.
Returns:
the chronology in UTC/**
     * Gets the Chronology in the UTC time zone.
     * 
     * @return the chronology in UTC
     */
    public Chronology withUTC() {
        return INSTANCE_UTC;
    }

    
Gets the Chronology in a specific time zone.
Params:
zone –  the zone to get the chronology in, null is default
Returns:
the chronology/**
     * Gets the Chronology in a specific time zone.
     * 
     * @param zone  the zone to get the chronology in, null is default
     * @return the chronology
     */
    public Chronology withZone(DateTimeZone zone) {
        if (zone == null) {
            zone = DateTimeZone.getDefault();
        }
        if (zone == getZone()) {
            return this;
        }
        return getInstance(zone);
    }

    protected void assemble(Fields fields) {
        if (getBase() == null) {
            super.assemble(fields);
        }
    }

    boolean isLeapYear(int year) {
        return ((year & 3) == 0) && ((year % 100) != 0 || (year % 400) == 0);
    }

    long calculateFirstDayOfYearMillis(int year) {
        // Initial value is just temporary.
        int leapYears = year / 100;
        if (year < 0) {
            // Add 3 before shifting right since /4 and >>2 behave differently
            // on negative numbers. When the expression is written as
            // (year / 4) - (year / 100) + (year / 400),
            // it works for both positive and negative values, except this optimization
            // eliminates two divisions.
            leapYears = ((year + 3) >> 2) - leapYears + ((leapYears + 3) >> 2) - 1;
        } else {
            leapYears = (year >> 2) - leapYears + (leapYears >> 2);
            if (isLeapYear(year)) {
                leapYears--;
            }
        }

        return (year * 365L + (leapYears - DAYS_0000_TO_1970)) * DateTimeConstants.MILLIS_PER_DAY;
    }

    int getMinYear() {
        return MIN_YEAR;
    }

    int getMaxYear() {
        return MAX_YEAR;
    }

    long getAverageMillisPerYear() {
        return MILLIS_PER_YEAR;
    }

    long getAverageMillisPerYearDividedByTwo() {
        return MILLIS_PER_YEAR / 2;
    }

    long getAverageMillisPerMonth() {
        return MILLIS_PER_MONTH;
    }

    long getApproxMillisAtEpochDividedByTwo() {
        return (1970L * MILLIS_PER_YEAR) / 2;
    }

}