/*
 *  Copyright 2001-2006 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;

import java.io.Serializable;

import org.joda.time.base.AbstractInstant;
import org.joda.time.chrono.ISOChronology;
import org.joda.time.convert.ConverterManager;
import org.joda.time.convert.InstantConverter;

/**
 * Instant is the standard implementation of a fully immutable instant in time.
 * <p>
 * <code>Instant</code> is an implementation of {@link ReadableInstant}.
 * As with all instants, it represents an exact point on the time-line,
 * but limited to the precision of milliseconds. An <code>Instant</code>
 * should be used to represent a point in time irrespective of any other
 * factor, such as chronology or time zone.
 * <p>
 * Internally, the class holds one piece of data, the instant as milliseconds
 * from the Java epoch of 1970-01-01T00:00:00Z.
 * <p>
 * For example, an Instant can be used to compare two <code>DateTime</code>
 * objects irrespective of chronology or time zone.
 * <pre>
 * boolean sameInstant = dt1.toInstant().equals(dt2.toInstant());
 * </pre>
 * Note that the following code will also perform the same check:
 * <pre>
 * boolean sameInstant = dt1.isEqual(dt2);
 * </pre>
 * <p>
 * Instant is thread-safe and immutable.
 *
 * @author Stephen Colebourne
 * @since 1.0
 */
public final class Instant
        extends AbstractInstant
        implements ReadableInstant, Serializable {

    /** Serialization lock */
    private static final long serialVersionUID = 3299096530934209741L;

    /** The millis from 1970-01-01T00:00:00Z */
    private final long iMillis;

    //-----------------------------------------------------------------------
    /**
     * Constructs an instance set to the current system millisecond time.
     */
    public Instant() {
        super();
        iMillis = DateTimeUtils.currentTimeMillis();
    }

    /**
     * Constructs an instance set to the milliseconds from 1970-01-01T00:00:00Z.
     * 
     * @param instant  the milliseconds from 1970-01-01T00:00:00Z
     */
    public Instant(long instant) {
        super();
        iMillis = instant;
    }

    /**
     * Constructs an instance from an Object that represents a datetime.
     * <p>
     * The recognised object types are defined in {@link ConverterManager} and
     * include String, Calendar and Date.
     *
     * @param instant  the datetime object, null means now
     * @throws IllegalArgumentException if the instant is invalid
     */
    public Instant(Object instant) {
        super();
        InstantConverter converter = ConverterManager.getInstance().getInstantConverter(instant);
        iMillis = converter.getInstantMillis(instant, ISOChronology.getInstanceUTC());
    }

    //-----------------------------------------------------------------------
    /**
     * Get this object as an Instant by returning <code>this</code>.
     * 
     * @return <code>this</code>
     */
    public Instant toInstant() {
        return this;
    }

    //-----------------------------------------------------------------------
    /**
     * Gets a copy of this instant with different millis.
     * <p>
     * The returned object will be either be a new Instant or <code>this</code>.
     *
     * @param newMillis  the new millis, from 1970-01-01T00:00:00Z
     * @return a copy of this instant with different millis
     */
    public Instant withMillis(long newMillis) {
        return (newMillis == iMillis ? this : new Instant(newMillis));
    }

    /**
     * Gets a copy of this instant with the specified duration added.
     * <p>
     * If the addition is zero, then <code>this</code> is returned.
     * 
     * @param durationToAdd  the duration to add to this one
     * @param scalar  the amount of times to add, such as -1 to subtract once
     * @return a copy of this instant with the duration added
     * @throws ArithmeticException if the new instant exceeds the capacity of a long
     */
    public Instant withDurationAdded(long durationToAdd, int scalar) {
        if (durationToAdd == 0 || scalar == 0) {
            return this;
        }
        long instant = getChronology().add(getMillis(), durationToAdd, scalar);
        return withMillis(instant);
    }

    /**
     * Gets a copy of this instant with the specified duration added.
     * <p>
     * If the addition is zero, then <code>this</code> is returned.
     * 
     * @param durationToAdd  the duration to add to this one, null means zero
     * @param scalar  the amount of times to add, such as -1 to subtract once
     * @return a copy of this instant with the duration added
     * @throws ArithmeticException if the new instant exceeds the capacity of a long
     */
    public Instant withDurationAdded(ReadableDuration durationToAdd, int scalar) {
        if (durationToAdd == null || scalar == 0) {
            return this;
        }
        return withDurationAdded(durationToAdd.getMillis(), scalar);
    }

    //-----------------------------------------------------------------------
    /**
     * Gets a copy of this instant with the specified duration added.
     * <p>
     * If the amount is zero or null, then <code>this</code> is returned.
     * 
     * @param duration  the duration to add to this one
     * @return a copy of this instant with the duration added
     * @throws ArithmeticException if the new instant exceeds the capacity of a long
     */
    public Instant plus(long duration) {
        return withDurationAdded(duration, 1);
    }

    /**
     * Gets a copy of this instant with the specified duration added.
     * <p>
     * If the amount is zero or null, then <code>this</code> is returned.
     * 
     * @param duration  the duration to add to this one, null means zero
     * @return a copy of this instant with the duration added
     * @throws ArithmeticException if the new instant exceeds the capacity of a long
     */
    public Instant plus(ReadableDuration duration) {
        return withDurationAdded(duration, 1);
    }

    //-----------------------------------------------------------------------
    /**
     * Gets a copy of this instant with the specified duration taken away.
     * <p>
     * If the amount is zero or null, then <code>this</code> is returned.
     * 
     * @param duration  the duration to reduce this instant by
     * @return a copy of this instant with the duration taken away
     * @throws ArithmeticException if the new instant exceeds the capacity of a long
     */
    public Instant minus(long duration) {
        return withDurationAdded(duration, -1);
    }

    /**
     * Gets a copy of this instant with the specified duration taken away.
     * <p>
     * If the amount is zero or null, then <code>this</code> is returned.
     * 
     * @param duration  the duration to reduce this instant by
     * @return a copy of this instant with the duration taken away
     * @throws ArithmeticException if the new instant exceeds the capacity of a long
     */
    public Instant minus(ReadableDuration duration) {
        return withDurationAdded(duration, -1);
    }

    //-----------------------------------------------------------------------
    /**
     * Gets the milliseconds of the instant.
     * 
     * @return the number of milliseconds since 1970-01-01T00:00:00Z
     */
    public long getMillis() {
        return iMillis;
    }

    /**
     * Gets the chronology of the instant, which is ISO in the UTC zone.
     * <p>
     * This method returns {@link ISOChronology#getInstanceUTC()} which
     * corresponds to the definition of the Java epoch 1970-01-01T00:00:00Z.
     * 
     * @return ISO in the UTC zone
     */
    public Chronology getChronology() {
        return ISOChronology.getInstanceUTC();
    }

    //-----------------------------------------------------------------------
    /**
     * Get this object as a DateTime using ISOChronology in the default zone.
     * <p>
     * This method returns a DateTime object in the default zone.
     * This differs from the similarly named method on DateTime, DateMidnight
     * or MutableDateTime which retains the time zone. The difference is
     * because Instant really represents a time <i>without</i> a zone,
     * thus calling this method we really have no zone to 'retain' and
     * hence expect to switch to the default zone.
     * <p>
     * This method definition preserves compatability with earlier versions
     * of Joda-Time.
     *
     * @return a DateTime using the same millis
     */
    public DateTime toDateTime() {
        return new DateTime(getMillis(), ISOChronology.getInstance());
    }

    /**
     * Get this object as a DateTime using ISOChronology in the default zone.
     * This method is identical to <code>toDateTime()</code>.
     * <p>
     * This method returns a DateTime object in the default zone.
     * This differs from the similarly named method on DateTime, DateMidnight
     * or MutableDateTime which retains the time zone. The difference is
     * because Instant really represents a time <i>without</i> a zone,
     * thus calling this method we really have no zone to 'retain' and
     * hence expect to switch to the default zone.
     * <p>
     * This method definition preserves compatability with earlier versions
     * of Joda-Time.
     *
     * @return a DateTime using the same millis with ISOChronology
     * @deprecated Use toDateTime() as it is identical
     */
    public DateTime toDateTimeISO() {
        return toDateTime();
    }

    /**
     * Get this object as a MutableDateTime using ISOChronology in the default zone.
     * <p>
     * This method returns a MutableDateTime object in the default zone.
     * This differs from the similarly named method on DateTime, DateMidnight
     * or MutableDateTime which retains the time zone. The difference is
     * because Instant really represents a time <i>without</i> a zone,
     * thus calling this method we really have no zone to 'retain' and
     * hence expect to switch to the default zone.
     * <p>
     * This method definition preserves compatability with earlier versions
     * of Joda-Time.
     *
     * @return a MutableDateTime using the same millis
     */
    public MutableDateTime toMutableDateTime() {
        return new MutableDateTime(getMillis(), ISOChronology.getInstance());
    }

    /**
     * Get this object as a MutableDateTime using ISOChronology in the default zone.
     * This method is identical to <code>toMutableDateTime()</code>.
     * <p>
     * This method returns a MutableDateTime object in the default zone.
     * This differs from the similarly named method on DateTime, DateMidnight
     * or MutableDateTime which retains the time zone. The difference is
     * because Instant really represents a time <i>without</i> a zone,
     * thus calling this method we really have no zone to 'retain' and
     * hence expect to switch to the default zone.
     * <p>
     * This method definition preserves compatability with earlier versions
     * of Joda-Time.
     *
     * @return a MutableDateTime using the same millis with ISOChronology
     * @deprecated Use toMutableDateTime() as it is identical
     */
    public MutableDateTime toMutableDateTimeISO() {
        return toMutableDateTime();
    }

}
