ZoneOffsetTransition.java

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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Copyright (c) 2009-2012, Stephen Colebourne & Michael Nascimento Santos
 *
 * All rights reserved.
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package build.tools.tzdb;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Objects;

/**
 * A transition between two offsets caused by a discontinuity in the local time-line.
 *
 * @since 1.8
 */
final class ZoneOffsetTransition implements Comparable<ZoneOffsetTransition> {

    /**
     * The local transition date-time at the transition.
     */
    private final LocalDateTime transition;
    /**
     * The offset before transition.
     */
    private final ZoneOffset offsetBefore;
    /**
     * The offset after transition.
     */
    private final ZoneOffset offsetAfter;

    /**
     * Creates an instance defining a transition between two offsets.
     *
     * @param transition  the transition date-time with the offset before the transition, not null
     * @param offsetBefore  the offset before the transition, not null
     * @param offsetAfter  the offset at and after the transition, not null
     */
    ZoneOffsetTransition(LocalDateTime transition, ZoneOffset offsetBefore, ZoneOffset offsetAfter) {
        Objects.requireNonNull(transition, "transition");
        Objects.requireNonNull(offsetBefore, "offsetBefore");
        Objects.requireNonNull(offsetAfter, "offsetAfter");
        if (offsetBefore.equals(offsetAfter)) {
            throw new IllegalArgumentException("Offsets must not be equal");
        }
        this.transition = transition;
        this.offsetBefore = offsetBefore;
        this.offsetAfter = offsetAfter;
    }

    /**
     * Creates an instance from epoch-second and offsets.
     *
     * @param epochSecond  the transition epoch-second
     * @param offsetBefore  the offset before the transition, not null
     * @param offsetAfter  the offset at and after the transition, not null
     */
    ZoneOffsetTransition(long epochSecond, ZoneOffset offsetBefore, ZoneOffset offsetAfter) {
        this.transition = LocalDateTime.ofEpochSecond(epochSecond, 0, offsetBefore);
        this.offsetBefore = offsetBefore;
        this.offsetAfter = offsetAfter;
    }

    /**
     * Gets the transition instant as an epoch second.
     *
     * @return the transition epoch second
     */
    public long toEpochSecond() {
        return transition.toEpochSecond(offsetBefore);
    }

    /**
     * Gets the local transition date-time, as would be expressed with the 'before' offset.
     * <p>
     * This is the date-time where the discontinuity begins expressed with the 'before' offset.
     * At this instant, the 'after' offset is actually used, therefore the combination of this
     * date-time and the 'before' offset will never occur.
     * <p>
     * The combination of the 'before' date-time and offset represents the same instant
     * as the 'after' date-time and offset.
     *
     * @return the transition date-time expressed with the before offset, not null
     */
    public LocalDateTime getDateTimeBefore() {
        return transition;
    }

    /**
     * Gets the local transition date-time, as would be expressed with the 'after' offset.
     * <p>
     * This is the first date-time after the discontinuity, when the new offset applies.
     * <p>
     * The combination of the 'before' date-time and offset represents the same instant
     * as the 'after' date-time and offset.
     *
     * @return the transition date-time expressed with the after offset, not null
     */
    public LocalDateTime getDateTimeAfter() {
        return transition.plusSeconds(getDurationSeconds());
    }

    /**
     * Gets the offset before the transition.
     * <p>
     * This is the offset in use before the instant of the transition.
     *
     * @return the offset before the transition, not null
     */
    public ZoneOffset getOffsetBefore() {
        return offsetBefore;
    }

    /**
     * Gets the offset after the transition.
     * <p>
     * This is the offset in use on and after the instant of the transition.
     *
     * @return the offset after the transition, not null
     */
    public ZoneOffset getOffsetAfter() {
        return offsetAfter;
    }

    /**
     * Gets the duration of the transition in seconds.
     *
     * @return the duration in seconds
     */
    private int getDurationSeconds() {
        return getOffsetAfter().getTotalSeconds() - getOffsetBefore().getTotalSeconds();
    }

    /**
     * Does this transition represent a gap in the local time-line.
     * <p>
     * Gaps occur where there are local date-times that simply do not not exist.
     * An example would be when the offset changes from {@code +01:00} to {@code +02:00}.
     * This might be described as 'the clocks will move forward one hour tonight at 1am'.
     *
     * @return true if this transition is a gap, false if it is an overlap
     */
    public boolean isGap() {
        return getOffsetAfter().getTotalSeconds() > getOffsetBefore().getTotalSeconds();
    }

    /**
     * Does this transition represent a gap in the local time-line.
     * <p>
     * Overlaps occur where there are local date-times that exist twice.
     * An example would be when the offset changes from {@code +02:00} to {@code +01:00}.
     * This might be described as 'the clocks will move back one hour tonight at 2am'.
     *
     * @return true if this transition is an overlap, false if it is a gap
     */
    public boolean isOverlap() {
        return getOffsetAfter().getTotalSeconds() < getOffsetBefore().getTotalSeconds();
    }

    /**
     * Checks if the specified offset is valid during this transition.
     * <p>
     * This checks to see if the given offset will be valid at some point in the transition.
     * A gap will always return false.
     * An overlap will return true if the offset is either the before or after offset.
     *
     * @param offset  the offset to check, null returns false
     * @return true if the offset is valid during the transition
     */
    public boolean isValidOffset(ZoneOffset offset) {
        return isGap() ? false : (getOffsetBefore().equals(offset) || getOffsetAfter().equals(offset));
    }

    /**
     * Gets the valid offsets during this transition.
     * <p>
     * A gap will return an empty list, while an overlap will return both offsets.
     *
     * @return the list of valid offsets
     */
    List<ZoneOffset> getValidOffsets() {
        if (isGap()) {
            return Collections.emptyList();
        }
        return Arrays.asList(getOffsetBefore(), getOffsetAfter());
    }

    /**
     * Compares this transition to another based on the transition instant.
     * <p>
     * This compares the instants of each transition.
     * The offsets are ignored, making this order inconsistent with equals.
     *
     * @param transition  the transition to compare to, not null
     * @return the comparator value, negative if less, positive if greater
     */
    @Override
    public int compareTo(ZoneOffsetTransition transition) {
        return Long.compare(this.toEpochSecond(), transition.toEpochSecond());
    }

    /**
     * Checks if this object equals another.
     * <p>
     * The entire state of the object is compared.
     *
     * @param other  the other object to compare to, null returns false
     * @return true if equal
     */
    @Override
    public boolean equals(Object other) {
        if (other == this) {
            return true;
        }
        if (other instanceof ZoneOffsetTransition) {
            ZoneOffsetTransition d = (ZoneOffsetTransition) other;
            return transition.equals(d.transition) &&
                offsetBefore.equals(d.offsetBefore) && offsetAfter.equals(d.offsetAfter);
        }
        return false;
    }

    /**
     * Returns a suitable hash code.
     *
     * @return the hash code
     */
    @Override
    public int hashCode() {
        return transition.hashCode() ^ offsetBefore.hashCode() ^ Integer.rotateLeft(offsetAfter.hashCode(), 16);
    }

}