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Collections and collection utilities for Neo4j.
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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [https://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */
package org.neo4j.collection;

import java.util.NoSuchElementException;
import org.eclipse.collections.api.iterator.LongIterator;

/**
 * Simple array based FIFO queue for primitive longs.
 * Newly enqueued element is added into the end of the queue, and dequeue will return
 * element from the head of the queue. (See CLRS 10.1 for more detailed description)
 *
 * Queue capacity should always be power of two to be able to use
 * '&' mask operation with {@link #values} length.
 */
public class PrimitiveLongArrayQueue {
    private static final int DEFAULT_CAPACITY = 16;
    private long[] values;
    private int head;
    private int tail;

    public PrimitiveLongArrayQueue() {
        this(DEFAULT_CAPACITY);
    }

    PrimitiveLongArrayQueue(int capacity) {
        if (capacity == 0 || (capacity & (capacity - 1)) != 0) {
            throw new IllegalArgumentException("Capacity should be power of 2. Requested capacity: " + capacity);
        }
        initValues(capacity);
    }

    public boolean isEmpty() {
        return head == tail;
    }

    public int size() {
        return (tail - head) & (values.length - 1);
    }

    public LongIterator longIterator() {
        return new PrimitiveLongArrayQueueIterator();
    }

    public long dequeue() {
        if (isEmpty()) {
            throw new IllegalStateException("Fail to poll first element. Queue is empty.");
        }
        long value = values[head];
        head = (head + 1) & (values.length - 1);
        return value;
    }

    public void enqueue(long value) {
        values[tail] = value;
        tail = (tail + 1) & (values.length - 1);
        if (tail == head) {
            ensureCapacity();
        }
    }

    public void addAll(PrimitiveLongArrayQueue otherQueue) {
        while (!otherQueue.isEmpty()) {
            enqueue(otherQueue.dequeue());
        }
    }

    private void initValues(int capacity) {
        values = new long[capacity];
        head = 0;
        tail = 0;
    }

    private void ensureCapacity() {
        int newCapacity = values.length << 1;
        if (newCapacity < 0) {
            throw new IllegalStateException("Fail to increase queue capacity.");
        }
        long[] newValues = new long[newCapacity];
        int elementsFromHeadTillEnd = values.length - head;
        System.arraycopy(values, head, newValues, 0, elementsFromHeadTillEnd);
        System.arraycopy(values, 0, newValues, elementsFromHeadTillEnd, head);
        tail = values.length;
        head = 0;
        values = newValues;
    }

    private class PrimitiveLongArrayQueueIterator implements LongIterator {
        private int position;

        PrimitiveLongArrayQueueIterator() {
            this.position = head;
        }

        @Override
        public boolean hasNext() {
            return position != tail;
        }

        @Override
        public long next() {
            if (hasNext()) {
                long value = values[position];
                position = (position + 1) & (values.length - 1);
                return value;
            }
            throw new NoSuchElementException();
        }
    }
}