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Neo4j kernel is a lightweight, embedded Java database designed to store data structured as graphs rather than tables. For more information, see http://neo4j.org.
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/*
 * Copyright (c) 2002-2015 "Neo Technology,"
 * Network Engine for Objects in Lund AB [http://neotechnology.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.kernel.impl.util;

import java.util.Arrays;

import static java.lang.Math.max;

public class SequenceArray
{
    private static final long UNSET = -1L;
    // This is the backing store, treated as a ring, courtesy of cursor
    private long[] array;
    private int cursor;
    private int itemsAhead;
    private final int longsPerItem;
    private int capacity;

    public SequenceArray( int longsPerItem, int initialCapacity )
    {
        this.longsPerItem = longsPerItem;
        this.capacity = initialCapacity;
        this.array = new long[capacity*longsPerItem];
    }

    public void clear()
    {
        cursor = 0;
        itemsAhead = 0;
    }

    void offer( long baseNumber, long number, long[] meta )
    {
        int diff = (int) (number-baseNumber);
        ensureArrayCapacity( diff );
        int index = cursor+diff-1;

        // If we offer a value a bit ahead of the last offered value then clear the values in between
        for ( int i = cursor+itemsAhead; i < index; i++ )
        {
            array[index( i )] = UNSET;
        }

        int absIndex = index( index );
        array[absIndex] = number;
        System.arraycopy( meta, 0, array, absIndex+1, longsPerItem-1 );
        itemsAhead = max( itemsAhead, diff );
    }

    private int index( int logicalIndex )
    {
        return (logicalIndex % capacity) * longsPerItem;
    }

    long pollHighestGapFree( long given, long[] meta )
    {
        // assume that "given" would be placed at cursor
        long number = given;
        int length = itemsAhead-1;
        int absIndex = 0;
        for ( int i = 0; i < length; i++ )
        {
            advanceCursor();
            absIndex = index( cursor );
            if ( array[absIndex] == UNSET )
            {   // we found a gap, return the number before the gap
                break;
            }

            number++;
            assert array[absIndex] == number : "Expected index " + cursor + " to be " + number +
                    ", but was " + array[absIndex] + ". This is for i=" + i;
        }

        // copy the meta values into the supplied meta
        System.arraycopy( array, absIndex+1, meta, 0, longsPerItem-1 );
        return number;
    }

    private void advanceCursor()
    {
        assert itemsAhead > 0;
        itemsAhead--;
        cursor = (cursor+1)%capacity;
    }

    private void ensureArrayCapacity( int capacity )
    {
        while ( capacity > this.capacity )
        {
            int newCapacity = this.capacity*2;
            long[] newArray = new long[newCapacity*longsPerItem];
            // Copy contents to new array, newArray starting at 0
            for ( int i = 0; i < itemsAhead; i++ )
            {
                System.arraycopy( array, index( cursor + i ), newArray, index( i ), longsPerItem );
            }
            this.array = newArray;
            this.capacity = newCapacity;
            this.cursor = 0;
        }
    }

    @Override
    public String toString()
    {
        StringBuilder builder = new StringBuilder();
        for ( int i = 0; i < itemsAhead; i++ )
        {
            long value = array[index( cursor + i )];
            if ( value != UNSET )
            {
                builder.append( builder.length() > 0 ? "," : "" ).append( value );
            }
        }
        return builder.toString();
    }

    public boolean seen( long baseNumber, long number, long[] meta )
    {
        int diff = (int) (number - baseNumber);
        int index = cursor + diff - 1;

        if ( index >= cursor + itemsAhead )
        {
            return false;
        }

        int absIndex = index( index );
        long[] arrayRef = array;
        long num = arrayRef[absIndex];
        if ( num != number )
        {
            return false;
        }

        long[] metaCopy = Arrays.copyOfRange( arrayRef, absIndex + 1, absIndex + longsPerItem );
        return Arrays.equals( meta, metaCopy );
    }
}