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/**
 * JDBM LICENSE v1.00
 *
 * Redistribution and use of this software and associated documentation
 * ("Software"), with or without modification, are permitted provided
 * that the following conditions are met:
 *
 * 1. Redistributions of source code must retain copyright
 *    statements and notices.  Redistributions must also contain a
 *    copy of this document.
 *
 * 2. Redistributions in binary form must reproduce the
 *    above copyright notice, this list of conditions and the
 *    following disclaimer in the documentation and/or other
 *    materials provided with the distribution.
 *
 * 3. The name "JDBM" must not be used to endorse or promote
 *    products derived from this Software without prior written
 *    permission of Cees de Groot.  For written permission,
 *    please contact [email protected].
 *
 * 4. Products derived from this Software may not be called "JDBM"
 *    nor may "JDBM" appear in their names without prior written
 *    permission of Cees de Groot. 
 *
 * 5. Due credit should be given to the JDBM Project
 *    (http://jdbm.sourceforge.net/).
 *
 * THIS SOFTWARE IS PROVIDED BY THE JDBM PROJECT AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT
 * NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
 * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL
 * CEES DE GROOT OR ANY CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * Copyright 2000 (C) Cees de Groot. All Rights Reserved.
 * Contributions are Copyright (C) 2000 by their associated contributors.
 *
 * $Id: FreeLogicalRowIdPage.java,v 1.1 2000/05/06 00:00:31 boisvert Exp $
 */
package jdbm.recman;


/**
 * Class describing a page that holds logical rowids that were freed. Note
 * that the methods have *physical* rowids in their signatures - this is
 * because logical and physical rowids are internally the same, only their
 * external representation (i.e. in the client API) differs.
 */
class FreeLogicalRowIdPage extends PageHeader
{
    /** The offset for the number of free pages */
    private static final short O_COUNT = PageHeader.SIZE; // short count

    /** Offset of the number of free row Ids */
    static final short O_FREE = O_COUNT + Magic.SZ_SHORT;

    /** The number of elements by page */
    static final short ELEMS_PER_PAGE = ( RecordFile.BLOCK_SIZE - O_FREE ) / PhysicalRowId.SIZE;

    /** */
    final PhysicalRowId[] slots = new PhysicalRowId[ELEMS_PER_PAGE];


    /**
     * Constructs a data page view from the indicated block.
     */
    FreeLogicalRowIdPage( BlockIo blockIo )
    {
        super( blockIo );
    }


    /**
     * Factory method to create or return a data page for the indicated block.
     */
    static FreeLogicalRowIdPage getFreeLogicalRowIdPageView( BlockIo blockIo )
    {
        BlockView view = blockIo.getView();

        if ( ( view != null ) && ( view instanceof FreeLogicalRowIdPage ) )
        {
            return ( FreeLogicalRowIdPage ) view;
        }
        else
        {
            return new FreeLogicalRowIdPage( blockIo );
        }
    }


    /** 
     * @return the number of free rowids 
     */
    short getCount()
    {
        return blockIo.readShort( O_COUNT );
    }


    /** 
     * Sets the number of free rowids 
     */
    private void setCount( short i )
    {
        blockIo.writeShort( O_COUNT, i );
    }


    /** 
     * Frees a slot 
     */
    void free( int slot )
    {
        get( slot ).setBlock( 0 );
        setCount( ( short ) ( getCount() - 1 ) );
    }


    /** 
     * Allocates a slot 
     */
    PhysicalRowId alloc( int slot )
    {
        setCount( ( short ) ( getCount() + 1 ) );
        get( slot ).setBlock( -1 );

        return get( slot );
    }


    /** 
     * Returns true if a slot is allocated 
     */
    private boolean isAllocated( int slot )
    {
        return get( slot ).getBlock() > 0;
    }


    /** 
     * Returns true if a slot is free 
     */
    private boolean isFree( int slot )
    {
        return !isAllocated( slot );
    }


    /** 
     * Returns the value of the indicated slot 
     */
    PhysicalRowId get( int slot )
    {
        if ( slots[slot] == null )
        {
            slots[slot] = new PhysicalRowId( blockIo, slotToOffset( slot ) );
        }

        return slots[slot];
    }


    /** 
     * Converts slot to offset 
     */
    private short slotToOffset( int slot )
    {
        return ( short ) ( O_FREE + ( slot * PhysicalRowId.SIZE ) );
    }


    /**
     *  Returns first free slot, -1 if no slots are available
     */
    int getFirstFree()
    {
        for ( int i = 0; i < ELEMS_PER_PAGE; i++ )
        {
            if ( isFree( i ) )
            {
                return i;
            }
        }

        return -1;
    }


    /**
     * @return The first allocated slot, -1 if no slots are available.
     */
    int getFirstAllocated()
    {
        for ( int i = 0; i < ELEMS_PER_PAGE; i++ )
        {
            if ( isAllocated( i ) )
            {
                return i;
            }
        }

        return -1;
    }


    /**
     * {@inheritDoc}
     */
    public String toString()
    {
        StringBuilder sb = new StringBuilder();

        sb.append( "FreeLogRowIdPage ( " );

        // The blockIO
        sb.append( super.toString() ).append( ", " );

        // The first rowId
        sb.append( "count: " ).append( getCount() );

        // Dump the Physical row id
        for ( int i = 0; i < ELEMS_PER_PAGE; i++ )
        {
            if ( slots[i] != null )
            {
                sb.append( ", [" ).append( i ).append( "]=<" ).append( slots[i].getBlock() ).append( ", " )
                    .append( slots[i].getOffset() ).append( ">" );
            }
        }

        sb.append( ")" );

        return sb.toString();
    }
}