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Original JDBM Implementation
There is a newer version: 2.0.0-M3
/**
 * 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: FreePhysicalRowIdPage.java,v 1.1 2000/05/06 00:00:31 boisvert Exp $
 */
package jdbm.recman;


/**
 * Class describing a page that holds physical rowids that were freed.
 */
final class FreePhysicalRowIdPage extends PageHeader 
{
    // offsets
    private static final short O_COUNT = PageHeader.SIZE; // short count
    static final short O_FREE = O_COUNT + Magic.SZ_SHORT;
    static final short ELEMS_PER_PAGE =  ( RecordFile.BLOCK_SIZE - O_FREE ) / FreePhysicalRowId.SIZE;
    
    // slots we returned.
    FreePhysicalRowId[] slots = new FreePhysicalRowId[ELEMS_PER_PAGE];

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

    /**
     * Factory method to create or return a data page for the indicated block.
     */
    static FreePhysicalRowIdPage getFreePhysicalRowIdPageView( BlockIo block ) 
    {
        BlockView view = block.getView();
        if ( view != null && view instanceof FreePhysicalRowIdPage )
        {
            return ( FreePhysicalRowIdPage ) view;
        }
        else
        {
            return new FreePhysicalRowIdPage( block );
        }
    }
    

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

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

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

    
    /** Allocates a slot */
    FreePhysicalRowId alloc( int slot ) 
    {
        setCount( ( short ) ( getCount() + 1 ) );
        return get( slot );
    }

    
    /** Returns true if a slot is allocated */
    boolean isAllocated( int slot ) 
    {
        return get( slot ).getSize() != 0;
    }
    

    /** Returns true if a slot is free */
    boolean isFree( int slot ) 
    {
        return ! isAllocated( slot );
    }
    
    
    /** Returns the value of the indicated slot */
    FreePhysicalRowId get( int slot ) 
    {
        if ( slots[slot] == null )
        {
            slots[slot] = new FreePhysicalRowId( block, slotToOffset( slot ) ) ;
        }
  
        return slots[slot];
    }
    

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

    /** 
     * Returns first slot with available size >= indicated size, or -1 if no 
     * slots are available.
     */
    int getFirstLargerThan( int size ) 
    {
        for ( int i = 0; i < ELEMS_PER_PAGE; i++ ) 
        {
            if ( isAllocated( i ) && get( i ).getSize() >= size )
            {
                return i;
            }
        }
  
        return -1;
    }
}