translationpage.java

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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 [e-mail].
 *
 * 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
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 * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL
 * CEES DE GROOT OR ANY CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
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 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
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 * 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: TranslationPage.java,v 1.1 2000/05/06 00:00:31 boisvert Exp $
 */

package jdbm.recman;

/**
 *  Class describing a page that holds translations from physical rowids
 *  to logical rowids. In fact, the page just holds physical rowids - the
 *  page's block is the block for the logical rowid, the offset serve
 *  as offset for the rowids.
 */
final class TranslationPage extends PageHeader {
    // offsets
    static final short O_TRANS = PageHeader.SIZE; // short count
    static final short ELEMS_PER_PAGE = 
        (RecordFile.BLOCK_SIZE - O_TRANS) / PhysicalRowId.SIZE;
    
    // slots we returned.
    final PhysicalRowId[] slots = new PhysicalRowId[ELEMS_PER_PAGE];

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

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

    /** Returns the value of the indicated rowid on the page */
    PhysicalRowId get(short offset) {
        int slot = (offset - O_TRANS) / PhysicalRowId.SIZE;
        if (slots[slot] == null) 
            slots[slot] = new PhysicalRowId(block, offset);
        return slots[slot];
    }
}