datapage.java

Java Crawler with domain knowledge path

/**
 * 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
 * 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: DataPage.java,v 1.1 2000/05/06 00:00:31 boisvert Exp $
 */

package jdbm.recman;

/**
 *  Class describing a page that holds data.
 */
final class DataPage extends PageHeader {
    // offsets
    private static final short O_FIRST = PageHeader.SIZE; // short firstrowid
    static final short O_DATA = (short)(O_FIRST + Magic.SZ_SHORT);
    static final short DATA_PER_PAGE = (short)(RecordFile.BLOCK_SIZE - O_DATA);

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

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

    /** Returns the first rowid's offset */
    short getFirst() {
  return block.readShort(O_FIRST);
    }
    
    /** Sets the first rowid's offset */
    void setFirst(short value) {
  paranoiaMagicOk();
  if (value > 0 && value < O_DATA)
      throw new Error("DataPage.setFirst: offset " + value 
          + " too small");
  block.writeShort(O_FIRST, value);
    }
}