001    /**
002     * JDBM LICENSE v1.00
003     *
004     * Redistribution and use of this software and associated documentation
005     * ("Software"), with or without modification, are permitted provided
006     * that the following conditions are met:
007     *
008     * 1. Redistributions of source code must retain copyright
009     *    statements and notices.  Redistributions must also contain a
010     *    copy of this document.
011     *
012     * 2. Redistributions in binary form must reproduce the
013     *    above copyright notice, this list of conditions and the
014     *    following disclaimer in the documentation and/or other
015     *    materials provided with the distribution.
016     *
017     * 3. The name "JDBM" must not be used to endorse or promote
018     *    products derived from this Software without prior written
019     *    permission of Cees de Groot.  For written permission,
020     *    please contact cg@cdegroot.com.
021     *
022     * 4. Products derived from this Software may not be called "JDBM"
023     *    nor may "JDBM" appear in their names without prior written
024     *    permission of Cees de Groot. 
025     *
026     * 5. Due credit should be given to the JDBM Project
027     *    (http://jdbm.sourceforge.net/).
028     *
029     * THIS SOFTWARE IS PROVIDED BY THE JDBM PROJECT AND CONTRIBUTORS
030     * ``AS IS'' AND ANY EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT
031     * NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
032     * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL
033     * CEES DE GROOT OR ANY CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
034     * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
035     * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
036     * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
037     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
038     * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
039     * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
040     * OF THE POSSIBILITY OF SUCH DAMAGE.
041     *
042     * Copyright 2000 (C) Cees de Groot. All Rights Reserved.
043     * Contributions are Copyright (C) 2000 by their associated contributors.
044     *
045     * $Id: PageHeader.java,v 1.2 2003/09/21 15:47:01 boisvert Exp $
046     */
047    package jdbm.recman;
048    
049    
050    import org.apache.directory.server.i18n.I18n;
051    
052    
053    /**
054     * This class represents a page header. It is the common superclass for
055     * all different page views. It contains the following information:
056     * 
057     * <ol>
058     *   <li>2 bytes: the short block type code</li>
059     *   <li>8 bytes: the long block id of the next block in the block list</li>
060     *   <li>8 bytes: the long block id of the previous block in the block list</li>
061     * </ol>
062     * 
063     * The page header block view hence sees 18 bytes of page header data.
064     */
065    public class PageHeader implements BlockView 
066    {
067        // offsets into page header's (BlockIo's) buffer
068        
069        /** the page (BlockIo's type code) short magic code */
070        private static final short O_MAGIC = 0; 
071        /** the long block id of the next block in the block list */
072        private static final short O_NEXT = Magic.SZ_SHORT;  
073        /** the long block id of the previous block in the block list */
074        private static final short O_PREV = O_NEXT + Magic.SZ_LONG; 
075        
076        /** the size of this page header = 18 bytes */
077        protected static final short SIZE = O_PREV + Magic.SZ_LONG;
078    
079        /** the page header block this view is associated with */
080        protected BlockIo block;
081    
082        
083        /**
084         * Constructs a PageHeader object from a block
085         *
086         * @param block The block that contains the page header
087         * @throws IOException if the block is too short to keep the page header.
088         */
089        protected PageHeader( BlockIo block ) 
090        {
091            this.block = block;
092            block.setView( this );
093            if ( ! magicOk() )
094            {
095                throw new Error( I18n.err( I18n.ERR_546, block.getBlockId(), getMagic() ) );
096            }
097        }
098        
099        
100        /**
101         * Constructs a new PageHeader of the indicated type. Used for newly
102         * created pages.
103         */
104        PageHeader( BlockIo block, short type ) 
105        {
106            this.block = block;
107            block.setView( this );
108            setType( type );
109        }
110        
111        
112        /**
113         * Factory method to create or return a page header for the indicated block.
114         */
115        static PageHeader getView ( BlockIo block ) 
116        {
117            BlockView view = block.getView();
118            if ( view != null && view instanceof PageHeader )
119            {
120                return ( PageHeader ) view;
121            }
122            else
123            {
124                return new PageHeader( block );
125            }
126        }
127        
128        
129        /**
130         * Returns true if the magic corresponds with the fileHeader magic.
131         */
132        private boolean magicOk() 
133        {
134            int magic = getMagic();
135            return magic >= Magic.BLOCK
136                && magic <= ( Magic.BLOCK + Magic.FREEPHYSIDS_PAGE );
137        }
138        
139        
140        /**
141         * For paranoia mode
142         */
143        protected void paranoiaMagicOk() 
144        {
145            if ( ! magicOk() )
146            {
147                throw new Error( I18n.err( I18n.ERR_547, getMagic() ) );
148            }
149        }
150        
151        
152        /** Returns the magic code */
153        short getMagic() 
154        {
155            return block.readShort( O_MAGIC );
156        }
157    
158        
159        /** Returns the next block. */
160        long getNext() 
161        {
162            paranoiaMagicOk();
163            return block.readLong( O_NEXT );
164        }
165        
166        
167        /** Sets the next block. */
168        void setNext( long next ) 
169        {
170            paranoiaMagicOk();
171            block.writeLong( O_NEXT, next );
172        }
173        
174        
175        /** Returns the previous block. */
176        long getPrev() 
177        {
178            paranoiaMagicOk();
179            return block.readLong( O_PREV );
180        }
181        
182        
183        /** Sets the previous block. */
184        void setPrev( long prev ) 
185        {
186            paranoiaMagicOk();
187            block.writeLong( O_PREV, prev );
188        }
189        
190        
191        /** Sets the type of the page header */
192        void setType ( short type ) 
193        {
194            block.writeShort( O_MAGIC, ( short ) ( Magic.BLOCK + type ) );
195        }
196    }