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1   /**
2    * JDBM LICENSE v1.00
3    *
4    * Redistribution and use of this software and associated documentation
5    * ("Software"), with or without modification, are permitted provided
6    * that the following conditions are met:
7    *
8    * 1. Redistributions of source code must retain copyright
9    *    statements and notices.  Redistributions must also contain a
10   *    copy of this document.
11   *
12   * 2. Redistributions in binary form must reproduce the
13   *    above copyright notice, this list of conditions and the
14   *    following disclaimer in the documentation and/or other
15   *    materials provided with the distribution.
16   *
17   * 3. The name "JDBM" must not be used to endorse or promote
18   *    products derived from this Software without prior written
19   *    permission of Cees de Groot.  For written permission,
20   *    please contact cg@cdegroot.com.
21   *
22   * 4. Products derived from this Software may not be called "JDBM"
23   *    nor may "JDBM" appear in their names without prior written
24   *    permission of Cees de Groot. 
25   *
26   * 5. Due credit should be given to the JDBM Project
27   *    (http://jdbm.sourceforge.net/).
28   *
29   * THIS SOFTWARE IS PROVIDED BY THE JDBM PROJECT AND CONTRIBUTORS
30   * ``AS IS'' AND ANY EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT
31   * NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
32   * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL
33   * CEES DE GROOT OR ANY CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
34   * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
35   * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
36   * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
37   * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
38   * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
39   * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
40   * OF THE POSSIBILITY OF SUCH DAMAGE.
41   *
42   * Copyright 2000 (C) Cees de Groot. All Rights Reserved.
43   * Contributions are Copyright (C) 2000 by their associated contributors.
44   *
45   * $Id: PageHeader.java,v 1.2 2003/09/21 15:47:01 boisvert Exp $
46   */
47  
48  package jdbm.recman;
49  
50  /**
51   *  This class represents a page header. It is the common superclass for
52   *  all different page views.
53   */
54  public class PageHeader implements BlockView {
55      // offsets
56      private static final short O_MAGIC = 0; // short magic
57      private static final short O_NEXT = Magic.SZ_SHORT;  // long next
58      private static final short O_PREV = O_NEXT + Magic.SZ_LONG; // long prev
59      protected static final short SIZE = O_PREV + Magic.SZ_LONG;
60  
61      // my block
62      protected BlockIo block;
63  
64      /**
65       *  Constructs a PageHeader object from a block
66       *
67       *  @param block The block that contains the file header
68       *  @throws IOException if the block is too short to keep the file
69       *          header.
70       */
71      protected PageHeader(BlockIo block) {
72          initialize(block);
73          if (!magicOk())
74              throw new Error("CRITICAL: page header magic for block "
75                              + block.getBlockId() + " not OK "
76                              + getMagic());
77      }
78      
79      /**
80       *  Constructs a new PageHeader of the indicated type. Used for newly
81       *  created pages.
82       */
83      PageHeader(BlockIo block, short type) {
84          initialize(block);
85          setType(type);
86      }
87      
88      /**
89       *  Factory method to create or return a page header for the
90       *  indicated block.
91       */
92      static PageHeader getView(BlockIo block) {
93          BlockView view = block.getView();
94          if (view != null && view instanceof PageHeader)
95              return (PageHeader) view;
96          else
97              return new PageHeader(block);
98      }
99      
100     private void initialize(BlockIo block) {
101         this.block = block;
102         block.setView(this);
103     }
104     
105     /**
106      *  Returns true if the magic corresponds with the fileHeader magic.
107      */
108     private boolean magicOk() {
109         int magic = getMagic();
110         return magic >= Magic.BLOCK
111             && magic <= (Magic.BLOCK + Magic.FREEPHYSIDS_PAGE);
112     }
113     
114     /**
115      *  For paranoia mode
116      */
117     protected void paranoiaMagicOk() {
118         if (!magicOk())
119             throw new Error("CRITICAL: page header magic not OK "
120                             + getMagic());
121     }
122     
123     /** Returns the magic code */
124     short getMagic() {
125         return block.readShort(O_MAGIC);
126     }
127 
128     /** Returns the next block. */
129     long getNext() {
130         paranoiaMagicOk();
131         return block.readLong(O_NEXT);
132     }
133     
134     /** Sets the next block. */
135     void setNext(long next) {
136         paranoiaMagicOk();
137         block.writeLong(O_NEXT, next);
138     }
139     
140     /** Returns the previous block. */
141     long getPrev() {
142         paranoiaMagicOk();
143         return block.readLong(O_PREV);
144     }
145     
146     /** Sets the previous block. */
147     void setPrev(long prev) {
148         paranoiaMagicOk();
149         block.writeLong(O_PREV, prev);
150     }
151     
152     /** Sets the type of the page header */
153     void setType(short type) {
154         block.writeShort(O_MAGIC, (short) (Magic.BLOCK + type));
155     }
156 }