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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: FileHeader.java,v 1.3 2005/06/25 23:12:32 doomdark Exp $
46   */
47  
48  package jdbm.recman;
49  
50  /**
51   *  This class represents a file header. It is a 1:1 representation of
52   *  the data that appears in block 0 of a file.
53   */
54  class FileHeader implements BlockView {
55      // offsets
56      private static final short O_MAGIC = 0; // short magic
57      private static final short O_LISTS = Magic.SZ_SHORT; // long[2*NLISTS]
58      private static final int O_ROOTS = 
59          O_LISTS + (Magic.NLISTS * 2 * Magic.SZ_LONG);
60  
61      // my block
62      private BlockIo block;
63  
64      /** The number of "root" rowids available in the file. */
65      static final int NROOTS = 
66          (RecordFile.BLOCK_SIZE - O_ROOTS) / Magic.SZ_LONG;
67  
68      /**
69       *  Constructs a FileHeader object from a block.
70       *
71       *  @param block The block that contains the file header
72       *  @param isNew If true, the file header is for a new file.
73       *  @throws IOException if the block is too short to keep the file
74       *          header.
75       */
76      FileHeader(BlockIo block, boolean isNew) {
77          this.block = block;
78          if (isNew)
79              block.writeShort(O_MAGIC, Magic.FILE_HEADER);
80          else if (!magicOk())
81              throw new Error("CRITICAL: file header magic not OK " 
82                              + block.readShort(O_MAGIC));
83      }
84  
85      /** Returns true if the magic corresponds with the fileHeader magic.  */
86      private boolean magicOk() {
87          return block.readShort(O_MAGIC) == Magic.FILE_HEADER;
88      }
89  
90  
91      /** Returns the offset of the "first" block of the indicated list */
92      private short offsetOfFirst(int list) {
93          return (short) (O_LISTS + (2 * Magic.SZ_LONG * list));
94      }
95  
96      /** Returns the offset of the "last" block of the indicated list */
97      private short offsetOfLast(int list) {
98          return (short) (offsetOfFirst(list) + Magic.SZ_LONG);
99      }
100 
101     /** Returns the offset of the indicated root */
102     private short offsetOfRoot(int root) {
103         return (short) (O_ROOTS + (root * Magic.SZ_LONG));
104     }
105 
106     /**
107      *  Returns the first block of the indicated list
108      */
109     long getFirstOf(int list) {
110         return block.readLong(offsetOfFirst(list));
111     }
112     
113     /**
114      *  Sets the first block of the indicated list
115      */
116     void setFirstOf(int list, long value) {
117         block.writeLong(offsetOfFirst(list), value);
118     }
119     
120     /**
121      *  Returns the last block of the indicated list
122      */
123     long getLastOf(int list) {
124         return block.readLong(offsetOfLast(list));
125     }
126     
127     /**
128      *  Sets the last block of the indicated list
129      */
130     void setLastOf(int list, long value) {
131         block.writeLong(offsetOfLast(list), value);
132     }
133 
134     /**
135      *  Returns the indicated root rowid. A root rowid is a special rowid
136      *  that needs to be kept between sessions. It could conceivably be
137      *  stored in a special file, but as a large amount of space in the
138      *  block header is wasted anyway, it's more useful to store it where
139      *  it belongs.
140      *
141      *  @see #NROOTS
142      */
143     long getRoot(int root) {
144         return block.readLong(offsetOfRoot(root));
145     }
146 
147     /**
148      *  Sets the indicated root rowid.
149      *
150      *  @see #getRoot
151      *  @see #NROOTS
152      */
153     void setRoot(int root, long rowid) {
154         block.writeLong(offsetOfRoot(root), rowid);
155     }
156 }