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Fix a lot of things, and add a TODO.
author Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
date Fri, 17 Feb 2012 12:54:08 +0100
parents b1bec4b5ccf3
children
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1 <?xml version="1.0" encoding="utf-8"?>
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2 <?xml-stylesheet type="text/css" href="../style.css"?>
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3 <!DOCTYPE html>
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4 <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
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5 <head>
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6 <title>PBG3 format</title>
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7 </head>
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8 <body>
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9 <h1>PBG3 format</h1>
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10 <p>The PBG3 format is an archive format used by EoSD.</p>
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11
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12 <p>It is a bitstream composed of a header, a file table, and LZSS-compressed files.</p>
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13
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14
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15 <h2>Reading integers</h2>
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16 <p>Integers in PBG3 files are never signed, they are not byte-aligned, and have a variable size.<br/>
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17 Their size is given by two bits: 00 means the number is stored in one byte, 10 means it is stored in three bytes.</p>
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18
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19 <p>Ex:</p>
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20 <pre>
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21 0x0012 is stored as: 0000010010
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22 0x0112 is stored as: 010000000100010010
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23 </pre>
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24
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25
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26 <h2>Reading strings</h2>
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27 <p>Strings are stored as standard NULL-terminated sequences of bytes.<br/>
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28 The only catch is they are not byte-aligned.</p>
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29
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30
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31 <h2>Header</h2>
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32 <p>The header is composed of three fields:</p>
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33 <ul>
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34 <li>magic (string): "PBG3"</li>
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35 <li>number of entries (integer)</li>
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36 <li>offset of the file table (integer)</li>
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37 </ul>
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38
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39 <p>The size of the header is thus comprised between 52 bits and 100 bits.</p>
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40
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41
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42 <h2>File table</h2>
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43 <p>The file table starts at a byte boundary, but as the rest of the file, isn't byte-aligned.<br/>
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44 It consists of a sequence of entries.<br/>
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45 Each entry is composed of five fields:</p>
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46 <ul>
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47 <li>unknown1 (int): the same for every entry of an archive, and without much change between two archives #TODO</li>
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48 <li>unknown2 (int) #TODO</li>
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49 <li>checksum (int): simple checksum of compressed data</li>
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50 <li>size (int): size of uncompressed data</li>
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51 <li>name (string): name of the file</li>
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52 </ul>
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53
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54 <p>The checksum is a mere sum of the compressed data.<br/>
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55 Files are compressed using the LZSS algorithm, with a dictionary size of 8192 bytes and a minimum matching length of 4 bytes.<br/>
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56 The size of the offset component of (offset, length) tuples is 13 bits, whereas the size of the length component is 4 bits.<br/>
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57 A file ends with a (0, 0) tuple, that is, 18 zero bits.</p>
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58
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59 <p>Uncompressing a LZSS-compressed file is quite easy, see lzss.py.</p>
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60 </body>
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61 </html>