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1 The PBG3 format is an archive format used by Touhou 6 (The Embodiment of Scarlet Devil).
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2
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3 It is a bitstream composed of a header, a file table, and LZSS-compressed files.
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4
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5
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6
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7 Reading integers
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8 ----------------
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9
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10 Integers in PBG3 files are never signed, they are not byte-aligned, and have a variable size.
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11 Their size is given by two bits: 00 means the number is stored in one byte, 10 means it is stored in three bytes.
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12
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13 Ex:
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14 0x0012 is stored as: 0000010010
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15 0x0112 is stored as: 010000000100010010
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16
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17
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18
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19 Reading strings
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20 ---------------
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21
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22 Strings are stored as standard NULL-terminated sequences of bytes.
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23 The only catch is they are not byte-aligned.
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24
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25
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26
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27 Header
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28 ------
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29
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30 The header is composed of three fields:
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31 * magic (string): "PBG3"
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32 * number of entries (integer)
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33 * offset of the file table (integer)
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34
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35 The size of the header is thus comprised between 52 bits and 100 bits.
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36
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37
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38
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39 File table
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40 ----------
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41
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42 The file table starts at a byte boundary, but as the rest of the file, isn't byte-aligned.
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43 It consists of a sequence of entries.
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44 Each entry is composed of five fields:
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45 * unknown1 (int) #TODO
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46 * unknown2 (int) #TODO
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47 * checksum (int): simple checksum of compressed data
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48 * size (int): size of uncompressed data
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49 * name (string): name of the file
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50
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51 The checksum is a mere sum of the compressed data.
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52 Files are compressed using the LZSS algorithm, with a dictionary size of 8192 bytes and a minimum matching length of 4 bytes.
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53 The size of the offset component of (offset, length) tuples is 13 bits, whereas the size of the length component is 4 bits.
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54 A file ends with a (0, 0) tuple, that is, 18 zero bits.
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55
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56 Uncompressing a LZSS-compressed file is quite easy, see lzss.py.
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57
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