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55 lines
2.4 KiB
Plaintext
55 lines
2.4 KiB
Plaintext
About
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PalmDOC uses LZ77 compression techniques. DOC files can contain only compressed
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text. The format does not allow for any text formatting. This keeps files
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small, in keeping with the Palm philosophy. However, extensions to the format
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can use tags, such as HTML or PML, to include formatting within text. These
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extensions to PalmDoc are not interchangeable and are the basis for most eBook
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Reader formats on Palm devices.
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LZ77 algorithms achieve compression by replacing portions of the data with
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references to matching data that has already passed through both encoder and
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decoder. A match is encoded by a pair of numbers called a length-distance pair,
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which is equivalent to the statement "each of the next length characters is
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equal to the character exactly distance characters behind it in the
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uncompressed stream." (The "distance" is sometimes called the "offset" instead.)
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In the PalmDoc format, a length-distance pair is always encoded by a two-byte
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sequence. Of the 16 bits that make up these two bytes, 11 bits go to encoding
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the distance, 3 go to encoding the length, and the remaining two are used to
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make sure the decoder can identify the first byte as the beginning of such a
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two-byte sequence.
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PalmDoc combines LZ77 with a simple kind of byte pair compression.
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PalmDoc files are decoded as follows:
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-------------------------------------
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Read a byte from the compressed stream. If the byte is
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0x00: "1 literal" copy that byte unmodified to the decompressed stream.
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0x09 to 0x7f: "1 literal" copy that byte unmodified to the decompressed stream.
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0x01 to 0x08: "literals": the byte is interpreted as a count from 1 to 8, and
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that many literals are copied unmodified from the compressed stream to the
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decompressed stream.
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0x80 to 0xbf: "length, distance" pair: the 2 leftmost bits of this byte ('10')
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are discarded, and the following 6 bits are combined with the 8 bits of the
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next byte to make a 14 bit "distance, length" item. Those 14 bits are broken
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into 11 bits of distance backwards from the current location in the
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uncompressed text, and 3 bits of length to copy from that point
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(copying n+3 bytes, 3 to 10 bytes).
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0xc0 to 0xff: "byte pair": this byte is decoded into 2 characters: a space
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character, and a letter formed from this byte XORed with 0x80.
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Repeat from the beginning until there is no more bytes in the compressed file.
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PalmDOC data is always divided into 4096 byte blocks and the blocks are acted
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upon independently.
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