87 lines
3.0 KiB
Python
87 lines
3.0 KiB
Python
import struct
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# ported directly from the PalmDoc Perl library
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# http://kobesearch.cpan.org/htdocs/EBook-Tools/EBook/Tools/PalmDoc.pm.html
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def uncompress_lz77(data):
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length = len(data);
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offset = 0; # Current offset into data
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# char; # Character being examined
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# ord; # Ordinal of $char
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# lz77; # 16-bit Lempel-Ziv 77 length-offset pair
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# lz77offset; # LZ77 offset
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# lz77length; # LZ77 length
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# lz77pos; # Position inside $lz77length
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text = ''; # Output (uncompressed) text
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# textlength; # Length of uncompressed text during LZ77 pass
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# textpos; # Position inside $text during LZ77 pass
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while offset < length:
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# char = substr($data,$offset++,1);
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char = data[offset];
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offset += 1;
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ord_ = ord(char);
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# print " ".join([repr(char), hex(ord_)])
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# The long if-elsif chain is the best logic for $ord handling
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## no critic (Cascading if-elsif chain)
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if (ord_ == 0):
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# Nulls are literal
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text += char;
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elif (ord_ <= 8):
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# Next $ord bytes are literal
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text += data[offset:offset+ord_] # text .=substr($data,$offset,ord);
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offset += ord_;
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elif (ord_ <= 0x7f):
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# Values from 0x09 through 0x7f are literal
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text += char;
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elif (ord_ <= 0xbf):
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# Data is LZ77-compressed
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# From Wikipedia:
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# "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,
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# 11 bits go to encoding the distance, 3 go to encoding
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# the length, and the remaining two are used to make sure
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# the decoder can identify the first byte as the beginning
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# of such a two-byte sequence."
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offset += 1;
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if (offset > len(data)):
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print("WARNING: offset to LZ77 bits is outside of the data: %d" % offset);
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return text;
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lz77, = struct.unpack('>H', data[offset-2:offset])
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# Leftmost two bits are ID bits and need to be dropped
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lz77 &= 0x3fff;
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# Length is rightmost 3 bits + 3
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lz77length = (lz77 & 0x0007) + 3;
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# Remaining 11 bits are offset
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lz77offset = lz77 >> 3;
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if (lz77offset < 1):
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print("WARNING: LZ77 decompression offset is invalid!");
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return text;
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# Getting text from the offset is a little tricky, because
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# in theory you can be referring to characters you haven't
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# actually decompressed yet. You therefore have to check
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# the reference one character at a time.
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textlength = len(text);
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for lz77pos in range(lz77length): # for($lz77pos = 0; $lz77pos < $lz77length; $lz77pos++)
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textpos = textlength - lz77offset;
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if (textpos < 0):
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print("WARNING: LZ77 decompression reference is before"+
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" beginning of text! %x" % lz77);
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return;
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text += text[textpos:textpos+1]; #text .= substr($text,$textpos,1);
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textlength+=1;
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else:
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# 0xc0 - 0xff are single characters (XOR 0x80) preceded by
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# a space
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text += ' ' + chr(ord_ ^ 0x80);
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return text;
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