102 lines
3.2 KiB
Python
102 lines
3.2 KiB
Python
import logging
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from io import BytesIO
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from typing import BinaryIO, Iterator, List, Optional, cast
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logger = logging.getLogger(__name__)
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class CorruptDataError(Exception):
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pass
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class LZWDecoder:
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def __init__(self, fp: BinaryIO) -> None:
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self.fp = fp
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self.buff = 0
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self.bpos = 8
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self.nbits = 9
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# NB: self.table stores None only in indices 256 and 257
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self.table: Optional[List[Optional[bytes]]] = None
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self.prevbuf: Optional[bytes] = None
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def readbits(self, bits: int) -> int:
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v = 0
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while 1:
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# the number of remaining bits we can get from the current buffer.
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r = 8-self.bpos
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if bits <= r:
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# |-----8-bits-----|
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# |-bpos-|-bits-| |
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# | |----r----|
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v = (v << bits) | ((self.buff >> (r-bits)) & ((1 << bits)-1))
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self.bpos += bits
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break
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else:
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# |-----8-bits-----|
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# |-bpos-|---bits----...
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# | |----r----|
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v = (v << r) | (self.buff & ((1 << r)-1))
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bits -= r
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x = self.fp.read(1)
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if not x:
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raise EOFError
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self.buff = ord(x)
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self.bpos = 0
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return v
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def feed(self, code: int) -> bytes:
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x = b''
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if code == 256:
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self.table = [bytes((c,)) for c in range(256)] # 0-255
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self.table.append(None) # 256
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self.table.append(None) # 257
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self.prevbuf = b''
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self.nbits = 9
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elif code == 257:
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pass
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elif not self.prevbuf:
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assert self.table is not None
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x = self.prevbuf = cast(bytes, self.table[code]) # assume not None
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else:
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assert self.table is not None
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if code < len(self.table):
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x = cast(bytes, self.table[code]) # assume not None
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self.table.append(self.prevbuf+x[:1])
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elif code == len(self.table):
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self.table.append(self.prevbuf+self.prevbuf[:1])
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x = cast(bytes, self.table[code])
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else:
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raise CorruptDataError
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table_length = len(self.table)
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if table_length == 511:
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self.nbits = 10
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elif table_length == 1023:
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self.nbits = 11
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elif table_length == 2047:
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self.nbits = 12
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self.prevbuf = x
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return x
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def run(self) -> Iterator[bytes]:
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while 1:
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try:
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code = self.readbits(self.nbits)
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except EOFError:
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break
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try:
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x = self.feed(code)
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except CorruptDataError:
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# just ignore corrupt data and stop yielding there
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break
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yield x
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assert self.table is not None
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logger.debug('nbits=%d, code=%d, output=%r, table=%r'
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% (self.nbits, code, x, self.table[258:]))
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def lzwdecode(data: bytes) -> bytes:
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fp = BytesIO(data)
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s = LZWDecoder(fp).run()
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return b''.join(s)
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