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