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97 lines
2.7 KiB

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())