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@FiloSottile /lzs.py
Last active Nov 11, 2016

What would you like to do?
#!/usr/bin/env python
#-*- coding:utf-8 -*-
##############################################################
# Lempel-Ziv-Stac decompression
# BitReader and RingList classes
#
# Copyright (C) 2011 Filippo Valsorda - FiloSottile
# filosottile.wiki gmail.com - www.pytux.it
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program. If not, see <http://www.gnu.org/licenses/>.
#
##############################################################
import collections
class BitReader:
"""
Gets a string or a iterable of chars (also mmap)
representing bytes (ord) and permits to extract
bits one by one like a stream
"""
def __init__(self, bytes):
self._bits = collections.deque()
for byte in bytes:
byte = ord(byte)
for n in xrange(8):
self._bits.append(bool((byte >> (7-n)) & 1))
def getBit(self):
return self._bits.popleft()
def getBits(self, num):
res = 0
for i in xrange(num):
res += self.getBit() << num-1-i
return res
def getByte(self):
return self.getBits(8)
def __len__(self):
return len(self._bits)
class RingList:
"""
When the list is full, for every item appended
the older is removed
"""
def __init__(self, length):
self.__data__ = collections.deque()
self.__full__ = False
self.__max__ = length
def append(self, x):
if self.__full__:
self.__data__.popleft()
self.__data__.append(x)
if self.size() == self.__max__:
self.__full__ = True
def get(self):
return self.__data__
def size(self):
return len(self.__data__)
def maxsize(self):
return self.__max__
def __getitem__(self, n):
if n >= self.size():
return None
return self.__data__[n]
def LZSDecompress(data, window = RingList(2048)):
"""
Gets a string or a iterable of chars (also mmap)
representing bytes (ord) and an optional
pre-populated dictionary; return the decompressed
string and the final dictionary
"""
reader = BitReader(data)
result = ''
while True:
bit = reader.getBit()
if not bit:
char = reader.getByte()
result += chr(char)
window.append(char)
else:
bit = reader.getBit()
if bit:
offset = reader.getBits(7)
if offset == 0:
# EOF
break
else:
offset = reader.getBits(11)
lenField = reader.getBits(2)
if lenField < 3:
lenght = lenField + 2
else:
lenField <<= 2
lenField += reader.getBits(2)
if lenField < 15:
lenght = (lenField & 0x0f) + 5
else:
lenCounter = 0
lenField = reader.getBits(4)
while lenField == 15:
lenField = reader.getBits(4)
lenCounter += 1
lenght = 15*lenCounter + 8 + lenField
for i in xrange(lenght):
char = window[-offset]
result += chr(char)
window.append(char)
return result, window

Could you write some example code on how to use this to decompress files?
for instance the Rom-0 files from routers.

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