/
compressed_rtf.py
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/
compressed_rtf.py
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#!/usr/bin/env python
#-*- coding: utf8 -*-
"""
Compressed Rich Text Format (RTF) worker
Based on Rich Text Format (RTF) Compression Algorithm
https://msdn.microsoft.com/en-us/library/cc463890(v=exchg.80).aspx
"""
import struct
import sys
from io import BytesIO
from .crc32 import crc32
__all__ = ['compress', 'decompress']
PY3 = sys.version_info[0] == 3
INIT_DICT = (
b'{\\rtf1\\ansi\\mac\\deff0\\deftab720{\\fonttbl;}{\\f0\\fnil \\froman \\'
b'fswiss \\fmodern \\fscript \\fdecor MS Sans SerifSymbolArialTimes New '
b'RomanCourier{\\colortbl\\red0\\green0\\blue0\r\n\\par \\pard\\plain\\'
b'f0\\fs20\\b\\i\\u\\tab\\tx'
)
INIT_DICT_SIZE = 207
MAX_DICT_SIZE = 4096
COMPRESSED = b'LZFu'
UNCOMPRESSED = b'MELA'
def compress(data, compressed=True):
"""
Compress `data` using RTF compression algorithm
If `compressed` flag is False, data will be written uncompressed
"""
output_buffer = b''
# set init dict
init_dict = list(INIT_DICT + b' ' * (MAX_DICT_SIZE - INIT_DICT_SIZE))
write_offset = INIT_DICT_SIZE
# compressed
if compressed:
comp_type = COMPRESSED
# make stream
in_stream = BytesIO(data)
# init params
control_byte = 0
control_bit = 1
token_offset = 0
token_buffer = b''
match_len = 0
longest_match = 0
while True:
# find longest match
dict_offset, longest_match, write_offset = \
_find_longest_match(init_dict, in_stream, write_offset)
char = in_stream.read(longest_match if longest_match > 1 else 1)
# EOF input stream
if not char:
# update params
control_byte |= 1 << control_bit - 1
control_bit += 1
token_offset += 2
# add dict reference
dict_ref = (write_offset & 0xfff) << 4
token_buffer += struct.pack('>H', dict_ref)
# add to output
output_buffer += struct.pack('B', control_byte)
output_buffer += token_buffer[:token_offset]
break
else:
if longest_match > 1:
# update params
control_byte |= 1 << control_bit - 1
control_bit += 1
token_offset += 2
# add dict reference
dict_ref = (dict_offset & 0xfff) << 4 | (
longest_match - 2) & 0xf
token_buffer += struct.pack('>H', dict_ref)
else:
# character is not found in dictionary
if longest_match == 0:
init_dict[write_offset] = ord(char) if PY3 else char
write_offset = (write_offset + 1) % MAX_DICT_SIZE
# update params
control_byte |= 0 << control_bit - 1
control_bit += 1
token_offset += 1
# add literal
token_buffer += char
longest_match = 0
if control_bit > 8:
# add to output
output_buffer += struct.pack('B', control_byte)
output_buffer += token_buffer[:token_offset]
# reset params
control_byte = 0
control_bit = 1
token_offset = 0
token_buffer = b''
else:
# if uncompressed - copy data to output
comp_type = UNCOMPRESSED
output_buffer = data
# write compressed RTF header
comp_size = struct.pack('<I', len(output_buffer) + 12)
raw_size = struct.pack('<I', len(data))
crc_value = struct.pack('<I', crc32(output_buffer))
return comp_size + raw_size + comp_type + crc_value + output_buffer
def decompress(data):
"""
Decompress `data` using RTF compression algorithm
"""
# set init dict
init_dict = list(INIT_DICT)
init_dict += b' ' * (MAX_DICT_SIZE - INIT_DICT_SIZE)
if len(data) < 16:
raise Exception('Data must be at least 16 bytes long')
write_offset = INIT_DICT_SIZE
output_buffer = BytesIO()
# make stream
in_stream = BytesIO(data)
# read compressed RTF header
comp_size = struct.unpack('<I', in_stream.read(4))[0]
raw_size = struct.unpack('<I', in_stream.read(4))[0]
comp_type = in_stream.read(4)
crc_value = struct.unpack('<I', in_stream.read(4))[0]
# get only data
contents = BytesIO(in_stream.read(comp_size - 12))
if comp_type == COMPRESSED:
# check CRC
if crc_value != crc32(contents.read()):
raise Exception('CRC is invalid! The file is corrupt!')
contents.seek(0)
end = False
while not end:
val = contents.read(1)
if not val:
break
control = '{0:08b}'.format(ord(val))
# check bits from LSB to MSB
for i in range(1, 9):
if control[-i] == '1':
# token is reference (16 bit)
val = contents.read(2)
if not val:
break
token = struct.unpack('>H', val)[0] # big-endian
# extract [12 bit offset][4 bit length]
offset = (token >> 4) & 0b111111111111
length = token & 0b1111
# end indicator
if write_offset == offset:
end = True
break
actual_length = length + 2
for step in range(actual_length):
read_offset = (offset + step) % MAX_DICT_SIZE
char = init_dict[read_offset]
if PY3:
output_buffer.write(bytes([char]))
else:
output_buffer.write(char)
init_dict[write_offset] = char
write_offset = (write_offset + 1) % MAX_DICT_SIZE
else:
# token is literal (8 bit)
val = contents.read(1)
if not val:
break
output_buffer.write(val)
init_dict[write_offset] = ord(val) if PY3 else val
write_offset = (write_offset + 1) % MAX_DICT_SIZE
elif comp_type == UNCOMPRESSED:
return contents.read(raw_size)
else:
raise Exception('Unknown type of RTF compression!')
return output_buffer.getvalue()
def _find_longest_match(init_dict, stream, write_offset):
"""
Find the longest match
"""
# read the first char
char = stream.read(1)
if not char:
return 0, 0, write_offset
prev_write_offset = write_offset
dict_index = 0
match_len = 0
longest_match_len = 0
dict_offset = 0
# find the first char
while True:
if init_dict[dict_index % MAX_DICT_SIZE] == (ord(char) if PY3 else char):
match_len += 1
# if found longest match
if match_len <= 17 and match_len > longest_match_len:
dict_offset = dict_index - match_len + 1
# add to dictionary and update longest match
init_dict[write_offset] = ord(char) if PY3 else char
write_offset = (write_offset + 1) % MAX_DICT_SIZE
longest_match_len = match_len
# read the next char
char = stream.read(1)
if not char:
stream.seek(stream.tell() - match_len, 0)
return dict_offset, longest_match_len, write_offset
else:
stream.seek(stream.tell() - match_len - 1, 0)
match_len = 0
# read the first char
char = stream.read(1)
if not char:
break
dict_index += 1
if dict_index >= prev_write_offset + longest_match_len:
break
stream.seek(stream.tell() - match_len - 1, 0)
return dict_offset, longest_match_len, write_offset