用python编写一个字符串压缩程序(要求为自适应模型替代法)
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发布时间：2022-05-07 07:39
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时间：2022-04-18 09:42
import math
from bitarray import bitarray
class LZ77Compressor:
"""
A simplified implementation of the LZ77 Compression Algorithm
"""
MAX_WINDOW_SIZE = 400
def __init__(self, window_size=20):
self.window_size = min(window_size, self.MAX_WINDOW_SIZE)
self.lookahead_buffer_size = 15 # length of match is at most 4 bits
def compress(self, input_file_path, output_file_path=None, verbose=False):
"""
Given the path of an input file, its content is compressed by applying a simple
LZ77 compression algorithm.
The compressed format is:
0 bit followed by 8 bits (1 byte character) when there are no previous matches
within window
1 bit followed by 12 bits pointer (distance to the start of the match from the
current position) and 4 bits (length of the match)
If a path to the output file is provided, the compressed data is written into
a binary file. Otherwise, it is returned as a bitarray
if verbose is enabled, the compression description is printed to standard output
"""
data = None
i = 0
output_buffer = bitarray(endian='big')
# read the input file
try:
with open(input_file_path, 'rb') as input_file:
data = input_file.read()
except IOError:
print 'Could not open input file ...'
raise
while i < len(data):
#print i
match = self.findLongestMatch(data, i)
if match:
# Add 1 bit flag, followed by 12 bit for distance, and 4 bit for the length
# of the match
(bestMatchDistance, bestMatchLength) = match
output_buffer.append(True)
output_buffer.frombytes(chr(bestMatchDistance >> 4))
output_buffer.frombytes(chr(((bestMatchDistance & 0xf) << 4) | bestMatchLength))
if verbose:
print "<1, %i, %i>" % (bestMatchDistance, bestMatchLength),
i += bestMatchLength
else:
# No useful match was found. Add 0 bit flag, followed by 8 bit for the character
output_buffer.append(False)
output_buffer.frombytes(data[i])
if verbose:
print "<0, %s>" % data[i],
i += 1
# fill the buffer with zeros if the number of bits is not a multiple of 8
output_buffer.fill()
# write the compressed data into a binary file if a path is provided
if output_file_path:
try:
with open(output_file_path, 'wb') as output_file:
output_file.write(output_buffer.tobytes())
print "File was compressed successfully and saved to output path ..."
return None
except IOError:
print 'Could not write to output file path. Please check if the path is correct ...'
raise
# an output file path was not provided, return the compressed data
return output_buffer
def decompress(self, input_file_path, output_file_path=None):
"""
Given a string of the compressed file path, the data is decompressed back to its
original form, and written into the output file path if provided. If no output
file path is provided, the decompressed data is returned as a string
"""
data = bitarray(endian='big')
output_buffer = []
# read the input file
try:
with open(input_file_path, 'rb') as input_file:
data.fromfile(input_file)
except IOError:
print 'Could not open input file ...'
raise
while len(data) >= 9:
flag = data.pop(0)
if not flag:
byte = data[0:8].tobytes()
output_buffer.append(byte)
del data[0:8]
else:
byte1 = ord(data[0:8].tobytes())
byte2 = ord(data[8:16].tobytes())
del data[0:16]
distance = (byte1 << 4) | (byte2 >> 4)
length = (byte2 & 0xf)
for i in range(length):
output_buffer.append(output_buffer[-distance])
out_data = ''.join(output_buffer)
if output_file_path:
try:
with open(output_file_path, 'wb') as output_file:
output_file.write(out_data)
print 'File was decompressed successfully and saved to output path ...'
return None
except IOError:
print 'Could not write to output file path. Please check if the path is correct ...'
raise
return out_data
def findLongestMatch(self, data, current_position):
"""
Finds the longest match to a substring starting at the current_position
in the lookahead buffer from the history window
"""
end_of_buffer = min(current_position + self.lookahead_buffer_size, len(data) + 1)
best_match_distance = -1
best_match_length = -1
# Optimization: Only consider substrings of length 2 and greater, and just
# output any substring of length 1 (8 bits uncompressed is better than 13 bits
# for the flag, distance, and length)
for j in range(current_position + 2, end_of_buffer):
start_index = max(0, current_position - self.window_size)
substring = data[current_position:j]
for i in range(start_index, current_position):
repetitions = len(substring) / (current_position - i)
last = len(substring) % (current_position - i)
matched_string = data[i:current_position] * repetitions + data[i:i+last]
if matched_string == substring and len(substring) > best_match_length:
best_match_distance = current_position - i
best_match_length = len(substring)
if best_match_distance > 0 and best_match_length > 0:
return (best_match_distance, best_match_length)
return None
