Python数据结构之霍夫曼压缩
霍夫曼编码
- 前言
- 变长前缀码
- 前缀码的实现
- 自制Huffman压缩和解压工具
- Python 代码
前言
无损压缩领域最为常见的算法当属霍夫曼压缩算法了。其主要思想是放弃文本文件的传统保存方式，不再使用八位二进制数表示每一个字符，而是用较少的比特表示出现频率较高的字符，用较多的比特表示出现频率较低的字符。
在图像数据压缩时，游程编码和霍夫曼编码也是十分常用的。
变长前缀码
和每个字符所相关的编码都是一个比特字符串，就好像有一个以字符为键、比特字符串为值得符号表一样。我们可以试着将最短得比特字符赋予最常用的字符，将A编码为0、B编码为1、R编码为00。这样一来问题就出现了，A的编码是0，R的编码是00，那么当0出现的时候，我们应该认为其是A还是R的前缀呢？如果你不想引入分隔符的话，这个时候就需要引入变长前缀码。
在变长前缀码中，所有字符编码都不会成为其它字符编码的前缀，那么如此就不需要分隔符了。
前缀码的实现
前缀码的实现采用了单词查找树。
自制Huffman压缩和解压工具
网址如下，点击此处跳转
https://download.csdn.net/download/m0_37772174/11965071
自制工具是exe文件
压缩命令 ：SZip A inputfilename outputfilename
解压缩命令：SZip X inputfilename outputfilename
Python 代码
'''
@file huffman.py
'''
import heapq
import os
from functools import total_ordering
@total_ordering
class HeapNode:
def __init__(self, char, freq):
self.char = char
self.freq = freq
self.left = None
self.right = None
# defining comparators less_than and equals
def __lt__(self, other):
return self.freq < other.freq
def __eq__(self, other):
if(other == None):
return False
if(not isinstance(other, HeapNode)):
return False
return self.freq == other.freq
class HuffmanCoding:
def __init__(self, path):
self.path = path
self.heap = []
self.codes = {}
self.reverse_mapping = {}
# functions for compression:
def make_frequency_dict(self, text):
frequency = {}
for character in text:
if not character in frequency:
frequency[character] = 0
frequency[character] += 1
return frequency
def make_heap(self, frequency):
for key in frequency:
node = HeapNode(key, frequency[key])
heapq.heappush(self.heap, node)
def merge_nodes(self):
while(len(self.heap)>1):
node1 = heapq.heappop(self.heap)
node2 = heapq.heappop(self.heap)
merged = HeapNode(None, node1.freq + node2.freq)
merged.left = node1
merged.right = node2
heapq.heappush(self.heap, merged)
def make_codes_helper(self, root, current_code):
if(root == None):
return
if(root.char != None):
self.codes[root.char] = current_code
self.reverse_mapping[current_code] = root.char
return
self.make_codes_helper(root.left, current_code + "0")
self.make_codes_helper(root.right, current_code + "1")
def make_codes(self):
root = heapq.heappop(self.heap)
current_code = ""
self.make_codes_helper(root, current_code)
def get_encoded_text(self, text):
encoded_text = ""
for character in text:
encoded_text += self.codes[character]
return encoded_text
def pad_encoded_text(self, encoded_text):
extra_padding = 8 - len(encoded_text) % 8
for i in range(extra_padding):
encoded_text += "0"
padded_info = "{0:08b}".format(extra_padding)
encoded_text = padded_info + encoded_text
return encoded_text
def get_byte_array(self, padded_encoded_text):
if(len(padded_encoded_text) % 8 != 0):
print("Encoded text not padded properly")
exit(0)
b = bytearray()
for i in range(0, len(padded_encoded_text), 8):
byte = padded_encoded_text[i:i+8]
b.append(int(byte, 2))
return b
def compress(self):
filename, file_extension = os.path.splitext(self.path)
output_path = filename + ".bin"
with open(self.path, 'r+') as file, open(output_path, 'wb') as output:
text = file.read()
text = text.rstrip()
frequency = self.make_frequency_dict(text)
self.make_heap(frequency)
self.merge_nodes()
self.make_codes()
encoded_text = self.get_encoded_text(text)
padded_encoded_text = self.pad_encoded_text(encoded_text)
b = self.get_byte_array(padded_encoded_text)
output.write(bytes(b))
print("Compressed")
return output_path
""" functions for decompression: """
def remove_padding(self, padded_encoded_text):
padded_info = padded_encoded_text[:8]
extra_padding = int(padded_info, 2)
padded_encoded_text = padded_encoded_text[8:]
encoded_text = padded_encoded_text[:-1*extra_padding]
return encoded_text
def decode_text(self, encoded_text):
current_code = ""
decoded_text = ""
for bit in encoded_text:
current_code += bit
if(current_code in self.reverse_mapping):
character = self.reverse_mapping[current_code]
decoded_text += character
current_code = ""
return decoded_text
def decompress(self, input_path):
filename, file_extension = os.path.splitext(self.path)
output_path = filename + "_decompressed" + ".txt"
with open(input_path, 'rb') as file, open(output_path, 'w') as output:
bit_string = ""
byte = file.read(1)
while(len(byte) > 0):
byte = ord(byte)
bits = bin(byte)[2:].rjust(8, '0')
bit_string += bits
byte = file.read(1)
encoded_text = self.remove_padding(bit_string)
decompressed_text = self.decode_text(encoded_text)
output.write(decompressed_text)
print("Decompressed")
return output_path
#@file main.py
from huffman import HuffmanCoding
path = "test.txt"
h = HuffmanCoding(path)
output_path = h.compress()
print("Compressed file path: " + output_path)
decom_path = h.decompress(output_path)
print("Decompressed file path: " + decom_path)