python自适应算术编码
时间: 2023-08-25 16:17:33 浏览: 98
Python自适应算术编码是一种用于数据压缩的无损压缩算法。它将一段数据中的符号(通常是字符或字节)映射到一个区间,并将该区间编码为一个二进制数。通过不断更新区间的上下限来逐步缩小该区间,最终得到一个紧凑的编码。与传统的霍夫曼编码相比,自适应算术编码可以自适应地调整编码表,从而更好地适应不同的数据分布。Python中可以使用第三方库bitarray来实现自适应算术编码,也可以自己实现。
相关问题
使用python,实现自适应算术编码
以下是一个简单的Python实现自适应算术编码的例子:
```python
import bisect
from collections import defaultdict
class ArithmeticEncoder:
def __init__(self, bitout):
self.low = 0
self.high = 0xFFFFFFFF
self.bitout = bitout
self.pending_bits = 0
self.pending_data = []
def get_scaled_value(self, value_range, offset, total_range):
return ((value_range * (self.high - self.low)) // total_range) + offset
def encode(self, symbol, freqs):
total_freq = sum(freqs.values())
if total_freq >= 0xFFFFFFFF:
raise ValueError("Cannot code symbol with frequency >= 2^32")
value_range = (self.high - self.low) + 1
symbol_low = 0
for k, v in freqs.items():
if k < symbol:
symbol_low += v
symbol_high = symbol_low + freqs[symbol] - 1
self.low = self.get_scaled_value(symbol_low, self.low, total_freq)
self.high = self.get_scaled_value(symbol_high, self.low, total_freq)
while ((self.low ^ self.high) & 0xFF000000) == 0:
self.bitout.write(self.high >> 24)
for b in self.pending_data:
self.bitout.write(b ^ 0xFF)
self.pending_data.clear()
self.low <<= 8
self.high <<= 8
self.high |= 0xFF
self.pending_bits += 8
while self.pending_bits > 0:
self.bitout.write(self.high >> 24)
self.pending_data.append((self.high >> 24) ^ 0xFF)
self.high <<= 8
self.pending_bits -= 8
def finish(self):
self.bitout.write((self.high >> 24) - 1)
for b in self.pending_data:
self.bitout.write(self.high >> 24)
self.bitout.write(self.high >> 24)
class ArithmeticDecoder:
def __init__(self, bitin):
self.low = 0
self.high = 0xFFFFFFFF
self.bitin = bitin
self.code = 0
for i in range(4):
self.code = (self.code << 8) | self.bitin.read()
def get_value(self, freqs):
total_freq = sum(freqs.values())
if total_freq >= 0xFFFFFFFF:
raise ValueError("Cannot decode symbol with frequency >= 2^32")
value_range = (self.high - self.low) + 1
offset = self.code - self.low
offset = ((offset * total_freq) // value_range)
symbol = 0
symbol_low = 0
for k, v in freqs.items():
symbol_high = symbol_low + v - 1
if symbol_low <= offset <= symbol_high:
symbol = k
break
symbol_low = symbol_high + 1
symbol_high = symbol_low + freqs[symbol] - 1
self.low = self.get_scaled_value(symbol_low, self.low, total_freq)
self.high = self.get_scaled_value(symbol_high, self.low, total_freq)
while ((self.low ^ self.high) & 0xFF000000) == 0:
self.code = (self.code << 8) | self.bitin.read()
self.low <<= 8
self.high <<= 8
self.high |= 0xFF
return symbol
def get_scaled_value(self, value_range, offset, total_range):
return ((value_range * (self.high - self.low)) // total_range) + offset
```
使用方法如下:
```python
from bitarray import bitarray
def encode_string(s):
freqs = defaultdict(int)
for c in s:
freqs[c] += 1
bitout = bitarray()
enc = ArithmeticEncoder(bitout)
for c in s:
enc.encode(c, freqs)
freqs[c] += 1
enc.finish()
return bitout
def decode_string(bitstring):
freqs = defaultdict(int)
for b in bitstring:
freqs[b] += 1
bitin = bitstring.itersearch(bitarray('1'))
dec = ArithmeticDecoder(bitin)
s = ''
while True:
try:
symbol = dec.get_value(freqs)
s += symbol
freqs[symbol] += 1
except IndexError:
break
return s
```
这里我们使用了bitarray库来处理比特流。encode_string函数将一个字符串进行编码,并返回一个比特流。decode_string函数接受一个比特流,并对其进行解码,返回原始字符串。
用python实现算术编码
好的,算术编码是一种无损数据压缩算法,它可以将一个符号串编码为一个实数,实现数据的压缩。下面是用 Python 实现算术编码的代码:
```python
# 算术编码实现
def arithmetic_encode(symbols, probabilities):
# 计算符号的累积概率
cumulative_probabilities = [0.0] * len(probabilities)
cumulative_probabilities[0] = probabilities[0]
for i in range(1, len(probabilities)):
cumulative_probabilities[i] = cumulative_probabilities[i - 1] + probabilities[i]
# 初始化区间
low = 0.0
high = 1.0
# 编码
for symbol in symbols:
symbol_index = symbol - 1
symbol_range = high - low
high = low + symbol_range * cumulative_probabilities[symbol_index + 1]
low = low + symbol_range * cumulative_probabilities[symbol_index]
# 返回编码结果
return low
# 测试代码
symbols = [1, 2, 3, 4, 5]
probabilities = [0.1, 0.2, 0.3, 0.15, 0.25]
encoded_result = arithmetic_encode(symbols, probabilities)
print(encoded_result)
```
在代码中,我们先计算了每个符号的累积概率,然后使用区间编码的方法对符号串进行编码,最后返回编码结果。需要注意的是,这里的符号需要是整数,概率列表中的索引对应符号减一的值。