编写 Python 程序，实现利用算术编码对文本文件的压缩

算术编码是一种无损数据压缩算法，可以对文本文件进行压缩。下面是一个简单的 Python 程序，利用算术编码对文本文件进行压缩：

import sys
from collections import Counter

def get_probabilities(text):
    # 计算每个字符在文本中出现的频率
    counter = Counter(text)
    total = sum(counter.values())
    probabilities = {ch: freq/total for ch, freq in counter.items()}
    return probabilities

def compress(text):
    # 计算每个字符的概率
    probabilities = get_probabilities(text)

    # 初始化编码区间
    low = 0
    high = 1

    # 迭代计算编码区间
    for ch in text:
        diff = high - low
        high = low + diff * probabilities[ch]
        low = low + diff * sum(probabilities[prev_ch] for prev_ch in probabilities if prev_ch < ch)

    # 输出编码结果
    return (low + high) / 2

if __name__ == '__main__':
    if len(sys.argv) != 2:
        print('Usage: python arithmetic_coding.py <filename>')
        sys.exit(1)

    filename = sys.argv[1]
    with open(filename, 'r') as f:
        text = f.read()

    compressed = compress(text)
    print(f'Compressed size: {compressed:.2f} bits')

这个程序接受一个文件名作为参数，并读取文件中的文本。然后，它计算每个字符在文本中出现的频率，并使用这些概率计算编码区间。最后，程序输出压缩后的文本大小（以位为单位）。