算术编码python

算术编码是一种无损数据压缩技术，可以将任何类型的数据流编码为一个单一的浮点数。Python中可以使用一些第三方库来实现算术编码，比如bitarray和numpy。

以下是一个使用bitarray库实现算术编码的示例代码：

from bitarray import bitarray

def arithmetic_encode(data, freqs):
    # 计算符号累积频率
    cum_freqs = [0] + list(accumulate(freqs))
    total_freq = cum_freqs[-1]

    # 初始化编码器
    low = 0
    high = 1
    code = bitarray()

    # 编码每个符号
    for symbol in data:
        # 计算符号的范围
        symbol_low = cum_freqs[symbol]
        symbol_high = cum_freqs[symbol+1]
        symbol_range = symbol_high - symbol_low

        # 更新编码器的上界和下界
        new_low = low + symbol_range * (high - low)
        new_high = low + symbol_range * (high - low)

        # 将位添加到编码中，如果范围太小，则向上或向下舍入
        while True:
            if new_high < 0.5:
                code.append(0)
                code.extend([1] * num_pending_bits)
                low = 2 * low
                high = 2 * high
            elif new_low >= 0.5:
                code.append(1)
                code.extend([0] * num_pending_bits)
                low = 2 * (low - 0.5)
                high = 2 * (high - 0.5)
            elif 0.25 <= new_low and new_high < 0.75:
                num_pending_bits += 1
                new_low = 2 * (new_low - 0.25)
                new_high = 2 * (new_high - 0.25)
            else:
                break

        # 更新编码器的上界和下界
        low = new_low
        high = new_high

    # 编码结束符号
    end_range = high - low
    end_low = low
    end_high = low + end_range * 0.99999999
    while True:
        if end_high < 0.5:
            code.append(0)
            code.extend([1] * num_pending_bits)
            end_low = 2 * end_low
            end_high = 2 * end_high
        elif end_low >= 0.5:
            code.append(1)
            code.extend([0] * num_pending_bits)
            end_low = 2 * (end_low - 0.5)
            end_high = 2 * (end_high - 0.5)
        elif 0.25 <= end_low and end_high < 0.75:
            num_pending_bits += 1
            end_low = 2 * (end_low - 0.25)
            end_high = 2 * (end_high - 0.25)
        else:
            break

    # 返回编码
    return code.tobytes()

在以上代码中，data是要进行编码的数据流，freqs是每个符号出现频率的列表。该函数将返回一个字节串，其中包含编码后的数据。

要解码数据，可以使用以下示例代码：

from bitarray import bitarray

def arithmetic_decode(code, freqs, num_symbols):
    # 计算符号累积频率
    cum_freqs = [0] + list(accumulate(freqs))
    total_freq = cum_freqs[-1]

    # 初始化解码器
    low = 0
    high = 1
    current_code = bitarray(endian='big')
    current_code.frombytes(code)
    current_code.extend([0] * (8 - len(current_code) % 8))
    current_code.append(1)

    # 解码每个符号
    data = []
    for i in range(num_symbols):
        # 计算解码符号的范围
        range_size = high - low
        offset = int(((current_code - low) / range_size) * total_freq)
        symbol = bisect_right(cum_freqs, offset) - 1

        # 更新解码器的上界和下界
        symbol_low = cum_freqs[symbol]
        symbol_high = cum_freqs[symbol+1]
        symbol_range = symbol_high - symbol_low
        new_low = low + symbol_range * (current_code - low) / range_size
        new_high = low + symbol_range * (current_code - low) / range_size

        # 将解码的符号添加到数据中
        data.append(symbol)

        # 更新解码器的上界和下界
        low = new_low
        high = new_high

        # 移除已解码的位
        while True:
            if high < 0.5:
                low = 2 * low
                high = 2 * high
                current_code = (current_code << 1) & 0xfffffffffffffffe
                current_code |= 0x01 if current_code[-1] else 0x00
            elif low >= 0.5:
                low = 2 * (low - 0.5)
                high = 2 * (high - 0.5)
                current_code = (current_code << 1) & 0xfffffffffffffffe
                current_code |= 0x01 if current_code[-1] else 0x00
            elif 0.25 <= low and high < 0.75:
                low = 2 * (low - 0.25)
                high = 2 * (high - 0.25)
                current_code = (current_code << 1) & 0xfffffffffffffffe
                current_code |= 0x01 if current_code[-1] else 0x00
            else:
                break

    # 返回解码数据
    return data

在以上代码中，code是要进行解码的字节串，freqs是每个符号出现频率的列表，num_symbols是要解码的符号数。该函数将返回一个包含解码数据的列表。