如何让JavaScript与Python的base64 atob得到相同结果

我知道Python有base64.urlsafe_b64decode()，但想深入理解Base64解码的底层逻辑，所以打算把一段JS实现的atob代码移植到Python，但遇到了结果不一致的问题。

原JS的atob实现代码 #

function atob (input) {
  var chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
  var str = String(input).replace(/=+$/, '');
  if (str.length % 4 == 1) {
    throw new InvalidCharacterError("'atob' failed: The string to be decoded is not correctly encoded.");
  }
  for (
    // initialize result and counters
    var bc = 0, bs, buffer, idx = 0, output = '';
    // get next character
    buffer = str.charAt(idx++);
    // character found in table? initialize bit storage and add its ascii value;
    ~buffer && (bs = bc % 4 ? bs * 64 + buffer : buffer,
      // and if not first of each 4 characters,
      // convert the first 8 bits to one ascii character
      bc++ % 4) ? output += String.fromCharCode(255 & bs >> (-2 * bc & 6)) : 0
  ) {
    // try to find character in table (0-63, not found => -1)
    buffer = chars.indexOf(buffer);
  }
  return output;
}

我的困惑点 #

我看不懂JS中for循环的逻辑，尤其是：

• bs = bc % 4 ? bs*64+buffer: buffer, bc++ %4里的逗号表达式
• String.fromCharCode(255 & bs >> (-2 * bc & 6))的位运算部分

我尝试编写的Python代码 #

import re

# Test subject
b64_str: str = "fwHzODWqgMH+NjBq02yeyQ=="
    
# Lookup table for characters
chars: str = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="

# Replace right padding with empty string
replaced = re.sub("=+$", '', b64_str)
if len(replaced) % 4 == 1:
    raise ValueError("atob failed. The string to be decoded is not valid base64")

# Bit storage and counters
bc = 0
out: str = ''
for i in replaced:

    # Get ascii value of character
    buffer = ord(i)

    # If counter is evenly divisible by 4, return buffer as is, else add the ascii value
    bs = bc * 64 + buffer if bc % 4 else buffer
    bc += 1 % 4 # Not sure I understand this part
    
    # Check if character is in the chars table
    if i in chars:

        # Check if the bit storage and bit counter are non-zero
        if bs and bc:
            # If so, convert the first 8 bits to an ascii character
            out += chr(255 & bs >> (-2 * bc & 6))
        else:
            out = 0
            
    # Set buffer to the index of where the first instance of the character is in the b64 string
    # print(f"before: {chr(buffer)}")
    buffer = chars.index(chr(buffer))
    # print(f"after: {buffer}")
    
print(out)

问题现状 #

测试字符串"fwHzODWqgMH+NjBq02yeyQ=="的解码结果：

• JS输出：ó85ªÁþ60jÓlÉ
• 我的Python代码输出：2:u1(²ë:ð1G>%Y

结果不一致，需要解决这个问题。

内容的提问来源于stack exchange，提问作者Coldchain9