Python解码旧文档二进制数据求助：Base64转译数值推导存疑

Base64编码 #

FBE80YlsYgAIhjELHyhxUyQsNyQrawAMOgAQEAAQEAAIDwAUrgARCQAQHAATiAAY1QcwGQ
AMMCbJCAARIgAMc4qUV4iRIgAUuAARtAAUV4iRUgAMPAAJKQ9sRoiRtAAUOCbBSJf0OwAJ
jYIJjYIChhCq+v1bS0tHR0Y=

Base64解码后的二进制字符串 #

0001010000010001001111001101000110001001011011000110001000000000000010
0010000110001100010000101100011111001010000111000101010011001001000010
1100001101110010010000101011011010110000000000001100001110100000000000
0100000001000000000000000100000001000000000000000010000000111100000000
0001010010101110000000000001000100001001000000000001000000011100000000
0000010011100010000000000000011000110101010000011100110000000110010000
0000000011000011000000100110110010010000100000000000000100010010001000
0000000000110001110011100010101001010001010111100010001001000100100010
0000000000010100101110000000000000010001101101000000000000010100010101
1110001000100100010101001000000000000011000011110000000000000010010010
1001000011110110110001000110100010001001000110110100000000000001010000
1110000010011011000001010010001001011111110100001110110000000000001001
1000110110000010000010011000110110000010000000101000011000010000101010
101111101011111101010110110100101101001011010001110100011101000110

已知参考信息 #

(length = 2, value = 2) (length = 1, value = 0)
00010 10 00001 0 

核心困惑 #

文档明确说明二进制片段00100 1111对应数值15，但我完全搞不懂这个推导逻辑。

自行编写的Python代码 #

def getDivision(code):
    if not code:
        return "Unknown division code"
    base64data = base64.b64decode(code)
    binaryData = ""
    for x in base64data:
        a = bin(x)[2:]
        while len(a) < 8:
            a = "0" + a
        binaryData = binaryData + a
    header = int(binaryData[12:17], 2)
    code = binaryData[17:17 + header]

    print(code)

希望有人能帮我解释这段二进制到数值的推导过程。

内容的提问来源于stack exchange，提问作者Lynchie