Python

# 字节到大整数的打包与解包

## 解法

``````data = b'\x00\x124V\x00x\x90\xab\x00\xcd\xef\x01\x00#\x004'
``````

``````>>> len(data)
16
>>> int.from_bytes(data, 'little')
69120565665751139577663547927094891008
>>> int.from_bytes(data, 'big')
94522842520747284487117727783387188
>>>
``````

``````>>> x = 94522842520747284487117727783387188
>>> x.to_bytes(16, 'big')
b'\x00\x124V\x00x\x90\xab\x00\xcd\xef\x01\x00#\x004'
>>> x.to_bytes(16, 'little')
b'4\x00#\x00\x01\xef\xcd\x00\xab\x90x\x00V4\x12\x00'
>>>
``````

## 讨论

``````>>> data
b'\x00\x124V\x00x\x90\xab\x00\xcd\xef\x01\x00#\x004'
>>> import struct
>>> hi, lo = struct.unpack('>QQ', data)
>>> (hi << 64) + lo
94522842520747284487117727783387188
>>>
``````

``````>>> x = 0x01020304
>>> x.to_bytes(4, 'big')
b'\x01\x02\x03\x04'
>>> x.to_bytes(4, 'little')
b'\x04\x03\x02\x01'
>>>
``````

``````>>> x = 523 ** 23
>>> x
335381300113661875107536852714019056160355655333978849017944067
>>> x.to_bytes(16, 'little')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
OverflowError: int too big to convert
>>> x.bit_length()
208
>>> nbytes, rem = divmod(x.bit_length(), 8)
>>> if rem:
... nbytes += 1
...
>>>
>>> x.to_bytes(nbytes, 'little')
b'\x03X\xf1\x82iT\x96\xac\xc7c\x16\xf3\xb9\xcf...\xd0'
>>>
``````

https://my.oschina.net/jallenkwong/blog/4720509