问在给定范围内查找掩码的子掩码

EN

>>> def submask( mask, r ) :
...     result = 0
...     for bit in range(32,-1,-1) :
...         value = 1 << bit
...         if value & mask == 0 : continue
...         if value | result <= r :
...             result |= value
...     return result
... 
>>> submask(5,1)
1
>>> submask(5,3)
1
>>> submask(5,4)
4
>>> submask(5,5)
5
>>> submask(7,2)
2
>>>