If it won't be simple, it simply won't be. [Hire me, source code] by Miki Tebeka, CEO, 353Solutions

Thursday, February 07, 2008

Playing with bits


def mask(size):
'''Mask for `size' bits

>>> mask(3)
7
'''
return (1L << size) - 1

def num2bits(num, width=32):
'''String represntation (in bits) of a number

>>> num2bits(3, 5)
'00011'
'''
s = ""
for bit in range(width - 1, -1, -1):
if num & (1L << bit):
s += "1"
else:
s += "0"
return s

def get_bit(value, bit):
'''Get value of bit

>>> num2bits(5, 5)
'00101'
>>> get_bit(5, 0)
1
>>> get_bit(5, 1)
0
'''
return (value >> bit) & 1

def get_range(value, start, end):
'''Get range of bits

>>> num2bits(5, 5)
'00101'
>>> get_range(5, 0, 1)
1
>>> get_range(5, 1, 2)
2
'''
return (value >> start) & mask(end - start + 1)

def set_bit(num, bit, value):
'''Set bit `bit' in num to `value'

>>> i = 5
>>> set_bit(i, 1, 1)
7
>>> set_bit(i, 0, 0)
4
'''
if value:
return num | (1L << bit)
else:
return num & (~(1L << bit))

def sign_extend(num, size):
'''Sign exten number who is `size' bits wide

>>> sign_extend(5, 2)
1
>>> sign_extend(5, 3)
-3
'''
m = mask(size - 1)
res = num & m
# Positive
if (num & (1L << (size - 1))) == 0:
return res

# Negative, 2's complement
res = ~res
res &= m
res += 1
return -res
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