bit operations ?
Anton Vredegoor
anton at vredegoor.doge.nl
Fri Sep 20 20:17:04 CEST 2002
On Fri, 20 Sep 2002 16:40:18 +0200, "Shagshag13" <shagshag13 at yahoo.fr>
wrote:
>i'm looking for any libs which could handle bit operations on integer.
>(i must be able to get 0 to n bits from any int., only bit j, etc.)
If you're interested in setting/getting a lot of bits of a long
integer at once, this might be faster than bitshifting. I am not
guaranteeing that it is failsafe, or even that it is fast. Just hobby
programmer code I wrote when I was chasing phantoms. It works only for
long integers >= 0L now, but this should be easy to correct.
The two functions are reciprocal, so if bits are set with the setbits
function the getbits function does the reverse.
Anton.
#bitset.py test script
from time import time
import dis
#list of bit positions that are set to one
bitget ={'0' : [], '1' : [0], '2' : [1], '3' : [0,1],
'4' : [2], '5' : [0,2], '6' : [1,2], '7' : [0,1,2],
'8' : [3], '9' : [0,3], 'A' : [1,3], 'B' : [0,1,3],
'C' : [2,3], 'D' : [0,2,3], 'E' : [1,2,3], 'F' : [0,1,2,3]}
#f.e. if bit at position 1 is set, '8' changes to 'A' etc.
bitset = { '0' : ['1','2','4','8'], '1' : ['1','3','5','9'],
'2' : ['3','2','6','A'], '3' : ['3','3','7','B'],
'4' : ['5','6','4','C'], '5' : ['5','7','5','D'],
'6' : ['7','6','6','E'], '7' : ['7','7','7','F'],
'8' : ['9','A','C','8'], '9' : ['9','B','D','9'],
'A' : ['B','A','E','A'], 'B' : ['B','B','F','B'],
'C' : ['D','B','E','C'], 'D' : ['D','F','D','D'],
'E' : ['F','E','E','E'], 'F' : ['F','F','F','F'] }
def setbits(ilist):
#set bits at the positions defined in ilist,
#returns a long integer
dd,m = divmod(max(ilist),4)
if m:
dd+=1
res = ['0'] * (dd+1)
for x in ilist:
d, m = divmod(x,4)
i = dd-d
res[i] = bitset[res[i]][m]
return long(''.join(res),16)
def getbits(along):
#return a list of integers corresponding to the
#bitpositions that are set
h = hex(along)[2:-1]
res = []
i = 0
for j in range(len(h)-1,-1,-1):
for pos in bitget[h[j]]:
res.append(i+pos)
i+=4
return res
def test():
n =1000
a = 2349384569856987569824652587349873498723498L
b = getbits(a)
t=time()
for i in range(n):
getbits(a)
print time()-t
t=time()
for i in range(n):
setbits(b)
print time()-t
print setbits(b) == a
# print dis.dis(setbits)
# print dis.dis(getbits)
if __name__ == '__main__':
test()
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