On Jun 19, 4:27 pm, godavemon <[EMAIL PROTECTED]> wrote:
> I need to calculate the Hamming Distance of two integers. The hamming
> distance is the number of bits in two integers that don't match. I
> thought there'd be a function in math or scipy but i haven't been able
> to find one. This is my function but it seems like there should be a
> faster way. I do this computation many times and speed up is
> important.
>
> def hamdist( a, b , bits = 32):
> def _hamdist( x, bits):
> if bits:
> return (x & 1) + _hamdist(x >> 1, bits-1)
> return x & 1
> return _hamdist( a ^ b, bits)
>
> Another alternative would be to convert the XOR to a binary string and
> count the # of 1's.
>
> Which would be fastest? Are there better alternatives?
>
> Thanks!
I see no good reason to use recursion for this type of thing. Here are
some of my attempts:
[code]
from math import log
def yours(a, b , bits = 32):
def _hamdist( x, bits):
if bits:
return (x & 1) + _hamdist(x >> 1, bits-1)
return x & 1
return _hamdist(a ^ b, bits)
def simple(a, b, bits=32):
x = a ^ b
return sum((x >> i & 1) for i in xrange(bits))
def lazy(a, b, bits=32):
x = (a ^ b) & ((1 << bits) - 1)
tot = 0
while x:
tot += x & 1
x >>= 1
return tot
def fancy(a, b, bits=32):
x = (a ^ b) & ((1 << bits) - 1)
tot = 0
while x:
tot += 1
x ^= 1 << int(log(x, 2))
return tot
test_vals = (
((0xffffffff, 0), 32),
((0,0), 0),
((1,0), 1),
((0x80000000, 0), 1),
((0x55555555, 0), 16)
)
def test(f):
test_vals = (
((0xffffffff, 0), 32), # ALL
((0,0), 0), # None
((1,0), 1), # First
((0x80000000, 0), 1), # Last
((0x55555555, 0), 16), # Every Other
((0xffff, 0), 16), # First Half
((0xffff0000, 0), 16), # Last Half
)
for i, (args, exp) in enumerate(test_vals):
if f(*args) != exp:
return 0
return 1
if __name__ == "__main__":
for f in (yours, simple, lazy, fancy):
if not test(f):
print "%s failed"%f.__name__
[/code]
The python module `timeit` is handy for testing speed:
python -mtimeit -s"from hamdist import *" "test(yours)"
10000 loops, best of 3: 95.1 usec per loop
python -mtimeit -s"from hamdist import *" "test(simple)"
10000 loops, best of 3: 65.3 usec per loop
python -mtimeit -s"from hamdist import *" "test(lazy)"
10000 loops, best of 3: 59.8 usec per loop
python -mtimeit -s"from hamdist import *" "test(fancy)"
10000 loops, best of 3: 77.2 usec per loop
Even the ridiculous `fancy` version beat the recursive version.
Matt
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