Bloom filters in Python
Bloom filters in Python - 张沈鹏,在路上... - ITeye技术网站
Python语言: 临时自用代码@代码发芽网
#coding:utf-8
# Bloom filters in Python
# Adam Langley <[email protected]>
# 给CountedBloom加了一个max_count 张沈鹏 <[email protected]>
# Bloom-Filter算法简介
# http://www.googlechinablog.com/2007/07/bloom-filter.html
# http://zh.wikipedia.org/wiki/%E5%B8%83%E9%9A%86%E8%BF%87%E6%BB%A4%E5%99%A8
# 这个计算器可以帮你求最佳的参数
# http://www.cc.gatech.edu/~manolios/bloom-filters/calculator.html
# CountedBloom 的 buckets 参数对应于计算器的m,也就是"m denotes the number of bits in the Bloom filter"
import array
import struct
mixarray = array.array ('B', '\x00' * 256)
# The mixarray is based on RC4 and is used as diffusion in the hashing function
def mixarray_init (mixarray):
for i in range (256):
mixarray[i] = i
k = 7
for j in range (4):
for i in range (256):
s = mixarray[i]
k = (k + s) % 256
mixarray[i] = mixarray[k]
mixarray[k] = s
mixarray_init (mixarray)
class Bloom (object):
'''Bloom filters provide a fast and compact way of checking set membership. They do this by introducing a risk of a
false positive (but there are no false negatives).
For more information see http://www.cs.wisc.edu/~cao/papers/summary-cache/node8.html'''
def __init__ (self, bytes, hashes, data = None):
'''@bytes is the size of the bloom filter in 8-bit bytes and @hashes is the number of hash functions to use. Consult the
web page linked above for values to use. If in doubt, bytes = num_elements and hashes = 4'''
self.hashes = hashes
self.bytes = bytes
if data == None:
self.a = self._make_array (bytes)
else:
assert len (data) == bytes
self.a = data
def init_from_counted (self, cnt):
'''Set the contents of this filter from the contents of the counted filter @cnt. You have to match sizes'''
if self.bytes * 8 != (len (cnt.a) * 2):
raise ValueError ('Filters are not the same size')
for i in xrange (len (cnt.a)):
b = cnt.a[i]
b1 = (b & 0xf0) >> 4
b2 = (b & 0x0f)
if b1:
self.a[(i * 2) // 8] |= self.bitmask[(i * 2) % 8]
if b2:
self.a[(i * 2 + 1) // 8] |= self.bitmask[(i * 2 + 1) % 8]
def _make_array (self, size):
a = array.array ('B')
# stupidly, there's no good way that I can see of resizing an array without allocing a huge string to do so
# thus I use this, slightly odd, method:
blocklen = 256
arrayblock = array.array ('B', '\x00' * blocklen)
todo = size
while (todo >= blocklen):
a.extend (arrayblock)
todo -= blocklen
if todo:
a.extend (array.array ('B', '\x00' * todo))
# now a is of the right length
return a
def _hashfunc (self, n, val):
'''Apply the nth hash function'''
global mixarray
b = [ord(x) for x in struct.pack ('I', val)]
c = array.array ('B', [0, 0, 0, 0])
for i in range (4):
c[i] = mixarray[(b[i] + n) % 256]
return struct.unpack ('I', c.tostring())[0]
bitmask = [0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, 0x01]
def insert (self, val):
for i in range (self.hashes):
n = self._hashfunc (i, val) % (self.bytes *
self.a[n // 8] |= self.bitmask[n % 8]
def __contains__ (self, val):
for i in range (self.hashes):
n = self._hashfunc (i, val) % (self.bytes *
if not self.a[n // 8] & self.bitmask[n % 8]:
return 0
return 1
MAX_COUNT = 15
class CountedBloom (Bloom):
'''Just like a Bloom filter, but provides counting (e.g. you can delete as well). This uses 4 bits per bucket, so is
generally four times larger than the same non-counted bloom filter.'''
def __init__ (self, buckets, hashes):
'''Please note that @buckets must be even. Also note that with a Bloom object you give the number of *bytes* and each byte is 8 buckets. Here you're giving the number of buckets.'''
assert buckets % 2 == 0
self.hashes = hashes
self.buckets = buckets
self.a = self._make_array (buckets // 2)
def insert (self, val):
masks = [(0x0f, 0xf0), (0xf0, 0x0f)]
shifts = [4, 0 ]
for i in range (self.hashes):
n = self._hashfunc (i, val) % self.buckets
byte = n // 2
bucket = n % 2
(notmask, mask) = masks[bucket]
shift = shifts[bucket]
bval = ((self.a[byte] & mask) >> shift)
if bval < MAX_COUNT: # we shouldn't increment it if it's at the maximum
bval += 1
self.a[byte] = (self.a[byte] & notmask) | (bval << shift)
def __contains__ (self, val):
masks = [(0x0f, 0xf0), (0xf0, 0x0f)]
shifts = [4, 0]
for i in range (self.hashes):
n = self._hashfunc (i, val) % self.buckets
byte = n // 2
bucket = n % 2
(notmask, mask) = masks[bucket]
shift = shifts[bucket]
bval = ((self.a[byte] & mask) >> shift)
if bval == 0:
return 0
return 1
def max_count(self, val):
masks = [(0x0f, 0xf0), (0xf0, 0x0f)]
shifts = [4, 0]
count_val = MAX_COUNT
for i in range (self.hashes):
n = self._hashfunc (i, val) % self.buckets
byte = n // 2
bucket = n % 2
(notmask, mask) = masks[bucket]
shift = shifts[bucket]
bval = ((self.a[byte] & mask) >> shift)
if bval < MAX_COUNT:
if bval == 0:
return 0
else:
count_val = bval
return count_val
def __delitem__ (self, val):
masks = [(0x0f, 0xf0), (0xf0, 0x0f)]
shifts = [4, 0]
for i in range (self.hashes):
n = self._hashfunc (i, val) % self.buckets
byte = n // 2
bucket = n % 2
(notmask, mask) = masks[bucket]
shift = shifts[bucket]
bval = ((self.a[byte] & mask) >> shift)
if bval < MAX_COUNT: # we shouldn't decrement it if it's at the maximum
bval -= 1
self.a[byte] = (self.a[byte] & notmask) | (bval << shift)
__all__ = ['Bloom']
if __name__ == '__main__':
print 'Testing bloom filter: there should be no assertion failures'
a = Bloom (3, 4)
a.insert (45)
print a.a
a.insert (17)
print a.a
a.insert (12)
print a.a
assert 45 in a
assert 45 in a
assert not 33 in a
assert 45 in a
assert 17 in a
assert 12 in a
c = 0
for x in range (255):
if x in a:
c += 1
print c
print float(c)/255
a = CountedBloom (24, 4)
a.insert (45)
print a.a
a.insert (17)
print a.a
a.insert (12)
a.insert (12)
print "a.max_count(12)", a.max_count(12)
a.insert ("张沈鹏")
a.insert ("张沈鹏")
a.insert ("张沈鹏")
print "a.max_count(zsp)", a.max_count(12)
print a.a
assert 45 in a
assert 45 in a
assert not 33 in a
assert 45 in a
assert 17 in a
assert 12 in a
c = 0
for x in range (255):
if x in a:
c += 1
print c
print float(c)/255
del a[45]
assert not 45 in a
a2 = Bloom (3, 4)
a2.init_from_counted (a)
print a2.a
assert 17 in a2
assert 12 in a2
assert not 45 in a