from array import array
from random import Random
class BloomFilter:
# http://en.wikipedia.org/wiki/Bloom_filter
def __init__(self, num_bits, num_probes):
num_words = (num_bits + 31) // 32
self.arr = array('L', [0]) * num_words
self.num_probes = num_probes
def get_probes(self, key):
hasher = Random(key).randrange
for _ in range(self.num_probes):
array_index = hasher(len(self.arr))
bit_index = hasher(32)
yield array_index, 1 << bit_index
def add(self, key):
for i, mask in self.get_probes(key):
self.arr[i] |= mask
def __contains__(self, key):
return all(self.arr[i] & mask for i, mask in self.get_probes(key))
if __name__ == '__main__':
from random import sample
from string import ascii_letters
states = '''Alabama Alaska Arizona Arkansas California Colorado Connecticut
Delaware Florida Georgia Hawaii Idaho Illinois Indiana Iowa Kansas
Kentucky Louisiana Maine Maryland Massachusetts Michigan Minnesota
Mississippi Missouri Montana Nebraska Nevada NewHampshire NewJersey
NewMexico NewYork NorthCarolina NorthDakota Ohio Oklahoma Oregon
Pennsylvania RhodeIsland SouthCarolina SouthDakota Tennessee Texas Utah
Vermont Virginia Washington WestVirginia Wisconsin Wyoming'''.split()
bf = BloomFilter(num_bits=1000, num_probes=14)
for state in states:
bf.add(state)
m = sum(state in bf for state in states)
print('%d true positives out of %d trials' % (m, len(states)))
trials = 100000
m = sum(''.join(sample(ascii_letters, 5)) in bf for i in range(trials))
print('%d true negatives and %d false negatives out of %d trials'
% (trials-m, m, trials))
Diff to Previous Revision
--- revision 2 2011-05-04 17:53:16
+++ revision 3 2011-05-05 01:13:15
@@ -1,12 +1,8 @@
from array import array
-from hashlib import sha512
-try:
- from pickle import dumps # py3.x version
-except ImportError:
- from cPickle import dumps # py2.x version
+from random import Random
class BloomFilter:
- 'http://en.wikipedia.org/wiki/Bloom_filter'
+ # http://en.wikipedia.org/wiki/Bloom_filter
def __init__(self, num_bits, num_probes):
num_words = (num_bits + 31) // 32
@@ -14,11 +10,10 @@
self.num_probes = num_probes
def get_probes(self, key):
- h = int(sha512(dumps(key)).hexdigest(), 16)
- num_words = len(self.arr)
- for i in range(self.num_probes):
- h, array_index = divmod(h, num_words)
- h, bit_index = divmod(h, 32)
+ hasher = Random(key).randrange
+ for _ in range(self.num_probes):
+ array_index = hasher(len(self.arr))
+ bit_index = hasher(32)
yield array_index, 1 << bit_index
def add(self, key):
@@ -46,8 +41,10 @@
for state in states:
bf.add(state)
- assert all(state in bf for state in states) # Verify True Positives
+ m = sum(state in bf for state in states)
+ print('%d true positives out of %d trials' % (m, len(states)))
- n = 100000
- m = sum(''.join(sample(ascii_letters, 5)) in bf for i in range(n))
- print('%d :: %d' % (n-m, m)) # Ratio of True Negatives to False Positives
+ trials = 100000
+ m = sum(''.join(sample(ascii_letters, 5)) in bf for i in range(trials))
+ print('%d true negatives and %d false negatives out of %d trials'
+ % (trials-m, m, trials))
