def __init__(self, capacity, error_rate=0.001):
"""Implements a space-efficient probabilistic data structure
capacity
this BloomFilter must be able to store at least *capacity* elements
while maintaining no more than *error_rate* chance of false
positives
error_rate
the error_rate of the filter returning false positives. This
determines the filters capacity. Inserting more than capacity
elements greatly increases the chance of false positives.
>>> b = BloomFilter(capacity=100000, error_rate=0.001)
>>> b.add("test")
False
>>> "test" in b
True
"""
if not (0 < error_rate < 1):
raise ValueError("Error_Rate must be between 0 and 1.")
if not capacity > 0:
raise ValueError("Capacity must be > 0")
# given M = num_bits, k = num_slices, P = error_rate, n = capacity
# k = log2(1/P)
# solving for m = bits_per_slice
# n ~= M * ((ln(2) ** 2) / abs(ln(P)))
# n ~= (k * m) * ((ln(2) ** 2) / abs(ln(P)))
# m ~= n * abs(ln(P)) / (k * (ln(2) ** 2))
num_slices = int(math.ceil(math.log(1.0 / error_rate, 2)))
bits_per_slice = int(math.ceil((capacity * abs(math.log(error_rate))) / (num_slices * (math.log(2) ** 2))))
self._setup(error_rate, num_slices, bits_per_slice, capacity, 0)
self.bitarray = bitarray.bitarray(self.num_bits, endian="little")
self.bitarray.setall(False)
def fromfile(cls, f, n=-1):
"""Read a bloom filter from file-object `f' serialized with
``BloomFilter.tofile''. If `n' > 0 read only so many bytes."""
headerlen = calcsize(cls.FILE_FMT)
if 0 < n < headerlen:
raise ValueError("n too small!")
filter = cls(1) # Bogus instantiation, we will `_setup'.
filter._setup(*unpack(cls.FILE_FMT, f.read(headerlen)))
filter.bitarray = bitarray.bitarray(endian="little")
if n > 0:
(
filter.bitarray.frombytes(f.read(n - headerlen))
if is_string_io(f)
else filter.bitarray.fromfile(f, n - headerlen)
)
else:
(filter.bitarray.frombytes(f.read()) if is_string_io(f) else filter.bitarray.fromfile(f))
if filter.num_bits != filter.bitarray.length() and (
filter.num_bits + (8 - filter.num_bits % 8) != filter.bitarray.length()
):
raise ValueError("Bit length mismatch!")
return filter
