Today's fun and educational Python recipe

[Terry Reedy]

On 5/4/2011 2:17 PM, Raymond Hettinger wrote:
> Here's a 22-line beauty for a classic and amazing algorithm:
> http://bit.ly/bloom_filter
>
> The wiki article on the algorithm is brief and well-written:
> http://en.wikipedia.org/wiki/Bloom_filter

As I understand the article, the array of num_bits should have 
num_probes (more or less independent) bits set for each key. But as I 
understand the code

         for i in range(self.num_probes):
             h, array_index = divmod(h, num_words)
             h, bit_index = divmod(h, 32)
             yield array_index, 1 << bit_index

the same bit is being set or tested num_probes times. The last three 
lines have no dependence on i that I can see, so they appear to do the 
same thing each time. This seems like a bug.

The article says "For a good hash function with a wide output, there 
should be little if any correlation between different bit-fields of such 
a hash, so this type of hash can be used to generate multiple 
"different" hash functions by slicing its output into multiple bit 
fields. Alternatively, one can pass k different initial values (such as 
0, 1, ..., k − 1) to a hash function that takes an initial value; or add 
(or append) these values to the key." I do not see the code doing either 
of these.

-- 
Terry Jan Reedy