[bkanber]

Definitely not clear from the article alone. You can use more than one hash algorithm to reduce the chance of collisions. You set the values in the bit array for both hashes, and then you check both again.

Practical, contrived example: the string "w" gives us "2" from fnv and "11" from murmur. When you add that to the filter with both hashes, bits 2 and 11 are set.

The string "h" gives us 2 from fnv and 10 from murmur. If you were using only the fnv hash, you'd get a "maybe exists" result. But since the murmur hash is different for "h" than it is for "w", you get a "definitely not" result.

Of course, you still have collisions, you just cut them down. Both fnv and murmur return the same hashes for "w" and "woot", so adding "w" then checking "woot" still gives you a "maybe" result, but at least checking "h" does not.