番石榴 - 布隆过滤器：是否有可能为 ** 以前** 误报的值获得真负？

Question

If I understand correctly, once an item is put inside a guava bloom filter, mightContain will always return true. If the filter returns false on mightContain , then the value has never been put inside the filter. What I'm wondering is for the values that might be a false positive at a given moment, as more values are put in, the once false-positives might become true-negatives later on (if they are not put in, of course).

Something like this:

GuavaBloomFilter<Integer> bf = new GuavaBloomFilter<>(blah, blah);
# if I start checking, none of the values should return tru at the monent
System.out.println(bd.mightContain(5)); // false
System.out.println(bd.mightContain(10)); // false
System.out.println(bd.mightContain(15)); // false
# fine
# let's put in a value now
bf.put(10);
System.out.println(bd.mightContain(5));
System.out.println(bd.mightContain(10)); // true, every time from now on
System.out.println(bd.mightContain(15));

On the last 3 checks, when checking for 10, it will always return true. For 5 and 15, it might return true. Suppose that for 5 we get false (never put inside), for 15 we get a false positive.

So, we continue:

bf.put(5);
System.out.println(bd.mightContain(5)); // true, every single time from now on
System.out.println(bd.mightContain(10)); // true, every time from now on
System.out.println(bd.mightContain(15));

So.... now, when checking for 5, we will always get true. Is it possible that because of the state change inside the bloom filter, the result for checking 15 which was previously a false-positive, might return a true-negative value?

Answer 1

For a true Bloom filter, the bits only ever go from 0 to 1, never back - so the result of a mightContain call can only ever go from false to true , never back, because mightContain returns true if a certain subset of all bits are 1, and once they're 1 they'll stay 1.

Guava's implementation is indeed a true Bloom filter, since the BloomFilter.put method ( source ) delegates to Strategy.put ( source ), an interface implemented in BloomFilterStrategies ( source ). The Bloom filter's bits are stored in a LockFreeBitArray named bits , and the strategy only calls its bitSize , set and get methods. Of those, only set changes bits ( source ), and it only uses the bitwise 'or' operator | to change them. This can never change a 1 back to a 0.

So, it is indeed impossible for a value which was previously a false-positive to later become a true-negative.