From 5e3ba4fb04e1509b0e43d75c5551f1f0c2fd5e2d Mon Sep 17 00:00:00 2001 From: Bruno BELANYI Date: Sun, 14 Jul 2024 17:56:33 +0100 Subject: [PATCH] posts: bloom-filter: add lookup --- content/posts/2024-07-14-bloom-filter/index.md | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/content/posts/2024-07-14-bloom-filter/index.md b/content/posts/2024-07-14-bloom-filter/index.md index 1d593a7..93107d4 100644 --- a/content/posts/2024-07-14-bloom-filter/index.md +++ b/content/posts/2024-07-14-bloom-filter/index.md @@ -77,3 +77,21 @@ def insert(self, val: T) -> None: # Set the corresponding bit self._bit |= 1 << n ``` + +### Querying a key + +Because the _Bloom Filter_ does not actually store its elements, but some +derived data from hashing them, it can only definitely say if an element _does +not_ belong to it. Otherwise, it _may_ be part of the set, and should be checked +against the actual underlying store. + +```python +def may_contain(self, val: T) -> bool: + for f in self._hash_functions: + n = f(val) % BIT_COUNT + # If one of the bits is unset, the value is definitely not present + if not (self._bit & (1 << n)): + return False + # All bits were matched, `val` is likely to be part of the set + return True +```