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Cuckoo Filter

· 3 min read
Steven Normore
Steven Normore
Author

Today I learned about cuckoo filters, a data structure used in computer science for probabilistic set membership queries. Cuckoo filters are similar to Bloom filters but offer more flexibility by allowing for deletion of items from the filter while maintaining good space efficiency.

The key idea behind cuckoo filters is the concept of "cuckoo hashing." This hashing technique involves using two hash functions and two hash tables. When inserting an item into the filter, it's hashed by both functions, and the item is placed in one of the tables based on the hashes. If a collision occurs during insertion, the existing item is "kicked out" to its alternative position in the other table, similar to how a cuckoo bird might push another bird's eggs out of its nest.

One of the advantages of cuckoo filters is their ability to support item deletions efficiently, unlike Bloom filters that only support insertions and queries. Deletions in cuckoo filters involve finding the item based on its hash values and marking it as deleted without affecting other items' positions.

Cuckoo filters are widely used in applications where approximate set membership queries are needed with low false-positive rates. They offer a good compromise between space efficiency, query performance, and support for deletions, making them a valuable tool in data structures and algorithms.

Examples​

Python​

import cuckoofilter

# Create a cuckoo filter with a capacity of 100 items and a fingerprint size of 4 bytes
cf = cuckoofilter.CuckooFilter(capacity=100, fingerprint_size=4)

# Insert items into the filter
cf.insert("item1")
cf.insert("item2")
cf.insert("item3")

# Check if an item is in the filter
print("Is item2 in the filter?", cf.contains("item2"))  # Output: True

# Delete an item from the filter
cf.delete("item2")

# Check if the deleted item is still in the filter
print("Is item2 in the filter after deletion?", cf.contains("item2"))  # Output: False

In this code snippet, we create a cuckoo filter using the cuckoofilter library in Python, insert items, check for membership, and demonstrate item deletion capability.

Rust​

use cuckoofilter::CuckooFilter;

fn main() {
    // Create a cuckoo filter with a capacity of 100 items and a fingerprint size of 4 bytes
    let mut cf = CuckooFilter::new(100, 4);

    // Insert items into the filter
    cf.insert("item1".as_bytes());
    cf.insert("item2".as_bytes());
    cf.insert("item3".as_bytes());

    // Check if an item is in the filter
    println!("Is item2 in the filter? {}", cf.contains("item2".as_bytes())); // Output: true

    // Delete an item from the filter
    cf.delete("item2".as_bytes());

    // Check if the deleted item is still in the filter
    println!("Is item2 in the filter after deletion? {}", cf.contains("item2".as_bytes())); // Output: false
}

In this Rust code, we use the cuckoofilter crate to create a cuckoo filter, insert items into it, check for membership, and demonstrate item deletion. The CuckooFilter::new function creates a new cuckoo filter with a specified capacity and fingerprint size. The insert method adds items to the filter, while contains checks if an item is present. The delete method removes an item from the filter.