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//! A concurrent implementation of Bloom filters. //! //! Bloom filters is a simple data structure, which is used in many different situations. It can //! neatly solve certain problems heaurustically without need for extreme memory usage. //! //! This implementation is fairly standard, except that it uses atomic integers to work //! concurrently. #![feature(integer_atomics)] use std::sync::atomic::{self, AtomicU64}; /// The atomic ordering used throughout the crate. const ORDERING: atomic::Ordering = atomic::Ordering::Relaxed; /// Hash an integer. /// /// This is a pseudorandom permutation of `u64` with high statistical quality. It can thus be used /// as a hash function. fn hash(mut x: u64) -> u64 { // The following is copied from SeaHash. x = x.wrapping_mul(0x6eed0e9da4d94a4f); let a = x >> 32; let b = x >> 60; x ^= a >> b; x = x.wrapping_mul(0x6eed0e9da4d94a4f); // We XOR with some constant to make it zero-sensitive. x ^ 0x11c92f7574d3e84f } /// A concurrent Bloom filter. /// /// Bloom filters are a probabilistic data structure, which allows you to insert elements, and /// later test if they were inserted. The filter will either know it doesn't contain the element, /// or that it might. It will never be "sure", hence the name "filter". /// /// It works by having an array of bits. Every element is hashed into a sequence of these bits. The /// bits of the inserted elements are set to 1. When testing for membership, we simply AND the /// bits. pub struct Filter { /// The bit array. /// /// We use `u64` to improve performance of `Filter::clear()`. bits: Vec<AtomicU64>, /// The number of hash functions. hashers: usize, } impl Filter { /// Get the chunk of a particular hash. #[inline] fn get(&self, hash: u64) -> &AtomicU64 { &self.bits[(hash as usize / 64) % self.bits.len()] } /// Create a new Bloom filter with the optimal number of hash functions. /// /// This creates a Bloom filter with `bytes` bytes of internal data, and optimal number (for /// `expected_elements` number of elements) of hash functions. pub fn new(bytes: usize, expected_elements: usize) -> Filter { // The number of hashers are calculated by multiplying the bits per element by ln(2), which // we approximate through multiplying by an integer, then shifting. Filter::with_size_and_hashers(bytes, (bytes / expected_elements * 45426) >> 16) } /// Create a new Bloom filter with some number of bytes and hashers. /// /// This creates a Bloom filter with at least `bytes` bytes of internal data and `hashers` /// number of hash functions. pub fn with_size_and_hashers(bytes: usize, hashers: usize) -> Filter { // Convert `bytes` to number of `u64`s, and ceil to avoid case where the output is 0. let len = (bytes + 7) / 8; // Initialize a vector with zeros. let mut vec = Vec::with_capacity(len); for _ in 0..len { vec.push(AtomicU64::new(0)); } Filter { bits: vec, hashers: hashers, } } /// Clear the Bloom filter. /// /// This removes every element from the Bloom filter. /// /// Note that it will not do so atomically, and it can remove elements inserted simulatenously /// to this function being called. pub fn clear(&self) { for i in &self.bits { // Clear the bits of this chunk. i.store(0, ORDERING); } } /// Insert an element into the Bloom filter. pub fn insert(&self, x: u64) { // Start at `x`. let mut h = x; // Run over the hashers. for _ in 0..self.hashers { // We use the hash function to generate a pseudorandom sequence, defining the different // hashes. h = hash(h); // Create a mask and OR the chunk chosen by `hash`. self.get(h).fetch_or(1 << (h % 8), ORDERING); } } /// Check if the Bloom filter potentially contains an element. /// /// This returns `true` if we're not sure if the filter contains `x` or not, and `false` if we /// know that the filter does not contain `x`. pub fn maybe_contains(&self, x: u64) -> bool { // Start at `x`. let mut h = x; // Go over the hashers. for _ in 0..self.hashers { // Again, the hashes are defined by a cuckoo sequence of repeatedly hashing. h = hash(h); // Short-circuit if the bit is not set. if self.get(h).load(ORDERING) & 1 << (h % 8) == 0 { // Since the bit of this hash value was not set, it is impossible that the filter // contains `x`, so we return `false`. return false; } } // Every bit was set, so the element might be in the filter. true } } #[cfg(test)] mod tests { use super::*; use std::sync::Arc; use std::thread; #[test] fn insert() { let filter = Filter::new(400, 4); filter.insert(3); filter.insert(5); filter.insert(7); filter.insert(13); assert!(!filter.maybe_contains(0)); assert!(!filter.maybe_contains(1)); assert!(!filter.maybe_contains(2)); assert!(filter.maybe_contains(3)); assert!(filter.maybe_contains(5)); assert!(filter.maybe_contains(7)); assert!(filter.maybe_contains(13)); for i in 14..60 { assert!(!filter.maybe_contains(!i)); } } #[test] fn spam() { let filter = Arc::new(Filter::new(2000, 100)); let mut joins = Vec::new(); for _ in 0..16 { let filter = filter.clone(); joins.push(thread::spawn(move || for i in 0..100 { filter.insert(i) })); } for i in joins { i.join().unwrap(); } for i in 0..100 { assert!(filter.maybe_contains(i)); } for i in 100..200 { assert!(!filter.maybe_contains(i)); } } }