161 lines
5.0 KiB
Rust
161 lines
5.0 KiB
Rust
//! Simple Bloom Filter
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use bv::BitVec;
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use fnv::FnvHasher;
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use rand::{self, Rng};
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use serde::{Deserialize, Serialize};
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use std::cmp;
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use std::hash::Hasher;
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use std::marker::PhantomData;
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/// Generate a stable hash of `self` for each `hash_index`
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/// Best effort can be made for uniqueness of each hash.
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pub trait BloomHashIndex {
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fn hash_at_index(&self, hash_index: u64) -> u64;
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}
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#[derive(Serialize, Deserialize, Default, Clone, Debug, PartialEq)]
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pub struct Bloom<T: BloomHashIndex> {
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pub keys: Vec<u64>,
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pub bits: BitVec<u64>,
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num_bits_set: u64,
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_phantom: PhantomData<T>,
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}
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impl<T: BloomHashIndex> Bloom<T> {
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pub fn new(num_bits: usize, keys: Vec<u64>) -> Self {
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let bits = BitVec::new_fill(false, num_bits as u64);
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Bloom {
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keys,
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bits,
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num_bits_set: 0,
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_phantom: PhantomData::default(),
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}
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}
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/// create filter optimal for num size given the `FALSE_RATE`
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/// the keys are randomized for picking data out of a collision resistant hash of size
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/// `keysize` bytes
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/// https://hur.st/bloomfilter/
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pub fn random(num_items: usize, false_rate: f64, max_bits: usize) -> Self {
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let m = Self::num_bits(num_items as f64, false_rate);
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let num_bits = cmp::max(1, cmp::min(m as usize, max_bits));
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let num_keys = Self::num_keys(num_bits as f64, num_items as f64) as usize;
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let keys: Vec<u64> = (0..num_keys).map(|_| rand::thread_rng().gen()).collect();
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Self::new(num_bits, keys)
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}
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pub fn num_bits(num_items: f64, false_rate: f64) -> f64 {
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let n = num_items;
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let p = false_rate;
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((n * p.ln()) / (1f64 / 2f64.powf(2f64.ln())).ln()).ceil()
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}
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pub fn num_keys(num_bits: f64, num_items: f64) -> f64 {
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let n = num_items;
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let m = num_bits;
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1f64.max(((m / n) * 2f64.ln()).round())
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}
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fn pos(&self, key: &T, k: u64) -> u64 {
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key.hash_at_index(k) % self.bits.len()
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}
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pub fn clear(&mut self) {
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self.bits = BitVec::new_fill(false, self.bits.len());
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self.num_bits_set = 0;
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}
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pub fn add(&mut self, key: &T) {
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for k in &self.keys {
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let pos = self.pos(key, *k);
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if !self.bits.get(pos) {
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self.num_bits_set += 1;
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self.bits.set(pos, true);
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}
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}
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}
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pub fn contains(&self, key: &T) -> bool {
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for k in &self.keys {
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let pos = self.pos(key, *k);
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if !self.bits.get(pos) {
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return false;
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}
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}
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true
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}
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}
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fn slice_hash(slice: &[u8], hash_index: u64) -> u64 {
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let mut hasher = FnvHasher::with_key(hash_index);
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hasher.write(slice);
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hasher.finish()
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}
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impl<T: AsRef<[u8]>> BloomHashIndex for T {
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fn hash_at_index(&self, hash_index: u64) -> u64 {
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slice_hash(self.as_ref(), hash_index)
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}
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}
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#[cfg(test)]
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mod test {
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use super::*;
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use solana_sdk::hash::{hash, Hash};
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#[test]
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fn test_bloom_filter() {
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//empty
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let bloom: Bloom<Hash> = Bloom::random(0, 0.1, 100);
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assert_eq!(bloom.keys.len(), 0);
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assert_eq!(bloom.bits.len(), 1);
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//normal
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let bloom: Bloom<Hash> = Bloom::random(10, 0.1, 100);
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assert_eq!(bloom.keys.len(), 3);
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assert_eq!(bloom.bits.len(), 48);
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//saturated
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let bloom: Bloom<Hash> = Bloom::random(100, 0.1, 100);
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assert_eq!(bloom.keys.len(), 1);
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assert_eq!(bloom.bits.len(), 100);
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}
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#[test]
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fn test_add_contains() {
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let mut bloom: Bloom<Hash> = Bloom::random(100, 0.1, 100);
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//known keys to avoid false positives in the test
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bloom.keys = vec![0, 1, 2, 3];
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let key = hash(b"hello");
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assert!(!bloom.contains(&key));
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bloom.add(&key);
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assert!(bloom.contains(&key));
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let key = hash(b"world");
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assert!(!bloom.contains(&key));
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bloom.add(&key);
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assert!(bloom.contains(&key));
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}
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#[test]
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fn test_random() {
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let mut b1: Bloom<Hash> = Bloom::random(10, 0.1, 100);
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let mut b2: Bloom<Hash> = Bloom::random(10, 0.1, 100);
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b1.keys.sort();
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b2.keys.sort();
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assert_ne!(b1.keys, b2.keys);
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}
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// Bloom filter math in python
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// n number of items
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// p false rate
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// m number of bits
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// k number of keys
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//
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// n = ceil(m / (-k / log(1 - exp(log(p) / k))))
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// p = pow(1 - exp(-k / (m / n)), k)
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// m = ceil((n * log(p)) / log(1 / pow(2, log(2))));
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// k = round((m / n) * log(2));
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#[test]
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fn test_filter_math() {
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assert_eq!(Bloom::<Hash>::num_bits(100f64, 0.1f64) as u64, 480u64);
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assert_eq!(Bloom::<Hash>::num_bits(100f64, 0.01f64) as u64, 959u64);
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assert_eq!(Bloom::<Hash>::num_keys(1000f64, 50f64) as u64, 14u64);
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assert_eq!(Bloom::<Hash>::num_keys(2000f64, 50f64) as u64, 28u64);
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assert_eq!(Bloom::<Hash>::num_keys(2000f64, 25f64) as u64, 55u64);
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//ensure min keys is 1
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assert_eq!(Bloom::<Hash>::num_keys(20f64, 1000f64) as u64, 1u64);
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}
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}
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