threadsafe_lru/lib.rs
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#![deny(unsafe_code)]
/// # threadsafe-lru
///
/// This is a thread-safe implementation of an LRU (Least Recently Used) cache in Rust.
/// The `LruCache` struct uses sharding to improve concurrency by splitting the cache into
/// multiple smaller segments, each protected by a mutex.
///
/// ## Example Usage
///
/// ```rust
/// use threadsafe_lru::LruCache;
///
/// fn main() {
/// // Create a new LRU cache with 4 shards and capacity of 2 per shard
/// let cache = LruCache::new(4, 2);
///
/// // Insert items into the cache
/// let five = 5;
/// let six = 6;
/// assert_eq!(cache.insert(five, 10), None);
/// assert_eq!(cache.insert(six, 20), None);
///
/// // Retrieve an item from the cache
/// assert_eq!(cache.get(&five), Some(10));
///
/// // Promote an item to make it more recently used
/// cache.promote(&five);
///
/// // Remove an item from the cache
/// assert_eq!(cache.remove(&five), Some(10));
/// }
/// ```
///
/// In this example, a new `LruCache` is created with 4 shards and a capacity of 2 entries per shard.
/// Items are inserted using the `insert` method.
/// The `get` method retrieves an item by key, promoting it to the most recently used position.
/// Finally, the `remove` method deletes an item from the cache.
///
/// This implementation ensures that operations on different keys can be performed concurrently
/// without causing race conditions due to shared state.
///
use std::{
borrow::Borrow,
hash::{DefaultHasher, Hash, Hasher},
sync::{
atomic::{AtomicUsize, Ordering},
Mutex,
},
};
use hashbrown::HashMap;
use indexlist::{Index, IndexList};
// A thread-safe LRU
pub struct LruCache<K, V> {
shards: Vec<Mutex<Shard<K, V>>>,
count: AtomicUsize,
shards_count: usize,
cap_per_shard: usize,
}
struct Shard<K, V> {
entries: HashMap<K, (Index<K>, V)>,
order: IndexList<K>,
count: usize,
}
impl<K, V> LruCache<K, V>
where
K: Clone + Eq + Hash,
{
/// Creates a new instance of `LruCache`.
///
/// The cache is divided into multiple shards to improve concurrency by distributing the entries
/// across different locks.
///
/// # Arguments
///
/// * `shards_count` - The number of shards in the cache. Each shard acts as an independent LRU
/// with its own capacity and order list.
/// * `cap_per_shard` - The capacity for each shard, representing the maximum number of items it
/// can hold before evicting the least recently used item(s).
///
/// # Returns
///
/// A new instance of `LruCache`.
///
/// # Examples
///
/// ```rust
/// use threadsafe_lru::LruCache;
///
/// let cache: LruCache<i32, i32> = LruCache::new(4, 2); // Creates a cache with 4 shards and capacity of 2 entries per shard.
/// ```
pub fn new(shards_count: usize, cap_per_shard: usize) -> LruCache<K, V> {
let mut shards = Vec::default();
for _ in 0..shards_count {
shards.push(Mutex::new(Shard {
entries: HashMap::with_capacity(cap_per_shard),
order: IndexList::with_capacity(cap_per_shard),
count: 0,
}));
}
LruCache {
shards,
count: AtomicUsize::default(),
shards_count,
cap_per_shard,
}
}
/// Inserts a new key-value pair into the cache.
///
/// If the key already exists in the cache, its value is updated and it is promoted to the most
/// recently used position.
/// If inserting the new item causes the cache to exceed its capacity, the least recently used
/// item will be evicted from its shard.
///
/// # Arguments
///
/// * `k` - The key to insert or update.
/// * `v` - The value associated with the key.
///
/// # Returns
///
/// If the key already existed in the cache and was updated, the previous value is returned.
/// Otherwise, `None` is returned.
///
/// # Examples
///
/// ```rust
/// use threadsafe_lru::LruCache;
///
/// let cache = LruCache::new(4, 2);
///
/// let five = 5;
/// let six = 6;
///
/// assert_eq!(cache.insert(five, 10), None); // Inserts a new key-value pair
/// assert_eq!(cache.insert(six, 20), None); // Inserts another new key-value pair
/// assert_eq!(cache.insert(five, 30), Some(10)); // Updates an existing key with a new value and returns the old value
/// ```
pub fn insert(&self, k: K, v: V) -> Option<V> {
let mut shard = self.shards[self.shard(&k)].lock().unwrap();
let index = shard.entries.get(&k).map(|v| v.0);
if shard.count == self.cap_per_shard && index.is_none() {
if let Some(index) = shard.order.head_index() {
shard.entries.remove(&k);
shard.order.remove(index);
self.count.fetch_sub(1, Ordering::Relaxed);
shard.count -= 1;
}
}
match index {
Some(index) => {
shard.order.remove(index);
let index = shard.order.push_back(k.clone());
shard.entries.insert(k, (index, v)).map(|v| v.1)
}
None => {
let index = shard.order.push_back(k.clone());
shard.entries.insert(k, (index, v));
self.count.fetch_add(1, Ordering::Relaxed);
shard.count += 1;
None
}
}
}
/// Retrieves a value from the cache by key.
///
/// When a key is accessed using this method, it is promoted to the most recently used position
/// in its shard. If the key does not exist in the cache, `None` is returned.
///
/// # Arguments
///
/// * `k` - The key of the item to retrieve.
///
/// # Returns
///
/// The value associated with the key if it exists; otherwise, `None`.
///
/// # Examples
///
/// ```rust
/// use threadsafe_lru::LruCache;
///
/// let cache = LruCache::new(4, 2);
///
/// let five = 5;
/// let six = 6;
///
/// assert_eq!(cache.insert(five, 10), None);
/// assert_eq!(cache.get(&five), Some(10));
/// ```
///
/// This method ensures that frequently accessed items remain more accessible while older or less
/// frequently accessed items are evicted when necessary.
pub fn get<Q>(&self, k: &Q) -> Option<V>
where
K: Borrow<Q>,
Q: Hash + Eq + ?Sized + ToOwned<Owned = K>,
V: Clone,
{
let mut shard = self.shards[self.shard(k)].lock().unwrap();
let index = shard.entries.get(k).map(|e| e.0);
match index {
Some(index) => {
shard.order.remove(index);
shard.order.push_back(k.to_owned());
shard.entries.get(k).map(|e| e.1.clone())
}
None => None,
}
}
/// Retrieves and mutates a value from the cache by key.
///
/// When a key is accessed using this method, it is promoted to the most recently used position
/// in its shard. If the key does not exist in the cache, `None` is returned.
///
/// This function provides mutable access to the value associated with a given key, allowing you
/// to modify its contents directly without needing to retrieve and re-insert it into the cache.
///
/// # Arguments
///
/// * `k` - The key of the item to retrieve and mutate.
/// * `func` - A closure that takes a mutable reference to the value (if present) and allows
/// for in-place modifications.
///
/// # Examples
///
/// ```rust
/// use threadsafe_lru::LruCache;
///
/// let cache = LruCache::new(4, 2);
///
/// let five = 5;
/// let six = 6;
///
/// assert_eq!(cache.insert(five, 10), None);
/// cache.get_mut(&five, |v| {
/// if let Some(v) = v {
/// *v += 1
/// }
/// });
/// ```
///
/// This method ensures that frequently accessed items remain more accessible while older or less
/// frequently accessed items are evicted when necessary.
pub fn get_mut<Q, F>(&self, k: &Q, mut func: F)
where
K: Borrow<Q>,
Q: Hash + Eq + ?Sized + ToOwned<Owned = K>,
F: FnMut(Option<&mut V>),
{
let mut shard = self.shards[self.shard(k)].lock().unwrap();
let index = shard.entries.get(k).map(|e| e.0);
if let Some(index) = index {
shard.order.remove(index);
shard.order.push_back(k.to_owned());
func(shard.entries.get_mut(k).map(|e| &mut e.1));
}
}
/// Removes a key-value pair from the cache by key.
///
/// When an item is removed from the cache using this method, its corresponding entry is deleted
/// along with any references to it in the order list. If the key does not exist in the cache,
/// `None` is returned.
///
/// This operation ensures that the cache maintains its integrity and correctly tracks the number of items stored.
///
/// # Arguments
///
/// * `k` - The key of the item to remove.
///
/// # Returns
///
/// The value associated with the key if it existed; otherwise, `None`.
///
/// # Examples
///
/// ```rust
/// use threadsafe_lru::LruCache;
///
/// let cache = LruCache::new(4, 2);
///
/// let five = 5;
/// let six = 6;
///
/// assert_eq!(cache.insert(five, 10), None); // Inserts a new key-value pair
/// assert_eq!(cache.insert(six, 20), None); // Inserts another new key-value pair
///
/// assert_eq!(cache.remove(&five), Some(10)); // Removes the item with key five and returns its value
/// assert_eq!(cache.get(&five), None); // Key five no longer exists in the cache
/// ```
///
pub fn remove<Q>(&self, k: &Q) -> Option<V>
where
K: Borrow<Q>,
Q: Hash + Eq + ?Sized,
{
let mut shard = self.shards[self.shard(k)].lock().unwrap();
let entry = shard.entries.remove(k);
match entry {
Some((index, value)) => {
shard.order.remove(index);
self.count.fetch_sub(1, Ordering::Relaxed);
shard.count -= 1;
Some(value)
}
None => None,
}
}
/// Promotes a key-value pair in the cache to the most recently used position.
///
/// When an item is accessed using this method, it is promoted to the most recently used position
/// in its shard. If the key does not exist in the cache, no action is taken.
///
/// # Arguments
///
/// * `k` - The key of the item to promote.
///
///
/// # Examples
///
/// ```rust
/// use threadsafe_lru::LruCache;
///
/// let cache = LruCache::new(4, 2);
///
/// let five = 5;
/// let six = 6;
/// let seven = 7;
///
/// assert_eq!(cache.insert(five, 10), None); // Inserts a new key-value pair
/// assert_eq!(cache.insert(six, 20), None); // Inserts another new key-value pair
///
/// cache.promote(&five);
///
/// assert_eq!(cache.insert(seven, 30), None); // Inserts another new key-value pair
/// assert_eq!(cache.get(&five), Some(10)); // Retrieving the promoted item
/// ```
///
pub fn promote<Q>(&self, k: &Q)
where
K: Borrow<Q>,
Q: Hash + Eq + ?Sized + ToOwned<Owned = K>,
{
let mut shard = self.shards[self.shard(k)].lock().unwrap();
let index = shard.entries.get(k).map(|v| v.0);
if let Some(index) = index {
shard.order.remove(index);
shard.order.push_back(k.to_owned());
}
}
/// Returns the total number of key-value pairs currently stored in the cache.
///
/// This method provides a quick way to check how many items are present in the cache without
/// iterating over its contents.
///
/// # Examples
///
/// ```rust
/// use threadsafe_lru::LruCache;
///
/// let cache = LruCache::new(4, 2);
///
/// let five = 5;
/// let six = 6;
///
/// assert_eq!(cache.insert(five, 10), None); // Inserts a new key-value pair
/// assert_eq!(cache.len(), 1); // Cache now has one item
///
/// cache.insert(six, 20);
/// assert_eq!(cache.len(), 2); // Cache now has two items
///
/// cache.remove(&five);
/// assert_eq!(cache.len(), 1); // Cache now has only one item
///
/// let new_cache: LruCache<i32, i32> = LruCache::new(4, 2);
/// assert_eq!(new_cache.len(), 0); // New cache is empty
/// ```
///
pub fn len(&self) -> usize {
self.count.load(Ordering::Relaxed)
}
/// Checks if the cache is empty.
///
/// This method provides a quick way to determine whether the cache contains any key-value pairs
/// without iterating over its contents.
///
/// # Examples
///
/// ```rust
/// use threadsafe_lru::LruCache;
///
/// let cache = LruCache::new(4, 2);
///
/// assert!(cache.is_empty()); // Cache is empty upon creation
///
/// let five = 5;
/// let six = 6;
///
/// cache.insert(five, 10); // Inserting a key-value pair
/// assert!(!cache.is_empty()); // Cache is no longer empty
///
/// cache.remove(&five); // Removing the key-value pair
/// assert!(cache.is_empty()); // Cache is empty again after removal
/// ```
///
pub fn is_empty(&self) -> bool {
self.len() == 0
}
fn shard<Q>(&self, k: &Q) -> usize
where
K: Borrow<Q>,
Q: Hash + Eq + ?Sized,
{
let mut hasher = DefaultHasher::new();
k.hash(&mut hasher);
hasher.finish() as usize % self.shards_count
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_new() {
let shards_count = 4;
let cap_per_shard = 10;
let cache: LruCache<u8, u8> = LruCache::new(shards_count, cap_per_shard);
assert_eq!(cache.shards.len(), shards_count);
for shard in &cache.shards {
let lock = shard.lock().unwrap();
assert!(lock.entries.capacity() >= cap_per_shard);
assert_eq!(lock.count, 0);
}
assert_eq!(cache.shards_count, shards_count);
assert_eq!(cache.cap_per_shard, cap_per_shard);
}
#[test]
fn test_insert() {
let shards_count = 4;
let cap_per_shard = 2;
let cache = LruCache::new(shards_count, cap_per_shard);
let five = 5;
let six = 6;
let nine = 9;
assert_eq!(cache.shard(&five), cache.shard(&six));
assert_eq!(cache.shard(&five), cache.shard(&nine));
assert_eq!(cache.insert(five, 10), None);
assert_eq!(cache.insert(five, 10), Some(10));
assert_eq!(cache.count.load(Ordering::Relaxed), 1);
assert_eq!(cache.insert(six, 20), None);
assert_eq!(cache.count.load(Ordering::Relaxed), 2);
assert_eq!(cache.insert(nine, 30), None);
assert_eq!(cache.count.load(Ordering::Relaxed), 2);
assert_eq!(cache.insert(six, 20), Some(20));
assert_eq!(cache.count.load(Ordering::Relaxed), 2);
}
#[test]
fn test_get() {
let shards_count = 4;
let cap_per_shard = 2;
let cache = LruCache::new(shards_count, cap_per_shard);
let five = 5;
let six = 6;
assert_eq!(cache.shard(&five), cache.shard(&six));
assert_eq!(cache.insert(five, 10), None);
assert_eq!(cache.insert(six, 20), None);
assert_eq!(cache.get(&five), Some(10));
assert_eq!(cache.get(&six), Some(20));
let shard = cache.shards[cache.shard(&five)].lock().unwrap();
assert_eq!(shard.order.head(), Some(&five));
assert_eq!(shard.order.tail(), Some(&six));
assert_eq!(cache.get(&3), None);
assert_eq!(cache.count.load(Ordering::Relaxed), 2);
}
#[test]
fn test_get_mut() {
let shards_count = 4;
let cap_per_shard = 2;
let cache = LruCache::new(shards_count, cap_per_shard);
let five = 5;
let six = 6;
assert_eq!(cache.shard(&five), cache.shard(&six));
assert_eq!(cache.insert(five, 10), None);
assert_eq!(cache.insert(six, 20), None);
cache.get_mut(&five, |v| {
if let Some(v) = v {
*v = 30
}
});
assert_eq!(cache.get(&five), Some(30));
assert_eq!(cache.count.load(Ordering::Relaxed), 2);
let shard = cache.shards[cache.shard(&five)].lock().unwrap();
assert_eq!(shard.order.tail(), Some(&five));
assert_eq!(shard.order.head(), Some(&six));
cache.get_mut(&3, |v| {
if let Some(v) = v {
*v = 10
}
});
assert_eq!(cache.count.load(Ordering::Relaxed), 2);
}
#[test]
fn test_remove() {
let shards_count = 4;
let cap_per_shard = 2;
let cache = LruCache::new(shards_count, cap_per_shard);
let five = 5;
let six = 6;
assert_eq!(cache.shard(&five), cache.shard(&six));
assert_eq!(cache.insert(five, 10), None);
assert_eq!(cache.insert(six, 20), None);
assert_eq!(cache.remove(&five), Some(10));
assert_eq!(cache.count.load(Ordering::Relaxed), 1);
let shard = cache.shards[cache.shard(&six)].lock().unwrap();
assert!(!shard.entries.contains_key(&five));
assert_eq!(shard.order.head(), Some(&six));
drop(shard);
assert_eq!(cache.remove(&5), None);
assert_eq!(cache.count.load(Ordering::Relaxed), 1);
let new_cache: LruCache<i32, i32> = LruCache::new(shards_count, cap_per_shard);
assert_eq!(new_cache.remove(&five), None);
assert_eq!(new_cache.count.load(Ordering::Relaxed), 0);
}
#[test]
fn test_promote() {
let shards_count = 4;
let cap_per_shard = 2;
let cache = LruCache::new(shards_count, cap_per_shard);
let five = 5;
let six = 6;
assert_eq!(cache.shard(&five), cache.shard(&six));
assert_eq!(cache.insert(five, 10), None);
assert_eq!(cache.insert(six, 20), None);
let shard = cache.shards[cache.shard(&five)].lock().unwrap();
assert_eq!(shard.order.head(), Some(&five));
assert_eq!(shard.order.tail(), Some(&six));
drop(shard);
cache.promote(&five);
let shard = cache.shards[cache.shard(&five)].lock().unwrap();
assert_eq!(shard.order.head(), Some(&six));
assert_eq!(shard.order.tail(), Some(&five));
drop(shard);
assert_eq!(cache.get(&five), Some(10));
let shard = cache.shards[cache.shard(&five)].lock().unwrap();
assert_eq!(shard.order.head(), Some(&six));
assert_eq!(shard.order.tail(), Some(&five));
drop(shard);
}
#[test]
fn test_is_empty() {
let shards_count = 4;
let cap_per_shard = 2;
let cache = LruCache::new(shards_count, cap_per_shard);
assert!(cache.is_empty());
assert_eq!(cache.len(), 0);
let five = 5;
assert_eq!(cache.insert(five, 10), None);
assert!(!cache.is_empty());
assert_eq!(cache.len(), 1);
cache.remove(&five);
assert!(cache.is_empty());
assert_eq!(cache.len(), 0);
}
#[test]
fn test_len() {
let shards_count = 4;
let cap_per_shard = 2;
let cache = LruCache::new(shards_count, cap_per_shard);
let five = 5;
let six = 6;
assert_eq!(cache.insert(five, 10), None);
assert_eq!(cache.len(), 1);
cache.insert(six, 20);
assert_eq!(cache.len(), 2);
cache.remove(&five);
assert_eq!(cache.len(), 1);
let new_cache: LruCache<i32, i32> = LruCache::new(shards_count, cap_per_shard);
assert_eq!(new_cache.len(), 0);
}
#[test]
fn test_concurrent_access() {
use std::sync::Arc;
use std::thread;
let shards_count = 4;
let cap_per_shard = 2;
let cache: Arc<LruCache<i32, i32>> = Arc::new(LruCache::new(shards_count, cap_per_shard));
const THREAD_COUNT: usize = 10;
const OPERATIONS_PER_THREAD: usize = 100;
let mut handles = vec![];
for _ in 0..THREAD_COUNT {
let cache = Arc::clone(&cache);
let handle = thread::spawn(move || {
for _ in 0..OPERATIONS_PER_THREAD {
let key = rand::random::<i32>();
let value = rand::random::<i32>();
let op_type = rand::random::<u8>() % 3;
match op_type {
0 => {
cache.insert(key, value);
}
1 => {
cache.get(&key);
}
2 => {
cache.remove(&key);
}
_ => unreachable!(),
}
}
});
handles.push(handle);
}
for handle in handles {
handle.join().unwrap();
}
}
}