389 lines
14 KiB
Rust
389 lines
14 KiB
Rust
//! Least recently used (LRU) cache
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//!
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//! Implements a cache with least recently used cache replacement policy.
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//! A HashMap is used for fast access by a given key and a doubly linked list
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//! is used to keep track of the cache access order.
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use std::collections::{HashMap, hash_map::Entry};
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use std::marker::PhantomData;
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/// Interface for getting values on cache misses.
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pub trait Cacher<K, V> {
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/// Fetch a value for key on cache miss.
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///
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/// Whenever a cache miss occurs, the fetch method provides a corresponding value.
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/// If no value can be obtained for the given key, None is returned, the cache is
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/// not updated in that case.
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fn fetch(&mut self, key: K) -> Result<Option<V>, anyhow::Error>;
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}
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/// Node of the doubly linked list storing key and value
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struct CacheNode<K, V> {
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// We need to additionally store the key to be able to remove it
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// from the HashMap when removing the tail.
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key: K,
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value: V,
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prev: *mut CacheNode<K, V>,
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next: *mut CacheNode<K, V>,
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// Dropcheck marker. See the phantom-data section in the rustonomicon.
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_marker: PhantomData<Box<CacheNode<K, V>>>,
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}
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impl<K, V> CacheNode<K, V> {
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fn new(key: K, value: V) -> Self {
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Self {
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key,
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value,
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prev: std::ptr::null_mut(),
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next: std::ptr::null_mut(),
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_marker: PhantomData,
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}
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}
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}
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/// LRU cache instance.
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///
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/// # Examples:
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/// ```
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/// # use self::proxmox_backup::tools::lru_cache::{Cacher, LruCache};
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/// # fn main() -> Result<(), anyhow::Error> {
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/// struct LruCacher {};
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///
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/// impl Cacher<u64, u64> for LruCacher {
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/// fn fetch(&mut self, key: u64) -> Result<Option<u64>, anyhow::Error> {
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/// Ok(Some(key))
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/// }
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/// }
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///
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/// let mut cache = LruCache::new(3);
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///
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/// assert_eq!(cache.get_mut(1), None);
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/// assert_eq!(cache.len(), 0);
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///
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/// cache.insert(1, 1);
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/// cache.insert(2, 2);
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/// cache.insert(3, 3);
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/// cache.insert(4, 4);
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/// assert_eq!(cache.len(), 3);
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///
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/// assert_eq!(cache.get_mut(1), None);
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/// assert_eq!(cache.get_mut(2), Some(&mut 2));
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/// assert_eq!(cache.get_mut(3), Some(&mut 3));
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/// assert_eq!(cache.get_mut(4), Some(&mut 4));
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///
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/// cache.remove(4);
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/// cache.remove(3);
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/// cache.remove(2);
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/// assert_eq!(cache.len(), 0);
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/// assert_eq!(cache.get_mut(2), None);
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/// // access will fill in missing cache entry by fetching from LruCacher
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/// assert_eq!(cache.access(2, &mut LruCacher {}).unwrap(), Some(&mut 2));
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///
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/// cache.insert(1, 1);
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/// assert_eq!(cache.get_mut(1), Some(&mut 1));
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///
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/// cache.clear();
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/// assert_eq!(cache.len(), 0);
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/// assert_eq!(cache.get_mut(1), None);
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/// # Ok(())
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/// # }
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/// ```
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pub struct LruCache<K, V> {
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/// Quick access to individual nodes via the node pointer.
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map: HashMap<K, *mut CacheNode<K, V>>,
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/// Actual nodes stored in a linked list.
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list: LinkedList<K, V>,
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/// Max nodes the cache can hold, temporarily exceeded by 1 due to
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/// implementation details.
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capacity: usize,
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// Dropcheck marker. See the phantom-data section in the rustonomicon.
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_marker: PhantomData<Box<CacheNode<K, V>>>,
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}
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// trivial: if our contents are Send, the whole cache is Send
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unsafe impl<K: Send, V: Send> Send for LruCache<K, V> {}
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impl<K: std::cmp::Eq + std::hash::Hash + Copy, V> LruCache<K, V> {
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/// Create LRU cache instance which holds up to `capacity` nodes at once.
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pub fn new(capacity: usize) -> Self {
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Self {
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map: HashMap::with_capacity(capacity),
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list: LinkedList::new(),
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capacity,
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_marker: PhantomData,
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}
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}
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/// Clear all the entries from the cache.
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pub fn clear(&mut self) {
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// This frees only the HashMap with the node pointers.
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self.map.clear();
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// This frees the actual nodes and resets the list head and tail.
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self.list.clear();
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}
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/// Insert or update an entry identified by `key` with the given `value`.
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/// This entry is placed as the most recently used node at the head.
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pub fn insert(&mut self, key: K, value: V) {
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match self.map.entry(key) {
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Entry::Occupied(mut o) => {
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// Node present, update value
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let node_ptr = *o.get_mut();
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self.list.bring_to_front(node_ptr);
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let mut node = unsafe { Box::from_raw(node_ptr) };
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node.value = value;
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let _node_ptr = Box::into_raw(node);
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}
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Entry::Vacant(v) => {
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// Node not present, insert a new one
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// Unfortunately we need a copy of the key here, therefore it has
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// to impl the copy trait
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let node = Box::new(CacheNode::new(key, value));
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let node_ptr = Box::into_raw(node);
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self.list.push_front(node_ptr);
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v.insert(node_ptr);
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// If we have more elements than capacity,
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// delete the lists tail node (= oldest node).
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// This needs to be executed after the insert in order to
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// avoid borrow conflict. This means there are temporarily
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// self.capacity + 1 cache nodes.
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if self.map.len() > self.capacity {
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self.pop_tail();
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}
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}
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}
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}
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/// Remove the given `key` and its `value` from the cache.
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pub fn remove(&mut self, key: K) -> Option<V> {
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// Remove node pointer from the HashMap and get ownership of the node
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let node_ptr = self.map.remove(&key)?;
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let node = self.list.remove(node_ptr);
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Some(node.value)
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}
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/// Remove the least recently used node from the cache.
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fn pop_tail(&mut self) {
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if let Some(old_tail) = self.list.pop_tail() {
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// Remove HashMap entry for old tail
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self.map.remove(&old_tail.key);
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}
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}
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/// Get a mutable reference to the value identified by `key`.
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/// This will update the cache entry to be the most recently used entry.
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/// On cache misses, None is returned.
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pub fn get_mut<'a>(&'a mut self, key: K) -> Option<&'a mut V> {
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let node_ptr = self.map.get(&key)?;
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self.list.bring_to_front(*node_ptr);
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Some(unsafe { &mut (*self.list.head).value })
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}
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/// Number of entries in the cache.
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pub fn len(&self) -> usize {
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self.map.len()
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}
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/// Get a mutable reference to the value identified by `key`.
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/// This will update the cache entry to be the most recently used entry.
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/// On cache misses, the cachers fetch method is called to get a corresponding
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/// value.
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/// If fetch returns a value, it is inserted as the most recently used entry
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/// in the cache.
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pub fn access<'a>(&'a mut self, key: K, cacher: &mut dyn Cacher<K, V>) -> Result<Option<&'a mut V>, anyhow::Error> {
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match self.map.entry(key) {
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Entry::Occupied(mut o) => {
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// Cache hit, birng node to front of list
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let node_ptr = *o.get_mut();
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self.list.bring_to_front(node_ptr);
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}
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Entry::Vacant(v) => {
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// Cache miss, try to fetch from cacher and insert at the front
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match cacher.fetch(key)? {
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None => return Ok(None),
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Some(value) => {
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// Unfortunately we need a copy of the key here, therefore it has
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// to impl the copy trait
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let node = Box::new(CacheNode::new(key, value));
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let node_ptr = Box::into_raw(node);
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self.list.push_front(node_ptr);
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v.insert(node_ptr);
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// If we have more elements than capacity,
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// delete the lists tail node (= oldest node).
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// This needs to be executed after the insert in order to
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// avoid borrow conflict. This means there are temporarily
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// self.capacity + 1 cache nodes.
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if self.map.len() > self.capacity {
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self.pop_tail();
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}
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}
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}
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}
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}
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Ok(Some(unsafe { &mut (*self.list.head).value }))
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}
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}
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/// Linked list holding the nodes of the LruCache.
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///
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/// This struct actually holds the CacheNodes via the raw linked list pointers
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/// and allows to define the access sequence of these via the list sequence.
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/// The LinkedList of the standard library unfortunately does not implement
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/// an efficient way to bring list entries to the front, therefore we need our own.
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struct LinkedList<K, V> {
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head: *mut CacheNode<K, V>,
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tail: *mut CacheNode<K, V>,
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}
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impl<K, V> LinkedList<K, V> {
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/// Create a new empty linked list.
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fn new() -> Self {
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Self {
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head: std::ptr::null_mut(),
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tail: std::ptr::null_mut(),
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}
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}
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/// Bring the CacheNode referenced by `node_ptr` to the front of the linked list.
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fn bring_to_front(&mut self, node_ptr: *mut CacheNode<K, V>) {
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if node_ptr == self.head {
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// node is already head, just return
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return;
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}
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let mut node = unsafe { Box::from_raw(node_ptr) };
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// Update the prev node to point to next (or null if current node is tail)
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unsafe { (*node.prev).next = node.next };
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// Update the next node or otherwise the tail
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if !node.next.is_null() {
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unsafe { (*node.next).prev = node.prev };
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} else {
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// No next node means this was the tail
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self.tail = node.prev;
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}
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node.prev = std::ptr::null_mut();
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node.next = self.head;
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// update the head and release ownership of the node again
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let node_ptr = Box::into_raw(node);
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// Update current head
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unsafe { (*self.head).prev = node_ptr };
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// Update to new head
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self.head = node_ptr;
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}
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/// Insert a new node at the front of the linked list.
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fn push_front(&mut self, node_ptr: *mut CacheNode<K, V>) {
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let mut node = unsafe { Box::from_raw(node_ptr) };
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// Old head gets new heads next
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node.next = self.head;
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// Release ownership of node, rest can be handled with just the pointer.
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let node_ptr = Box::into_raw(node);
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// Update the prev for the old head
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if !self.head.is_null() {
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unsafe { (*self.head).prev = node_ptr };
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}
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// Update the head to the new node pointer
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self.head = node_ptr;
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// If there was no old tail, this node will be the new tail too
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if self.tail.is_null() {
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self.tail = node_ptr;
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}
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}
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/// Remove the node referenced by `node_ptr` from the linke list and return it.
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fn remove(&mut self, node_ptr: *mut CacheNode<K, V>) -> Box<CacheNode<K, V>> {
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let node = unsafe { Box::from_raw(node_ptr) };
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// Update the previous node or otherwise the head
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if !node.prev.is_null() {
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unsafe { (*node.prev).next = node.next };
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} else {
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// No previous node means this was the head
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self.head = node.next;
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}
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// Update the next node or otherwise the tail
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if !node.next.is_null() {
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unsafe { (*node.next).prev = node.prev };
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} else {
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// No next node means this was the tail
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self.tail = node.prev;
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}
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node
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}
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/// Remove the tail node from the linked list and return it.
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fn pop_tail(&mut self) -> Option<Box<CacheNode<K, V>>> {
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if self.tail.is_null() {
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return None;
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}
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let old_tail = unsafe { Box::from_raw(self.tail) };
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self.tail = old_tail.prev;
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// Update next node for new tail
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if !self.tail.is_null() {
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unsafe { (*self.tail).next = std::ptr::null_mut() };
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}
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Some(old_tail)
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}
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/// Clear the linked list and free all the nodes.
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fn clear(&mut self) {
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let mut next = self.head;
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while !next.is_null() {
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// Taking ownership of node and drop it at the end of the block.
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let current = unsafe { Box::from_raw(next) };
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next = current.next;
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}
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// Reset head and tail pointers
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self.head = std::ptr::null_mut();
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self.tail = std::ptr::null_mut();
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}
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}
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#[test]
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fn test_linked_list() {
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let mut list = LinkedList::new();
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for idx in 0..3 {
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let node = Box::new(CacheNode::new(idx, idx + 1));
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// Get pointer, release ownership.
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let node_ptr = Box::into_raw(node);
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list.push_front(node_ptr);
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}
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assert_eq!(unsafe { (*list.head).key }, 2);
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assert_eq!(unsafe { (*list.head).value }, 3);
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assert_eq!(unsafe { (*list.tail).key }, 0);
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assert_eq!(unsafe { (*list.tail).value }, 1);
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list.bring_to_front(list.tail);
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assert_eq!(unsafe { (*list.head).key }, 0);
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assert_eq!(unsafe { (*list.head).value }, 1);
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assert_eq!(unsafe { (*list.tail).key }, 1);
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assert_eq!(unsafe { (*list.tail).value }, 2);
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list.bring_to_front(list.tail);
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assert_eq!(unsafe { (*list.head).key }, 1);
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assert_eq!(unsafe { (*list.head).value }, 2);
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assert_eq!(unsafe { (*list.tail).key }, 2);
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assert_eq!(unsafe { (*list.tail).value }, 3);
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let tail = list.pop_tail().unwrap();
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assert_eq!(tail.key, 2);
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assert_eq!(tail.value, 3);
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assert_eq!(unsafe { (*list.head).key }, 1);
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assert_eq!(unsafe { (*list.head).value }, 2);
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assert_eq!(unsafe { (*list.tail).key }, 0);
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assert_eq!(unsafe { (*list.tail).value }, 1);
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list.clear();
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assert!(list.head.is_null());
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assert!(list.tail.is_null());
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}
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