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