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basiclru.go
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basiclru.go
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// Copyright 2022 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
// Package lru implements generically-typed LRU caches.
package lru
// BasicLRU is a simple LRU cache.
//
// This type is not safe for concurrent use.
// The zero value is not valid, instances must be created using NewCache.
type BasicLRU[K comparable, V any] struct {
list *list[K]
items map[K]cacheItem[K, V]
cap int
}
type cacheItem[K any, V any] struct {
elem *listElem[K]
value V
}
// NewBasicLRU creates a new LRU cache.
func NewBasicLRU[K comparable, V any](capacity int) BasicLRU[K, V] {
if capacity <= 0 {
capacity = 1
}
c := BasicLRU[K, V]{
items: make(map[K]cacheItem[K, V]),
list: newList[K](),
cap: capacity,
}
return c
}
// Add adds a value to the cache. Returns true if an item was evicted to store the new item.
func (c *BasicLRU[K, V]) Add(key K, value V) (evicted bool) {
item, ok := c.items[key]
if ok {
// Already exists in cache.
item.value = value
c.items[key] = item
c.list.moveToFront(item.elem)
return false
}
var elem *listElem[K]
if c.Len() >= c.cap {
elem = c.list.removeLast()
delete(c.items, elem.v)
evicted = true
} else {
elem = new(listElem[K])
}
// Store the new item.
// Note that, if another item was evicted, we re-use its list element here.
elem.v = key
c.items[key] = cacheItem[K, V]{elem, value}
c.list.pushElem(elem)
return evicted
}
// Contains reports whether the given key exists in the cache.
func (c *BasicLRU[K, V]) Contains(key K) bool {
_, ok := c.items[key]
return ok
}
// Get retrieves a value from the cache. This marks the key as recently used.
func (c *BasicLRU[K, V]) Get(key K) (value V, ok bool) {
item, ok := c.items[key]
if !ok {
return value, false
}
c.list.moveToFront(item.elem)
return item.value, true
}
// GetOldest retrieves the least-recently-used item.
// Note that this does not update the item's recency.
func (c *BasicLRU[K, V]) GetOldest() (key K, value V, ok bool) {
lastElem := c.list.last()
if lastElem == nil {
return key, value, false
}
key = lastElem.v
item := c.items[key]
return key, item.value, true
}
// Len returns the current number of items in the cache.
func (c *BasicLRU[K, V]) Len() int {
return len(c.items)
}
// Peek retrieves a value from the cache, but does not mark the key as recently used.
func (c *BasicLRU[K, V]) Peek(key K) (value V, ok bool) {
item, ok := c.items[key]
return item.value, ok
}
// Purge empties the cache.
func (c *BasicLRU[K, V]) Purge() {
c.list.init()
for k := range c.items {
delete(c.items, k)
}
}
// Remove drops an item from the cache. Returns true if the key was present in cache.
func (c *BasicLRU[K, V]) Remove(key K) bool {
item, ok := c.items[key]
if ok {
delete(c.items, key)
c.list.remove(item.elem)
}
return ok
}
// RemoveOldest drops the least recently used item.
func (c *BasicLRU[K, V]) RemoveOldest() (key K, value V, ok bool) {
lastElem := c.list.last()
if lastElem == nil {
return key, value, false
}
key = lastElem.v
item := c.items[key]
delete(c.items, key)
c.list.remove(lastElem)
return key, item.value, true
}
// Keys returns all keys in the cache.
func (c *BasicLRU[K, V]) Keys() []K {
keys := make([]K, 0, len(c.items))
return c.list.appendTo(keys)
}
// list is a doubly-linked list holding items of type he.
// The zero value is not valid, use newList to create lists.
type list[T any] struct {
root listElem[T]
}
type listElem[T any] struct {
next *listElem[T]
prev *listElem[T]
v T
}
func newList[T any]() *list[T] {
l := new(list[T])
l.init()
return l
}
// init reinitializes the list, making it empty.
func (l *list[T]) init() {
l.root.next = &l.root
l.root.prev = &l.root
}
// push adds an element to the front of the list.
func (l *list[T]) pushElem(e *listElem[T]) {
e.prev = &l.root
e.next = l.root.next
l.root.next = e
e.next.prev = e
}
// moveToFront makes 'node' the head of the list.
func (l *list[T]) moveToFront(e *listElem[T]) {
e.prev.next = e.next
e.next.prev = e.prev
l.pushElem(e)
}
// remove removes an element from the list.
func (l *list[T]) remove(e *listElem[T]) {
e.prev.next = e.next
e.next.prev = e.prev
e.next, e.prev = nil, nil
}
// removeLast removes the last element of the list.
func (l *list[T]) removeLast() *listElem[T] {
last := l.last()
if last != nil {
l.remove(last)
}
return last
}
// last returns the last element of the list, or nil if the list is empty.
func (l *list[T]) last() *listElem[T] {
e := l.root.prev
if e == &l.root {
return nil
}
return e
}
// appendTo appends all list elements to a slice.
func (l *list[T]) appendTo(slice []T) []T {
for e := l.root.prev; e != &l.root; e = e.prev {
slice = append(slice, e.v)
}
return slice
}