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124 lines
3.2 KiB
Go
124 lines
3.2 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE_list file.
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package nonblocking
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// entry is an LRU entry
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type entry[K comparable, V any] struct {
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// Next and previous pointers in the doubly-linked list of elements.
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// To simplify the implementation, internally a list l is implemented
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// as a ring, such that &l.root is both the next element of the last
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// list element (l.Back()) and the previous element of the first list
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// element (l.Front()).
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next, prev *entry[K, V]
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// The list to which this element belongs.
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list *lruList[K, V]
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// The LRU key of this element.
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key K
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// The value stored with this element.
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value V
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}
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// lruList represents a doubly linked list.
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// The zero value for lruList is an empty list ready to use.
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type lruList[K comparable, V any] struct {
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root entry[K, V] // sentinel list element, only &root, root.prev, and root.next are used
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len int // current list length excluding (this) sentinel element
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}
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// init initializes or clears list l.
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func (l *lruList[K, V]) init() *lruList[K, V] {
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l.root.next = &l.root
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l.root.prev = &l.root
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l.len = 0
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return l
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}
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// newList returns an initialized list.
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func newList[K comparable, V any]() *lruList[K, V] { return new(lruList[K, V]).init() }
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// length returns the number of elements of list l.
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// The complexity is O(1).
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func (l *lruList[K, V]) length() int { return l.len }
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// back returns the last element of list l or nil if the list is empty.
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func (l *lruList[K, V]) back() *entry[K, V] {
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if l.len == 0 {
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return nil
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}
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return l.root.prev
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}
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// lazyInit lazily initializes a zero List value.
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func (l *lruList[K, V]) lazyInit() {
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if l.root.next == nil {
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l.init()
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}
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}
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// insert inserts e after at, increments l.len, and returns e.
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func (l *lruList[K, V]) insert(e, at *entry[K, V]) *entry[K, V] {
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e.prev = at
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e.next = at.next
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e.prev.next = e
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e.next.prev = e
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e.list = l
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l.len++
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return e
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}
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// insertValue is a convenience wrapper for insert(&Element{Value: v}, at).
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func (l *lruList[K, V]) insertValue(k K, v V, at *entry[K, V]) *entry[K, V] {
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return l.insert(&entry[K, V]{value: v, key: k}, at)
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}
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// remove removes e from its list, decrements l.len
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func (l *lruList[K, V]) remove(e *entry[K, V]) V {
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// If already removed, do nothing.
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if e.prev == nil && e.next == nil {
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return e.value
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}
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e.prev.next = e.next
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e.next.prev = e.prev
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e.next = nil // avoid memory leaks
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e.prev = nil // avoid memory leaks
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e.list = nil
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l.len--
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return e.value
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}
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// move moves e to next to at.
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func (*lruList[K, V]) move(e, at *entry[K, V]) {
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if e == at {
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return
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}
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e.prev.next = e.next
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e.next.prev = e.prev
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e.prev = at
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e.next = at.next
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e.prev.next = e
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e.next.prev = e
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}
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// pushFront inserts a new element e with value v at the front of list l and returns e.
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func (l *lruList[K, V]) pushFront(k K, v V) *entry[K, V] {
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l.lazyInit()
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return l.insertValue(k, v, &l.root)
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}
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// moveToFront moves element e to the front of list l.
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// If e is not an element of l, the list is not modified.
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// The element must not be nil.
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func (l *lruList[K, V]) moveToFront(e *entry[K, V]) {
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if e.list != l || l.root.next == e {
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return
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}
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// see comment in List.Remove about initialization of l
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l.move(e, &l.root)
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}
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