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// Copyright (C) 2023 Luke Shumaker <lukeshu@lukeshu.com>
//
// SPDX-License-Identifier: GPL-2.0-or-later
package containers
import (
"git.lukeshu.com/go/typedsync"
)
// LinkedListEntry [T] is an entry in a LinkedList [T].
type LinkedListEntry[T any] struct {
older, newer *LinkedListEntry[T]
Value T
}
// LinkedList is a doubly-linked list.
//
// Rather than "head/tail", "front/back", or "next/prev", it has
// "oldest" and "newest". This is for to make code using it clearer;
// as the motivation for the LinkedList is as an implementation detail
// in LRU caches and FIFO queues, where this temporal naming is
// meaningful. Similarly, it does not implement many common features
// of a linked-list, because these applications do not need such
// features.
//
// An advantage over `container/list.List` is that LinkedList
// maintains a Pool of entries, so churning through the list does not
// churn out garbage. However, LinkedList also has the disadvantages
// that it has fewer safety checks and fewer features in general.
type LinkedList[T any] struct {
oldest, newest *LinkedListEntry[T]
pool typedsync.Pool[*LinkedListEntry[T]]
}
// IsEmpty returns whether the list empty or not.
func (l *LinkedList[T]) IsEmpty() bool {
return l.oldest == nil
}
// Delete removes an entry from the list. The entry is invalid once
// Delete returns, and should not be reused or have its .Value
// accessed.
//
// It is invalid (runtime-panic) to call Delete on a nil entry.
//
// It is invalid (corrupt the list) to call Delete on an entry that
// isn't in the list.
func (l *LinkedList[T]) Delete(entry *LinkedListEntry[T]) {
if entry.newer == nil {
l.newest = entry.older
} else {
entry.newer.older = entry.older
}
if entry.older == nil {
l.oldest = entry.newer
} else {
entry.older.newer = entry.newer
}
*entry = LinkedListEntry[T]{} // no memory leaks
l.pool.Put(entry)
}
// Store appends a value to the "newest" end of the list, returning
// the created entry.
func (l *LinkedList[T]) Store(val T) *LinkedListEntry[T] {
entry, ok := l.pool.Get()
if !ok {
entry = new(LinkedListEntry[T])
}
*entry = LinkedListEntry[T]{
older: l.newest,
Value: val,
}
l.newest = entry
if entry.older == nil {
l.oldest = entry
} else {
entry.older.newer = entry
}
return entry
}
// MoveToNewest moves an entry fron any position in the list to the
// "newest" end of the list. If the entry is already in the "newest"
// position, then MoveToNewest is a no-op.
//
// It is invalid (runtime-panic) to call MoveToNewest on a nil entry.
//
// It is invalid (corrupt the list) to call MoveToNewest on an entry
// that isn't in the list.
func (l *LinkedList[T]) MoveToNewest(entry *LinkedListEntry[T]) {
if entry.newer == nil {
// Already newest.
return
}
entry.newer.older = entry.older
if entry.older == nil {
l.oldest = entry.newer
} else {
entry.older.newer = entry.newer
}
entry.older = l.newest
l.newest.newer = entry
entry.newer = nil
l.newest = entry
}
// Oldest returns the entry at the "oldest" end of the list, or nil if
// the list is empty.
func (l *LinkedList[T]) Oldest() *LinkedListEntry[T] {
return l.oldest
}
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