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// Copyright (C) 2023 Luke Shumaker <lukeshu@lukeshu.com>
//
// SPDX-License-Identifier: GPL-2.0-or-later
package caching
import (
"context"
"fmt"
"sync"
)
type lruEntry[K comparable, V any] struct {
key K
val V
refs int
del chan struct{} // non-nil if a delete is waiting on .refs to drop to zero
}
type lruCache[K comparable, V any] struct {
cap int
src Source[K, V]
mu sync.Mutex
unused LinkedList[lruEntry[K, V]]
evictable LinkedList[lruEntry[K, V]] // only entries with .refs==0
byName map[K]*LinkedListEntry[lruEntry[K, V]]
waiters LinkedList[chan *LinkedListEntry[lruEntry[K, V]]]
}
// NewLRUCache returns a new thread-safe Cache with a simple Least-Recently-Used
// eviction policy.
//
// It is invalid (runtime-panic) to call NewLRUCache with a non-positive
// capacity or a nil source.
func NewLRUCache[K comparable, V any](cap int, src Source[K, V]) Cache[K, V] {
if cap <= 0 {
panic(fmt.Errorf("caching.NewLRUCache: invalid capacity: %v", cap))
}
if src == nil {
panic(fmt.Errorf("caching.NewLRUCache: nil source"))
}
ret := &lruCache[K, V]{
cap: cap,
src: src,
}
for i := 0; i < cap; i++ {
c.unused.Store(new(LinkedListEntry[lruEntry[K, V]]))
}
return ret
}
// Acquire implements Cache.
func (c *lruCache[K, V]) Acquire(ctx context.Context, k K) *V {
c.mu.Lock()
defer c.mu.Unlock()
if c.byName == nil {
c.byName = make(map[K]*LinkedListEntry[lruEntry[K, V]], c.cap)
}
entry, ok := c.byName[k]
if ok {
if entry.Value.refs == 0 {
c.evictable.Delete(entry)
}
entry.Value.refs++
} else {
switch {
case !c.unused.IsEmpty():
entry = c.unused.Oldest()
c.unused.Delete(entry)
case !c.evictable.IsEmpty():
entry = c.evictable.Oldest()
c.evictable.Delete(entry)
delete(c.byName, entry.Value.key)
default:
ch := make(chan *LinkedListEntry[lruEntry[K, V]])
c.waiters.Store(&LinkedListEntry[chan *LinkedListEntry[lruEntry[K, V]]]{Value: ch})
c.mu.Unlock()
entry = <-ch
c.mu.Lock()
}
entry.Value.key = k
c.src.Load(ctx, k, &entry.Value.val)
entry.Value.refs = 1
c.byName[k] = entry
}
return &entry.Value.val
}
// Release implements Cache.
func (c *lruCache[K, V]) Release(k K) {
c.mu.Lock()
defer c.mu.Unlock()
entry, ok := c.byName[k]
if !ok || entry.Value.refs <= 0 {
panic(fmt.Errorf("caching.lruCache.Release called on key that is not held: %v", k))
}
entry.Value.refs--
if entry.Value.refs == 0 {
del := entry.Value.del != nil
if del {
close(entry.Value.del)
entry.Value.del = nil
}
if c.waiters.IsEmpty() {
// Add it to the free-list.
if del {
delete(c.byName, k)
c.unused.Store(entry)
} else {
c.evictable.Store(entry)
}
} else {
// Someone's waiting to pop something off of the
// free-list; bypass the free-list and hand it directly
// to them.
// Make sure that no one aquires this entry between us
// writing it to the channel and the waiter calling
// c.mu.Lock().
delete(c.byName, k)
// Pass it to the waiter.
waiter := c.waiters.Oldest()
c.waiters.Delete(waiter)
waiter.Value <- entry
}
}
}
// Delete implements Cache.
func (c *lruCache[K, V]) Delete(k K) {
c.mu.Lock()
entry, ok := c.byName[k]
if !ok {
return
}
if entry.Value.refs == 0 {
delete(c.byName, k)
if c.waiters.IsEmpty() {
c.unused.Store(entry)
} else {
waiter := c.waiters.Oldest()
c.waiters.Delete(waiter)
waiter.Value <- entry
}
c.mu.Unlock()
} else {
if entry.Value.del == nil {
entry.Value.del = make(chan struct{})
}
ch := entry.Value.del
c.mu.Unlock()
<-ch
}
}
// Flush implements Cache.
func (c *lruCache[K, V]) Flush(ctx context.Context) {
c.mu.Lock()
defer c.mu.Unlock()
for _, entry := range c.byName {
c.src.Flush(ctx, &entry.Value.val)
}
for entry := c.unused.Oldest(); entry != nil; entry = entry.Newer() {
c.src.Flush(ctx, &entry.Value.val)
}
}
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