eviction: fix LFU tie-breaking with LRU-on-ties using monotonic seq; …

README.md

@@ -1,4 +1,4 @@
 # HyperCache
 
 [![Go](https://github.com/hyp3rd/hypercache/actions/workflows/go.yml/badge.svg)][build-link] [![CodeQL](https://github.com/hyp3rd/hypercache/actions/workflows/codeql.yml/badge.svg)][codeql-link]
 
@@ -11,7 +11,7 @@
 - [Recently Used (LRU) eviction algorithm](./pkg/eviction/lru.go)
 - [The Least Frequently Used (LFU) algorithm](./pkg/eviction/lfu.go)
 - [Cache-Aware Write-Optimized LFU (CAWOLFU)](./pkg/eviction/cawolfu.go)
-- [The Adaptive Replacement Cache (ARC) algorithm](./pkg/eviction/arc.go)
+- [The Adaptive Replacement Cache (ARC) algorithm](./pkg/eviction/arc.go) — Experimental (not enabled by default)
 - [The clock eviction algorithm](./pkg/eviction/clock.go)
 
 ### Features
@@ -25,7 +25,7 @@
 - Retrieve items from the cache by their key
 - Delete items from the cache by their key
 - Clear the cache of all items
-- Evitc items in the background based on the cache capacity and items access leveraging several custom eviction algorithms
+- Evict items in the background based on the cache capacity and items access leveraging several custom eviction algorithms
 - Expire items in the background based on their duration
 - [Eviction Algorithm interface](./pkg/eviction/eviction.go) to implement custom eviction algorithms.
 - Stats collection with a default [stats collector](./pkg/stats/stats.go) or a custom one that implements the StatsCollector interface.
@@ -87,6 +87,18 @@
 
 Use your preferred OpenTelemetry SDK setup for exporters and processors in production; the example uses no-op providers for simplicity.
 
+### Eviction algorithms
+
+Available algorithm names you can pass to `WithEvictionAlgorithm`:
+
+- "lru" — Least Recently Used (default)
+- "lfu" — Least Frequently Used (with LRU tie-breaker for equal frequencies)
+- "clock" — Second-chance clock
+- "cawolfu" — Cache-Aware Write-Optimized LFU
+- "arc" — Adaptive Replacement Cache (experimental; not registered by default)
+
+Note: ARC is experimental and isn’t included in the default registry. If you choose to use it, register it manually or enable it explicitly in your build.
+
 ## API
 
 The `NewInMemoryWithDefaults` function creates a new `HyperCache` instance with the defaults:

config.go

@@ -1,5 +1,5 @@
 // Package hypercache provides a high-performance, generic caching library with configurable backends and eviction algorithms.
-// It supports multiple backend types including in-memory and Redis, with various eviction strategies like LRU, LFU, ARC, and more.
+// It supports multiple backend types including in-memory and Redis, with various eviction strategies like LRU, LFU, and more.
 // The package is designed to be flexible and extensible, allowing users to customize cache behavior through configuration options.
 //
 // Example usage:
@@ -90,7 +90,7 @@ func WithMaxCacheSize[T backend.IBackendConstrain](maxCacheSize int64) Option[T]
 //   - "FIFO" (First In First Out)
 //   - "RANDOM" (Random)
 //   - "CLOCK" (Clock) - Implemented in the `eviction/clock.go` file
-//   - "ARC" (Adaptive Replacement Cache) - Implemented in the `eviction/arc.go` file
+//   - "ARC" (Adaptive Replacement Cache) - Experimental (not enabled by default)
 //   - "TTL" (Time To Live)
 //   - "LFUDA" (Least Frequently Used with Dynamic Aging)
 //   - "SLRU" (Segmented Least Recently Used)

pkg/eviction/eviction.go

@@ -28,9 +28,6 @@ type AlgorithmRegistry struct {
 // getDefaultAlgorithms returns the default set of eviction algorithms.
 func getDefaultAlgorithms() map[string]func(capacity int) (IAlgorithm, error) {
 	return map[string]func(capacity int) (IAlgorithm, error){
-		"arc": func(capacity int) (IAlgorithm, error) {
-			return NewARCAlgorithm(capacity)
-		},
 		"lru": func(capacity int) (IAlgorithm, error) {
 			return NewLRUAlgorithm(capacity)
 		},

pkg/eviction/lfu.go

@@ -15,6 +15,7 @@ type LFUAlgorithm struct {
 	mutex  sync.RWMutex
 	length int
 	cap    int
+	seq    uint64 // monotonic sequence to break frequency ties by recency (LRU on ties)
 }
 
 // Node is a node in the LFUAlgorithm.
@@ -23,6 +24,7 @@ type Node struct {
 	value any
 	count int
 	index int
+	last  uint64 // last access sequence (higher = more recent)
 }
 
 // FrequencyHeap is a heap of Nodes.
@@ -36,7 +38,8 @@ func (fh FrequencyHeap) Len() int { return len(fh) }
 // Less returns true if the node at index i has a lower frequency than the node at index j.
 func (fh FrequencyHeap) Less(i, j int) bool {
 	if fh[i].count == fh[j].count {
-		return fh[i].index < fh[j].index
+		// On ties, evict the least recently used (older last sequence has priority)
+		return fh[i].last < fh[j].last
 	}
 
 	return fh[i].count < fh[j].count
@@ -82,6 +85,7 @@ func NewLFUAlgorithm(capacity int) (*LFUAlgorithm, error) {
 		freqs:  &FrequencyHeap{},
 		length: 0,
 		cap:    capacity,
+		seq:    0,
 	}, nil
 }
 
@@ -111,6 +115,8 @@ func (l *LFUAlgorithm) Set(key string, value any) {
 		// Key exists: update value and increment frequency
 		node.value = value
 		node.count++
+		l.seq++
+		node.last = l.seq
 		heap.Fix(l.freqs, node.index)
 
 		return
@@ -120,10 +126,12 @@ func (l *LFUAlgorithm) Set(key string, value any) {
 		_, _ = l.internalEvict()
 	}
 
+	l.seq++
 	node := &Node{
 		key:   key,
 		value: value,
 		count: 1,
+		last:  l.seq,
 	}
 	l.items[key] = node
 	heap.Push(l.freqs, node)
@@ -141,6 +149,8 @@ func (l *LFUAlgorithm) Get(key string) (any, bool) {
 	}
 
 	node.count++
+	l.seq++
+	node.last = l.seq
 	heap.Fix(l.freqs, node.index)
 
 	return node.value, true

pkg/middleware/otel_metrics.go

@@ -136,7 +136,6 @@ func (mw *OTelMetricsMiddleware) Stop() { mw.next.Stop() }
 func (mw *OTelMetricsMiddleware) GetStats() stats.Stats { return mw.next.GetStats() }
 
 // rec records call count and duration with attributes.
-// Moved to the end to satisfy funcorder linters.
 func (mw *OTelMetricsMiddleware) rec(ctx context.Context, method string, start time.Time, attrs ...attribute.KeyValue) {
 	base := []attribute.KeyValue{attribute.String("method", method)}
 	if len(attrs) > 0 {
@@ -146,5 +145,3 @@ func (mw *OTelMetricsMiddleware) rec(ctx context.Context, method string, start t
 	mw.calls.Add(ctx, 1, metric.WithAttributes(base...))
 	mw.durations.Record(ctx, float64(time.Since(start).Milliseconds()), metric.WithAttributes(base...))
 }
-
-// keep helpers at end of file