Buckets must be a power of 2. Move from % to & for determining the bucket.

cache.go

@@ -12,7 +12,7 @@ type Cache struct {
 	*Configuration
 	list        *list.List
 	buckets     []*Bucket
-	bucketCount uint32
+	bucketMask  uint32
 	deletables  chan *Item
 	promotables chan *Item
 }
@@ -21,7 +21,7 @@ func New(config *Configuration) *Cache {
 	c := &Cache{
 		list:          list.New(),
 		Configuration: config,
-		bucketCount:   uint32(config.buckets),
+		bucketMask:    uint32(config.buckets) - 1,
 		buckets:       make([]*Bucket, config.buckets),
 		deletables:    make(chan *Item, config.deleteBuffer),
 		promotables:   make(chan *Item, config.promoteBuffer),
@@ -102,7 +102,7 @@ func (c *Cache) deleteItem(bucket *Bucket, item *Item) {
 func (c *Cache) bucket(key string) *Bucket {
 	h := fnv.New32a()
 	h.Write([]byte(key))
-	return c.buckets[h.Sum32()%c.bucketCount]
+	return c.buckets[h.Sum32()&c.bucketMask]
 }
 
 func (c *Cache) conditionalPromote(item *Item) {

configuration.go

@@ -30,9 +30,12 @@ func (c *Configuration) MaxItems(max uint64) *Configuration {
 }
 
 // Keys are hashed into % bucket count to provide greater concurrency (every set
-// requires a write lock on the bucket)
-// [64]
+// requires a write lock on the bucket). Must be a power of 2 (1, 2, 4, 8, 16, ...)
+// [16]
 func (c *Configuration) Buckets(count uint32) *Configuration {
+	if count == 0 || ((count&(^count+1)) == count) == false {
+		count = 16
+	}
 	c.buckets = int(count)
 	return c
 }

configuration_test.go

@@ -0,0 +1,23 @@
+package ccache
+
+import (
+	. "github.com/karlseguin/expect"
+	"testing"
+)
+
+type ConfigurationTests struct{}
+
+func Test_Configuration(t *testing.T) {
+	Expectify(new(ConfigurationTests), t)
+}
+
+func (_ *ConfigurationTests) BucketsPowerOf2() {
+	for i := uint32(0); i < 31; i++ {
+		c := Configure().Buckets(i)
+		if i == 1 || i == 2 || i == 4 || i == 8 || i == 16 {
+			Expect(c.buckets).ToEqual(int(i))
+		} else {
+			Expect(c.buckets).ToEqual(16)
+		}
+	}
+}

item.go

@@ -6,7 +6,6 @@ import (
 	"time"
 )
 
-
 type TrackedItem interface {
 	Value() interface{}
 	Release()
@@ -80,7 +79,7 @@ func (i *Item) Expired() bool {
 
 func (i *Item) TTL() time.Duration {
 	expires := atomic.LoadInt64(&i.expires)
-	return time.Second * time.Duration(expires - time.Now().Unix())
+	return time.Second * time.Duration(expires-time.Now().Unix())
 }
 
 func (i *Item) Expires() time.Time {

item_test.go

@@ -40,7 +40,6 @@ func (i *ItemTests) Expires() {
 	Expect(item.Expires().Unix()).To.Equal(now + 10)
 }
 
-
 func (i *ItemTests) Extend() {
 	item := &Item{expires: time.Now().Unix() + 10}
 	item.Extend(time.Minute * 2)

layeredcache.go

@@ -12,7 +12,7 @@ type LayeredCache struct {
 	*Configuration
 	list        *list.List
 	buckets     []*LayeredBucket
-	bucketCount uint32
+	bucketMask  uint32
 	deletables  chan *Item
 	promotables chan *Item
 }
@@ -21,7 +21,7 @@ func Layered(config *Configuration) *LayeredCache {
 	c := &LayeredCache{
 		list:          list.New(),
 		Configuration: config,
-		bucketCount:   uint32(config.buckets),
+		bucketMask:    uint32(config.buckets) - 1,
 		buckets:       make([]*LayeredBucket, config.buckets),
 		deletables:    make(chan *Item, config.deleteBuffer),
 		promotables:   make(chan *Item, config.promoteBuffer),
@@ -101,7 +101,7 @@ func (c *LayeredCache) Clear() {
 func (c *LayeredCache) bucket(key string) *LayeredBucket {
 	h := fnv.New32a()
 	h.Write([]byte(key))
-	return c.buckets[h.Sum32()%c.bucketCount]
+	return c.buckets[h.Sum32()&c.bucketMask]
 }
 
 func (c *LayeredCache) conditionalPromote(item *Item) {

readme.md

@@ -39,7 +39,7 @@ The most likely configuration options to tweak are:
 
 Configurations that change the internals of the cache, which aren't as likely to need tweaking:
 
-* `Buckets` - ccache shards its internal map to provide a greater amount of concurrency. The number of buckets is configurable (default: 16)
+* `Buckets` - ccache shards its internal map to provide a greater amount of concurrency. Must be a power of 2 (default: 16).
 * `PromoteBuffer(int)` - the size of the buffer to use to queue promotions (default: 1024)
 * `DeleteBuffer(int)` the size of the buffer to use to queue deletions (default: 1024)