diff --git a/2q.go b/2q.go
new file mode 100644
index 0000000..fa5780f
--- /dev/null
+++ b/2q.go
@@ -0,0 +1,223 @@
+package lru
+
+import (
+	"fmt"
+	"sync"
+
+	"google3/third_party/golang/lru/simplelru/simplelru"
+)
+
+const (
+	// Default2QRecentRatio is the ratio of the 2Q cache dedicated
+	// to recently added entries that have only been accessed once.
+	Default2QRecentRatio = 0.25
+
+	// Default2QGhostEntries is the default ratio of ghost
+	// entries kept to track entries recently evicted
+	Default2QGhostEntries = 0.50
+)
+
+// TwoQueueCache is a thread-safe fixed size 2Q cache.
+// 2Q is an enhancement over the standard LRU cache
+// in that it tracks both frequently and recently used
+// entries separately. This avoids a burst in access to new
+// entries from evicting frequently used entries. It adds some
+// additional tracking overhead to the standard LRU cache, and is
+// computationally about 2x the cost, and adds some metadata over
+// head. The ARCCache is similar, but does not require setting any
+// parameters.
+type TwoQueueCache struct {
+	size       int
+	recentSize int
+
+	recent      simplelru.LRUCache
+	frequent    simplelru.LRUCache
+	recentEvict simplelru.LRUCache
+	lock        sync.RWMutex
+}
+
+// New2Q creates a new TwoQueueCache using the default
+// values for the parameters.
+func New2Q(size int) (*TwoQueueCache, error) {
+	return New2QParams(size, Default2QRecentRatio, Default2QGhostEntries)
+}
+
+// New2QParams creates a new TwoQueueCache using the provided
+// parameter values.
+func New2QParams(size int, recentRatio float64, ghostRatio float64) (*TwoQueueCache, error) {
+	if size <= 0 {
+		return nil, fmt.Errorf("invalid size")
+	}
+	if recentRatio < 0.0 || recentRatio > 1.0 {
+		return nil, fmt.Errorf("invalid recent ratio")
+	}
+	if ghostRatio < 0.0 || ghostRatio > 1.0 {
+		return nil, fmt.Errorf("invalid ghost ratio")
+	}
+
+	// Determine the sub-sizes
+	recentSize := int(float64(size) * recentRatio)
+	evictSize := int(float64(size) * ghostRatio)
+
+	// Allocate the LRUs
+	recent, err := simplelru.NewLRU(size, nil)
+	if err != nil {
+		return nil, err
+	}
+	frequent, err := simplelru.NewLRU(size, nil)
+	if err != nil {
+		return nil, err
+	}
+	recentEvict, err := simplelru.NewLRU(evictSize, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	// Initialize the cache
+	c := &TwoQueueCache{
+		size:        size,
+		recentSize:  recentSize,
+		recent:      recent,
+		frequent:    frequent,
+		recentEvict: recentEvict,
+	}
+	return c, nil
+}
+
+// Get looks up a key's value from the cache.
+func (c *TwoQueueCache) Get(key interface{}) (value interface{}, ok bool) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+
+	// Check if this is a frequent value
+	if val, ok := c.frequent.Get(key); ok {
+		return val, ok
+	}
+
+	// If the value is contained in recent, then we
+	// promote it to frequent
+	if val, ok := c.recent.Peek(key); ok {
+		c.recent.Remove(key)
+		c.frequent.Add(key, val)
+		return val, ok
+	}
+
+	// No hit
+	return nil, false
+}
+
+// Add adds a value to the cache.
+func (c *TwoQueueCache) Add(key, value interface{}) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+
+	// Check if the value is frequently used already,
+	// and just update the value
+	if c.frequent.Contains(key) {
+		c.frequent.Add(key, value)
+		return
+	}
+
+	// Check if the value is recently used, and promote
+	// the value into the frequent list
+	if c.recent.Contains(key) {
+		c.recent.Remove(key)
+		c.frequent.Add(key, value)
+		return
+	}
+
+	// If the value was recently evicted, add it to the
+	// frequently used list
+	if c.recentEvict.Contains(key) {
+		c.ensureSpace(true)
+		c.recentEvict.Remove(key)
+		c.frequent.Add(key, value)
+		return
+	}
+
+	// Add to the recently seen list
+	c.ensureSpace(false)
+	c.recent.Add(key, value)
+	return
+}
+
+// ensureSpace is used to ensure we have space in the cache
+func (c *TwoQueueCache) ensureSpace(recentEvict bool) {
+	// If we have space, nothing to do
+	recentLen := c.recent.Len()
+	freqLen := c.frequent.Len()
+	if recentLen+freqLen < c.size {
+		return
+	}
+
+	// If the recent buffer is larger than
+	// the target, evict from there
+	if recentLen > 0 && (recentLen > c.recentSize || (recentLen == c.recentSize && !recentEvict)) {
+		k, _, _ := c.recent.RemoveOldest()
+		c.recentEvict.Add(k, nil)
+		return
+	}
+
+	// Remove from the frequent list otherwise
+	c.frequent.RemoveOldest()
+}
+
+// Len returns the number of items in the cache.
+func (c *TwoQueueCache) Len() int {
+	c.lock.RLock()
+	defer c.lock.RUnlock()
+	return c.recent.Len() + c.frequent.Len()
+}
+
+// Keys returns a slice of the keys in the cache.
+// The frequently used keys are first in the returned slice.
+func (c *TwoQueueCache) Keys() []interface{} {
+	c.lock.RLock()
+	defer c.lock.RUnlock()
+	k1 := c.frequent.Keys()
+	k2 := c.recent.Keys()
+	return append(k1, k2...)
+}
+
+// Remove removes the provided key from the cache.
+func (c *TwoQueueCache) Remove(key interface{}) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+	if c.frequent.Remove(key) {
+		return
+	}
+	if c.recent.Remove(key) {
+		return
+	}
+	if c.recentEvict.Remove(key) {
+		return
+	}
+}
+
+// Purge is used to completely clear the cache.
+func (c *TwoQueueCache) Purge() {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+	c.recent.Purge()
+	c.frequent.Purge()
+	c.recentEvict.Purge()
+}
+
+// Contains is used to check if the cache contains a key
+// without updating recency or frequency.
+func (c *TwoQueueCache) Contains(key interface{}) bool {
+	c.lock.RLock()
+	defer c.lock.RUnlock()
+	return c.frequent.Contains(key) || c.recent.Contains(key)
+}
+
+// Peek is used to inspect the cache value of a key
+// without updating recency or frequency.
+func (c *TwoQueueCache) Peek(key interface{}) (value interface{}, ok bool) {
+	c.lock.RLock()
+	defer c.lock.RUnlock()
+	if val, ok := c.frequent.Peek(key); ok {
+		return val, ok
+	}
+	return c.recent.Peek(key)
+}
diff --git a/2q_test.go b/2q_test.go
new file mode 100644
index 0000000..1b0f351
--- /dev/null
+++ b/2q_test.go
@@ -0,0 +1,306 @@
+package lru
+
+import (
+	"math/rand"
+	"testing"
+)
+
+func Benchmark2Q_Rand(b *testing.B) {
+	l, err := New2Q(8192)
+	if err != nil {
+		b.Fatalf("err: %v", err)
+	}
+
+	trace := make([]int64, b.N*2)
+	for i := 0; i < b.N*2; i++ {
+		trace[i] = rand.Int63() % 32768
+	}
+
+	b.ResetTimer()
+
+	var hit, miss int
+	for i := 0; i < 2*b.N; i++ {
+		if i%2 == 0 {
+			l.Add(trace[i], trace[i])
+		} else {
+			_, ok := l.Get(trace[i])
+			if ok {
+				hit++
+			} else {
+				miss++
+			}
+		}
+	}
+	b.Logf("hit: %d miss: %d ratio: %f", hit, miss, float64(hit)/float64(miss))
+}
+
+func Benchmark2Q_Freq(b *testing.B) {
+	l, err := New2Q(8192)
+	if err != nil {
+		b.Fatalf("err: %v", err)
+	}
+
+	trace := make([]int64, b.N*2)
+	for i := 0; i < b.N*2; i++ {
+		if i%2 == 0 {
+			trace[i] = rand.Int63() % 16384
+		} else {
+			trace[i] = rand.Int63() % 32768
+		}
+	}
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		l.Add(trace[i], trace[i])
+	}
+	var hit, miss int
+	for i := 0; i < b.N; i++ {
+		_, ok := l.Get(trace[i])
+		if ok {
+			hit++
+		} else {
+			miss++
+		}
+	}
+	b.Logf("hit: %d miss: %d ratio: %f", hit, miss, float64(hit)/float64(miss))
+}
+
+func Test2Q_RandomOps(t *testing.T) {
+	size := 128
+	l, err := New2Q(128)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	n := 200000
+	for i := 0; i < n; i++ {
+		key := rand.Int63() % 512
+		r := rand.Int63()
+		switch r % 3 {
+		case 0:
+			l.Add(key, key)
+		case 1:
+			l.Get(key)
+		case 2:
+			l.Remove(key)
+		}
+
+		if l.recent.Len()+l.frequent.Len() > size {
+			t.Fatalf("bad: recent: %d freq: %d",
+				l.recent.Len(), l.frequent.Len())
+		}
+	}
+}
+
+func Test2Q_Get_RecentToFrequent(t *testing.T) {
+	l, err := New2Q(128)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Touch all the entries, should be in t1
+	for i := 0; i < 128; i++ {
+		l.Add(i, i)
+	}
+	if n := l.recent.Len(); n != 128 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.frequent.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Get should upgrade to t2
+	for i := 0; i < 128; i++ {
+		_, ok := l.Get(i)
+		if !ok {
+			t.Fatalf("missing: %d", i)
+		}
+	}
+	if n := l.recent.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.frequent.Len(); n != 128 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Get be from t2
+	for i := 0; i < 128; i++ {
+		_, ok := l.Get(i)
+		if !ok {
+			t.Fatalf("missing: %d", i)
+		}
+	}
+	if n := l.recent.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.frequent.Len(); n != 128 {
+		t.Fatalf("bad: %d", n)
+	}
+}
+
+func Test2Q_Add_RecentToFrequent(t *testing.T) {
+	l, err := New2Q(128)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Add initially to recent
+	l.Add(1, 1)
+	if n := l.recent.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.frequent.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Add should upgrade to frequent
+	l.Add(1, 1)
+	if n := l.recent.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.frequent.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Add should remain in frequent
+	l.Add(1, 1)
+	if n := l.recent.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.frequent.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+}
+
+func Test2Q_Add_RecentEvict(t *testing.T) {
+	l, err := New2Q(4)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Add 1,2,3,4,5 -> Evict 1
+	l.Add(1, 1)
+	l.Add(2, 2)
+	l.Add(3, 3)
+	l.Add(4, 4)
+	l.Add(5, 5)
+	if n := l.recent.Len(); n != 4 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.recentEvict.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.frequent.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Pull in the recently evicted
+	l.Add(1, 1)
+	if n := l.recent.Len(); n != 3 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.recentEvict.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.frequent.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Add 6, should cause another recent evict
+	l.Add(6, 6)
+	if n := l.recent.Len(); n != 3 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.recentEvict.Len(); n != 2 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.frequent.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+}
+
+func Test2Q(t *testing.T) {
+	l, err := New2Q(128)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	for i := 0; i < 256; i++ {
+		l.Add(i, i)
+	}
+	if l.Len() != 128 {
+		t.Fatalf("bad len: %v", l.Len())
+	}
+
+	for i, k := range l.Keys() {
+		if v, ok := l.Get(k); !ok || v != k || v != i+128 {
+			t.Fatalf("bad key: %v", k)
+		}
+	}
+	for i := 0; i < 128; i++ {
+		_, ok := l.Get(i)
+		if ok {
+			t.Fatalf("should be evicted")
+		}
+	}
+	for i := 128; i < 256; i++ {
+		_, ok := l.Get(i)
+		if !ok {
+			t.Fatalf("should not be evicted")
+		}
+	}
+	for i := 128; i < 192; i++ {
+		l.Remove(i)
+		_, ok := l.Get(i)
+		if ok {
+			t.Fatalf("should be deleted")
+		}
+	}
+
+	l.Purge()
+	if l.Len() != 0 {
+		t.Fatalf("bad len: %v", l.Len())
+	}
+	if _, ok := l.Get(200); ok {
+		t.Fatalf("should contain nothing")
+	}
+}
+
+// Test that Contains doesn't update recent-ness
+func Test2Q_Contains(t *testing.T) {
+	l, err := New2Q(2)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	l.Add(1, 1)
+	l.Add(2, 2)
+	if !l.Contains(1) {
+		t.Errorf("1 should be contained")
+	}
+
+	l.Add(3, 3)
+	if l.Contains(1) {
+		t.Errorf("Contains should not have updated recent-ness of 1")
+	}
+}
+
+// Test that Peek doesn't update recent-ness
+func Test2Q_Peek(t *testing.T) {
+	l, err := New2Q(2)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	l.Add(1, 1)
+	l.Add(2, 2)
+	if v, ok := l.Peek(1); !ok || v != 1 {
+		t.Errorf("1 should be set to 1: %v, %v", v, ok)
+	}
+
+	l.Add(3, 3)
+	if l.Contains(1) {
+		t.Errorf("should not have updated recent-ness of 1")
+	}
+}
diff --git a/GOIMPORT/CONFIGURATION b/GOIMPORT/CONFIGURATION
new file mode 100644
index 0000000..5af4313
--- /dev/null
+++ b/GOIMPORT/CONFIGURATION
@@ -0,0 +1,24 @@
+# This file configures imports managed by //third_party/golang/import.go.
+# See go/thirdpartygo for more information.
+
+# ImportFiles specifies which files to import from the upstream source.
+[ImportFiles]
+  exclude: /[.]        # exclude hidden files and folders (e.g., ".gitignore")
+  exclude: /METADATA$  # see go/metadata
+  exclude: /OWNERS$    # see go/owners
+  exclude: /BUILD$     # see go/build
+  include: .*
+
+# ImportRenames specifies whether to rename any source files or directories.
+[ImportRenames]
+  sed: s:^/LICENSE([.](gpl|md|txt))?$:/LICENSE:I  # see go/thirdpartylicenses
+
+# GoogleFiles specifies files added to the import to support use within google3.
+[GoogleFiles]
+  include: ^/GOIMPORT/    # configuration and metadata for import.go tool
+  include: /METADATA$     # see go/metadata
+  include: /OWNERS$       # see go/owners
+  include: /BUILD$        # see go/build
+  include: /g3doc/        # see go/g3doc
+  include: /google_[^/]+  # treat files with google_ prefix as internal
+  exclude: .*
diff --git a/GOIMPORT/MANIFEST b/GOIMPORT/MANIFEST
new file mode 100644
index 0000000..37592fc
--- /dev/null
+++ b/GOIMPORT/MANIFEST
@@ -0,0 +1,25 @@
+# This file contains a manifest of all files imported by import.go.
+# This is for both human review and machine consumption.
+# Please see go/thirdpartygo for more information.
+# DO NOT EDIT. This must only be generated by //third_party/golang/import.go.
+
+= 76bb67cc7ab9bcac642c18c3e4899ad9 2q.go
+= 8b12af7f2c9fa3b4784553889a9237d1 2q_test.go
++ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx BUILD
++ 751f0e7cbc883b54a122bd4f01dd2a4c GOIMPORT/CONFIGURATION
++ 28fd12eb056a5bb8ebed19a2bcacd3b2 GOIMPORT/MANIFEST
++ 140fd5f3ecd2e8c98dc669b78fd3686a GOIMPORT/PATCHES/arc-cache-evict-callback.patch
+= a1608f08271c33c1ce42efa1d1e4def2 LICENSE
++ 5b640b88e2c81ec20a67a375a5910911 METADATA
++ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx OWNERS
+= 987ce2f7c18b660cae7643bd08a9c084 README.md
+= e92c1472a9d9f3ba9dbda0d204cb1972 arc.go
+= 2f4a51b3e51c7b52dd8ab46aeb8af1ba arc_test.go
+= ae9c3f66d678be12e34e4b7573a37e11 doc.go
+= b13049f1c2e66e5218d8ff60892b7583 go.mod
+= 0b0d13db404aa8e325e29f1ac920bac6 lru.go
+= f1e2b3daeda7634853c0eccc3af0b3ba lru_test.go
++ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx simplelru/BUILD
+= 3b46c3adff93aa36ef7b2fe1528406b8 simplelru/lru.go
+= f4b6494ce8149fc0132416315bba812a simplelru/lru_interface.go
+= 677f464589d003875d2191699f4e0417 simplelru/lru_test.go
diff --git a/GOIMPORT/PATCHES/arc-cache-evict-callback.patch b/GOIMPORT/PATCHES/arc-cache-evict-callback.patch
new file mode 100644
index 0000000..56bdcb8
--- /dev/null
+++ b/GOIMPORT/PATCHES/arc-cache-evict-callback.patch
@@ -0,0 +1,243 @@
+diff --git a/arc.go b/arc.go
+index 555225a..f4aaf38 100644
+--- a/arc.go
++++ b/arc.go
+@@ -18,17 +18,23 @@ type ARCCache struct {
+ 	size int // Size is the total capacity of the cache
+ 	p    int // P is the dynamic preference towards T1 or T2
+ 
+-	t1 simplelru.LRUCache // T1 is the LRU for recently accessed items
+-	b1 simplelru.LRUCache // B1 is the LRU for evictions from t1
++	t1 *simplelru.LRU // T1 is the LRU for recently accessed items
++	b1 *simplelru.LRU // B1 is the LRU for evictions from t1
+ 
+-	t2 simplelru.LRUCache // T2 is the LRU for frequently accessed items
+-	b2 simplelru.LRUCache // B2 is the LRU for evictions from t2
++	t2 *simplelru.LRU // T2 is the LRU for frequently accessed items
++	b2 *simplelru.LRU // B2 is the LRU for evictions from t2
+ 
+ 	lock sync.RWMutex
+ }
+ 
+ // NewARC creates an ARC of the given size
+ func NewARC(size int) (*ARCCache, error) {
++	return NewARCWithEvict(size, nil)
++}
++
++// NewARCWithEvict creates an ARC of the given size with onEvict callback.
++func NewARCWithEvict(size int, onEvicted func(key interface{}, value interface{})) (*ARCCache, error) {
++	callback := simplelru.EvictCallback(onEvicted)
+ 	// Create the sub LRUs
+ 	b1, err := simplelru.NewLRU(size, nil)
+ 	if err != nil {
+@@ -38,11 +44,11 @@ func NewARC(size int) (*ARCCache, error) {
+ 	if err != nil {
+ 		return nil, err
+ 	}
+-	t1, err := simplelru.NewLRU(size, nil)
++	t1, err := simplelru.NewLRU(size, callback)
+ 	if err != nil {
+ 		return nil, err
+ 	}
+-	t2, err := simplelru.NewLRU(size, nil)
++	t2, err := simplelru.NewLRU(size, callback)
+ 	if err != nil {
+ 		return nil, err
+ 	}
+@@ -67,7 +73,7 @@ func (c *ARCCache) Get(key interface{}) (value interface{}, ok bool) {
+ 	// If the value is contained in T1 (recent), then
+ 	// promote it to T2 (frequent)
+ 	if val, ok := c.t1.Peek(key); ok {
+-		c.t1.Remove(key)
++		c.t1.RemoveWithoutEvict(key)
+ 		c.t2.Add(key, val)
+ 		return val, ok
+ 	}
+@@ -89,7 +95,7 @@ func (c *ARCCache) Add(key, value interface{}) {
+ 	// Check if the value is contained in T1 (recent), and potentially
+ 	// promote it to frequent T2
+ 	if c.t1.Contains(key) {
+-		c.t1.Remove(key)
++		c.t1.RemoveWithoutEvict(key)
+ 		c.t2.Add(key, value)
+ 		return
+ 	}
+@@ -209,21 +215,18 @@ func (c *ARCCache) Keys() []interface{} {
+ 	return append(k1, k2...)
+ }
+ 
++func (c *ARCCache) subcaches() [4]simplelru.LRUCache {
++	return [4]simplelru.LRUCache{c.t1, c.t2, c.b1, c.b2}
++}
++
+ // Remove is used to purge a key from the cache
+ func (c *ARCCache) Remove(key interface{}) {
+ 	c.lock.Lock()
+ 	defer c.lock.Unlock()
+-	if c.t1.Remove(key) {
+-		return
+-	}
+-	if c.t2.Remove(key) {
+-		return
+-	}
+-	if c.b1.Remove(key) {
+-		return
+-	}
+-	if c.b2.Remove(key) {
+-		return
++	for _, cache := range c.subcaches() {
++		if cache.Remove(key) {
++			return
++		}
+ 	}
+ }
+ 
+@@ -231,10 +234,9 @@ func (c *ARCCache) Remove(key interface{}) {
+ func (c *ARCCache) Purge() {
+ 	c.lock.Lock()
+ 	defer c.lock.Unlock()
+-	c.t1.Purge()
+-	c.t2.Purge()
+-	c.b1.Purge()
+-	c.b2.Purge()
++	for _, cache := range c.subcaches() {
++		cache.Purge()
++	}
+ }
+ 
+ // Contains is used to check if the cache contains a key
+diff --git a/arc_test.go b/arc_test.go
+index e2d9b68..e597eda 100644
+--- a/arc_test.go
++++ b/arc_test.go
+@@ -4,6 +4,8 @@ import (
+ 	"math/rand"
+ 	"testing"
+ 	"time"
++
++	"google3/third_party/golang/cmp/cmp"
+ )
+ 
+ func init() {
+@@ -375,3 +377,66 @@ func TestARC_Peek(t *testing.T) {
+ 		t.Errorf("should not have updated recent-ness of 1")
+ 	}
+ }
++
++func TestARC_OnEvictedCallback(t *testing.T) {
++	type testCase struct {
++		size    int
++		data    []int
++		evicted []int
++	}
++
++	testCases := map[string]testCase{
++		"basic-lru-like": {
++			size:    2,
++			data:    []int{1, 2, 3, 4},
++			evicted: []int{1, 2},
++		},
++		"t1-to-t2-promotion-simple": {
++			size:    4,
++			data:    []int{1, 2, 1, 2, 3, 4, 5, 6},
++			evicted: []int{3, 4},
++		},
++		"adaptive-replacement-simple": {
++			size:    4,
++			data:    []int{1, 2, 3, 4, 5, 6, 1, 2, 7, 8},
++			evicted: []int{1, 2, 3, 4, 1, 5},
++		},
++		"MFU-not-evicted-by-new-entries": {
++			size:    4,
++			data:    []int{4, 4, 1, 2, 3, 4, 5, 6, 7, 8},
++			evicted: []int{1, 2, 3, 5},
++		},
++		"t2-full-b2-empty-new-items": {
++			size: 4,
++			data: []int{
++				// fill t2, b2 should be empty since its target size is zero
++				1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8,
++				// insert "new" items
++				1, 2, 3, 4,
++			},
++			evicted: []int{1, 2, 3, 4, 5, 1, 2, 3},
++		},
++	}
++
++	runTest := func(t *testing.T, tc testCase) {
++		evicted := []int{}
++		c, err := NewARCWithEvict(tc.size, func(key interface{}, _ interface{}) {
++			evicted = append(evicted, key.(int))
++		})
++		if err != nil {
++			t.Fatal(err)
++		}
++
++		for _, val := range tc.data {
++			c.Add(val, val)
++		}
++
++		if !cmp.Equal(tc.evicted, evicted) {
++			t.Errorf("Expectations mismatch.\nWant: %+v\nGot:  %+v", tc.evicted, evicted)
++		}
++	}
++
++	for name, tc := range testCases {
++		t.Run(name, func(t *testing.T) { runTest(t, tc) })
++	}
++}
+diff --git a/simplelru/lru.go b/simplelru/lru.go
+index 5673773..0b40413 100644
+--- a/simplelru/lru.go
++++ b/simplelru/lru.go
+@@ -1,3 +1,4 @@
++// Package simplelru contains intefrace and implementation of linked-list based LRU cache.
+ package simplelru
+ 
+ import (
+@@ -98,11 +99,21 @@ func (c *LRU) Peek(key interface{}) (value interface{}, ok bool) {
+ // Remove removes the provided key from the cache, returning if the
+ // key was contained.
+ func (c *LRU) Remove(key interface{}) (present bool) {
+-	if ent, ok := c.items[key]; ok {
++	ent, present := c.items[key]
++	if present {
+ 		c.removeElement(ent)
+-		return true
+ 	}
+-	return false
++	return present
++}
++
++// RemoveWithoutEvict removes the provided key from the cache without triggering
++// onEvict callback, returning if the key was contained.
++func (c *LRU) RemoveWithoutEvict(key interface{}) (present bool) {
++	ent, present := c.items[key]
++	if present {
++		c.removeWithoutEvict(ent)
++	}
++	return present
+ }
+ 
+ // RemoveOldest removes the oldest item from the cache.
+@@ -152,10 +163,17 @@ func (c *LRU) removeOldest() {
+ 
+ // removeElement is used to remove a given list element from the cache
+ func (c *LRU) removeElement(e *list.Element) {
+-	c.evictList.Remove(e)
+-	kv := e.Value.(*entry)
+-	delete(c.items, kv.key)
++	kv := c.removeWithoutEvict(e)
+ 	if c.onEvict != nil {
+ 		c.onEvict(kv.key, kv.value)
+ 	}
+ }
++
++// removeWithoutEvict is used to remove a given list element from the cache
++// without triggering onEvict callback to be called.
++func (c *LRU) removeWithoutEvict(e *list.Element) *entry {
++	c.evictList.Remove(e)
++	kv := e.Value.(*entry)
++	delete(c.items, kv.key)
++	return kv
++}
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..be2cc4d
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,362 @@
+Mozilla Public License, version 2.0
+
+1. Definitions
+
+1.1. "Contributor"
+
+     means each individual or legal entity that creates, contributes to the
+     creation of, or owns Covered Software.
+
+1.2. "Contributor Version"
+
+     means the combination of the Contributions of others (if any) used by a
+     Contributor and that particular Contributor's Contribution.
+
+1.3. "Contribution"
+
+     means Covered Software of a particular Contributor.
+
+1.4. "Covered Software"
+
+     means Source Code Form to which the initial Contributor has attached the
+     notice in Exhibit A, the Executable Form of such Source Code Form, and
+     Modifications of such Source Code Form, in each case including portions
+     thereof.
+
+1.5. "Incompatible With Secondary Licenses"
+     means
+
+     a. that the initial Contributor has attached the notice described in
+        Exhibit B to the Covered Software; or
+
+     b. that the Covered Software was made available under the terms of
+        version 1.1 or earlier of the License, but not also under the terms of
+        a Secondary License.
+
+1.6. "Executable Form"
+
+     means any form of the work other than Source Code Form.
+
+1.7. "Larger Work"
+
+     means a work that combines Covered Software with other material, in a
+     separate file or files, that is not Covered Software.
+
+1.8. "License"
+
+     means this document.
+
+1.9. "Licensable"
+
+     means having the right to grant, to the maximum extent possible, whether
+     at the time of the initial grant or subsequently, any and all of the
+     rights conveyed by this License.
+
+1.10. "Modifications"
+
+     means any of the following:
+
+     a. any file in Source Code Form that results from an addition to,
+        deletion from, or modification of the contents of Covered Software; or
+
+     b. any new file in Source Code Form that contains any Covered Software.
+
+1.11. "Patent Claims" of a Contributor
+
+      means any patent claim(s), including without limitation, method,
+      process, and apparatus claims, in any patent Licensable by such
+      Contributor that would be infringed, but for the grant of the License,
+      by the making, using, selling, offering for sale, having made, import,
+      or transfer of either its Contributions or its Contributor Version.
+
+1.12. "Secondary License"
+
+      means either the GNU General Public License, Version 2.0, the GNU Lesser
+      General Public License, Version 2.1, the GNU Affero General Public
+      License, Version 3.0, or any later versions of those licenses.
+
+1.13. "Source Code Form"
+
+      means the form of the work preferred for making modifications.
+
+1.14. "You" (or "Your")
+
+      means an individual or a legal entity exercising rights under this
+      License. For legal entities, "You" includes any entity that controls, is
+      controlled by, or is under common control with You. For purposes of this
+      definition, "control" means (a) the power, direct or indirect, to cause
+      the direction or management of such entity, whether by contract or
+      otherwise, or (b) ownership of more than fifty percent (50%) of the
+      outstanding shares or beneficial ownership of such entity.
+
+
+2. License Grants and Conditions
+
+2.1. Grants
+
+     Each Contributor hereby grants You a world-wide, royalty-free,
+     non-exclusive license:
+
+     a. under intellectual property rights (other than patent or trademark)
+        Licensable by such Contributor to use, reproduce, make available,
+        modify, display, perform, distribute, and otherwise exploit its
+        Contributions, either on an unmodified basis, with Modifications, or
+        as part of a Larger Work; and
+
+     b. under Patent Claims of such Contributor to make, use, sell, offer for
+        sale, have made, import, and otherwise transfer either its
+        Contributions or its Contributor Version.
+
+2.2. Effective Date
+
+     The licenses granted in Section 2.1 with respect to any Contribution
+     become effective for each Contribution on the date the Contributor first
+     distributes such Contribution.
+
+2.3. Limitations on Grant Scope
+
+     The licenses granted in this Section 2 are the only rights granted under
+     this License. No additional rights or licenses will be implied from the
+     distribution or licensing of Covered Software under this License.
+     Notwithstanding Section 2.1(b) above, no patent license is granted by a
+     Contributor:
+
+     a. for any code that a Contributor has removed from Covered Software; or
+
+     b. for infringements caused by: (i) Your and any other third party's
+        modifications of Covered Software, or (ii) the combination of its
+        Contributions with other software (except as part of its Contributor
+        Version); or
+
+     c. under Patent Claims infringed by Covered Software in the absence of
+        its Contributions.
+
+     This License does not grant any rights in the trademarks, service marks,
+     or logos of any Contributor (except as may be necessary to comply with
+     the notice requirements in Section 3.4).
+
+2.4. Subsequent Licenses
+
+     No Contributor makes additional grants as a result of Your choice to
+     distribute the Covered Software under a subsequent version of this
+     License (see Section 10.2) or under the terms of a Secondary License (if
+     permitted under the terms of Section 3.3).
+
+2.5. Representation
+
+     Each Contributor represents that the Contributor believes its
+     Contributions are its original creation(s) or it has sufficient rights to
+     grant the rights to its Contributions conveyed by this License.
+
+2.6. Fair Use
+
+     This License is not intended to limit any rights You have under
+     applicable copyright doctrines of fair use, fair dealing, or other
+     equivalents.
+
+2.7. Conditions
+
+     Sections 3.1, 3.2, 3.3, and 3.4 are conditions of the licenses granted in
+     Section 2.1.
+
+
+3. Responsibilities
+
+3.1. Distribution of Source Form
+
+     All distribution of Covered Software in Source Code Form, including any
+     Modifications that You create or to which You contribute, must be under
+     the terms of this License. You must inform recipients that the Source
+     Code Form of the Covered Software is governed by the terms of this
+     License, and how they can obtain a copy of this License. You may not
+     attempt to alter or restrict the recipients' rights in the Source Code
+     Form.
+
+3.2. Distribution of Executable Form
+
+     If You distribute Covered Software in Executable Form then:
+
+     a. such Covered Software must also be made available in Source Code Form,
+        as described in Section 3.1, and You must inform recipients of the
+        Executable Form how they can obtain a copy of such Source Code Form by
+        reasonable means in a timely manner, at a charge no more than the cost
+        of distribution to the recipient; and
+
+     b. You may distribute such Executable Form under the terms of this
+        License, or sublicense it under different terms, provided that the
+        license for the Executable Form does not attempt to limit or alter the
+        recipients' rights in the Source Code Form under this License.
+
+3.3. Distribution of a Larger Work
+
+     You may create and distribute a Larger Work under terms of Your choice,
+     provided that You also comply with the requirements of this License for
+     the Covered Software. If the Larger Work is a combination of Covered
+     Software with a work governed by one or more Secondary Licenses, and the
+     Covered Software is not Incompatible With Secondary Licenses, this
+     License permits You to additionally distribute such Covered Software
+     under the terms of such Secondary License(s), so that the recipient of
+     the Larger Work may, at their option, further distribute the Covered
+     Software under the terms of either this License or such Secondary
+     License(s).
+
+3.4. Notices
+
+     You may not remove or alter the substance of any license notices
+     (including copyright notices, patent notices, disclaimers of warranty, or
+     limitations of liability) contained within the Source Code Form of the
+     Covered Software, except that You may alter any license notices to the
+     extent required to remedy known factual inaccuracies.
+
+3.5. Application of Additional Terms
+
+     You may choose to offer, and to charge a fee for, warranty, support,
+     indemnity or liability obligations to one or more recipients of Covered
+     Software. However, You may do so only on Your own behalf, and not on
+     behalf of any Contributor. You must make it absolutely clear that any
+     such warranty, support, indemnity, or liability obligation is offered by
+     You alone, and You hereby agree to indemnify every Contributor for any
+     liability incurred by such Contributor as a result of warranty, support,
+     indemnity or liability terms You offer. You may include additional
+     disclaimers of warranty and limitations of liability specific to any
+     jurisdiction.
+
+4. Inability to Comply Due to Statute or Regulation
+
+   If it is impossible for You to comply with any of the terms of this License
+   with respect to some or all of the Covered Software due to statute,
+   judicial order, or regulation then You must: (a) comply with the terms of
+   this License to the maximum extent possible; and (b) describe the
+   limitations and the code they affect. Such description must be placed in a
+   text file included with all distributions of the Covered Software under
+   this License. Except to the extent prohibited by statute or regulation,
+   such description must be sufficiently detailed for a recipient of ordinary
+   skill to be able to understand it.
+
+5. Termination
+
+5.1. The rights granted under this License will terminate automatically if You
+     fail to comply with any of its terms. However, if You become compliant,
+     then the rights granted under this License from a particular Contributor
+     are reinstated (a) provisionally, unless and until such Contributor
+     explicitly and finally terminates Your grants, and (b) on an ongoing
+     basis, if such Contributor fails to notify You of the non-compliance by
+     some reasonable means prior to 60 days after You have come back into
+     compliance. Moreover, Your grants from a particular Contributor are
+     reinstated on an ongoing basis if such Contributor notifies You of the
+     non-compliance by some reasonable means, this is the first time You have
+     received notice of non-compliance with this License from such
+     Contributor, and You become compliant prior to 30 days after Your receipt
+     of the notice.
+
+5.2. If You initiate litigation against any entity by asserting a patent
+     infringement claim (excluding declaratory judgment actions,
+     counter-claims, and cross-claims) alleging that a Contributor Version
+     directly or indirectly infringes any patent, then the rights granted to
+     You by any and all Contributors for the Covered Software under Section
+     2.1 of this License shall terminate.
+
+5.3. In the event of termination under Sections 5.1 or 5.2 above, all end user
+     license agreements (excluding distributors and resellers) which have been
+     validly granted by You or Your distributors under this License prior to
+     termination shall survive termination.
+
+6. Disclaimer of Warranty
+
+   Covered Software is provided under this License on an "as is" basis,
+   without warranty of any kind, either expressed, implied, or statutory,
+   including, without limitation, warranties that the Covered Software is free
+   of defects, merchantable, fit for a particular purpose or non-infringing.
+   The entire risk as to the quality and performance of the Covered Software
+   is with You. Should any Covered Software prove defective in any respect,
+   You (not any Contributor) assume the cost of any necessary servicing,
+   repair, or correction. This disclaimer of warranty constitutes an essential
+   part of this License. No use of  any Covered Software is authorized under
+   this License except under this disclaimer.
+
+7. Limitation of Liability
+
+   Under no circumstances and under no legal theory, whether tort (including
+   negligence), contract, or otherwise, shall any Contributor, or anyone who
+   distributes Covered Software as permitted above, be liable to You for any
+   direct, indirect, special, incidental, or consequential damages of any
+   character including, without limitation, damages for lost profits, loss of
+   goodwill, work stoppage, computer failure or malfunction, or any and all
+   other commercial damages or losses, even if such party shall have been
+   informed of the possibility of such damages. This limitation of liability
+   shall not apply to liability for death or personal injury resulting from
+   such party's negligence to the extent applicable law prohibits such
+   limitation. Some jurisdictions do not allow the exclusion or limitation of
+   incidental or consequential damages, so this exclusion and limitation may
+   not apply to You.
+
+8. Litigation
+
+   Any litigation relating to this License may be brought only in the courts
+   of a jurisdiction where the defendant maintains its principal place of
+   business and such litigation shall be governed by laws of that
+   jurisdiction, without reference to its conflict-of-law provisions. Nothing
+   in this Section shall prevent a party's ability to bring cross-claims or
+   counter-claims.
+
+9. Miscellaneous
+
+   This License represents the complete agreement concerning the subject
+   matter hereof. If any provision of this License is held to be
+   unenforceable, such provision shall be reformed only to the extent
+   necessary to make it enforceable. Any law or regulation which provides that
+   the language of a contract shall be construed against the drafter shall not
+   be used to construe this License against a Contributor.
+
+
+10. Versions of the License
+
+10.1. New Versions
+
+      Mozilla Foundation is the license steward. Except as provided in Section
+      10.3, no one other than the license steward has the right to modify or
+      publish new versions of this License. Each version will be given a
+      distinguishing version number.
+
+10.2. Effect of New Versions
+
+      You may distribute the Covered Software under the terms of the version
+      of the License under which You originally received the Covered Software,
+      or under the terms of any subsequent version published by the license
+      steward.
+
+10.3. Modified Versions
+
+      If you create software not governed by this License, and you want to
+      create a new license for such software, you may create and use a
+      modified version of this License if you rename the license and remove
+      any references to the name of the license steward (except to note that
+      such modified license differs from this License).
+
+10.4. Distributing Source Code Form that is Incompatible With Secondary
+      Licenses If You choose to distribute Source Code Form that is
+      Incompatible With Secondary Licenses under the terms of this version of
+      the License, the notice described in Exhibit B of this License must be
+      attached.
+
+Exhibit A - Source Code Form License Notice
+
+      This Source Code Form is subject to the
+      terms of the Mozilla Public License, v.
+      2.0. If a copy of the MPL was not
+      distributed with this file, You can
+      obtain one at
+      http://mozilla.org/MPL/2.0/.
+
+If it is not possible or desirable to put the notice in a particular file,
+then You may include the notice in a location (such as a LICENSE file in a
+relevant directory) where a recipient would be likely to look for such a
+notice.
+
+You may add additional accurate notices of copyright ownership.
+
+Exhibit B - "Incompatible With Secondary Licenses" Notice
+
+      This Source Code Form is "Incompatible
+      With Secondary Licenses", as defined by
+      the Mozilla Public License, v. 2.0.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..33e58cf
--- /dev/null
+++ b/README.md
@@ -0,0 +1,25 @@
+golang-lru
+==========
+
+This provides the `lru` package which implements a fixed-size
+thread safe LRU cache. It is based on the cache in Groupcache.
+
+Documentation
+=============
+
+Full docs are available on [Godoc](http://godoc.org/github.com/hashicorp/golang-lru)
+
+Example
+=======
+
+Using the LRU is very simple:
+
+```go
+l, _ := New(128)
+for i := 0; i < 256; i++ {
+    l.Add(i, nil)
+}
+if l.Len() != 128 {
+    panic(fmt.Sprintf("bad len: %v", l.Len()))
+}
+```
diff --git a/arc.go b/arc.go
new file mode 100644
index 0000000..a5ed936
--- /dev/null
+++ b/arc.go
@@ -0,0 +1,259 @@
+package lru
+
+import (
+	"sync"
+
+	"google3/third_party/golang/lru/simplelru/simplelru"
+)
+
+// ARCCache is a thread-safe fixed size Adaptive Replacement Cache (ARC).
+// ARC is an enhancement over the standard LRU cache in that tracks both
+// frequency and recency of use. This avoids a burst in access to new
+// entries from evicting the frequently used older entries. It adds some
+// additional tracking overhead to a standard LRU cache, computationally
+// it is roughly 2x the cost, and the extra memory overhead is linear
+// with the size of the cache. ARC has been patented by IBM, but is
+// similar to the TwoQueueCache (2Q) which requires setting parameters.
+type ARCCache struct {
+	size int // Size is the total capacity of the cache
+	p    int // P is the dynamic preference towards T1 or T2
+
+	t1 *simplelru.LRU // T1 is the LRU for recently accessed items
+	b1 *simplelru.LRU // B1 is the LRU for evictions from t1
+
+	t2 *simplelru.LRU // T2 is the LRU for frequently accessed items
+	b2 *simplelru.LRU // B2 is the LRU for evictions from t2
+
+	lock sync.RWMutex
+}
+
+// NewARC creates an ARC of the given size
+func NewARC(size int) (*ARCCache, error) {
+	return NewARCWithEvict(size, nil)
+}
+
+// NewARCWithEvict creates an ARC of the given size with onEvict callback.
+func NewARCWithEvict(size int, onEvicted func(key interface{}, value interface{})) (*ARCCache, error) {
+	callback := simplelru.EvictCallback(onEvicted)
+	// Create the sub LRUs
+	b1, err := simplelru.NewLRU(size, nil)
+	if err != nil {
+		return nil, err
+	}
+	b2, err := simplelru.NewLRU(size, nil)
+	if err != nil {
+		return nil, err
+	}
+	t1, err := simplelru.NewLRU(size, callback)
+	if err != nil {
+		return nil, err
+	}
+	t2, err := simplelru.NewLRU(size, callback)
+	if err != nil {
+		return nil, err
+	}
+
+	// Initialize the ARC
+	c := &ARCCache{
+		size: size,
+		p:    0,
+		t1:   t1,
+		b1:   b1,
+		t2:   t2,
+		b2:   b2,
+	}
+	return c, nil
+}
+
+// Get looks up a key's value from the cache.
+func (c *ARCCache) Get(key interface{}) (value interface{}, ok bool) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+
+	// If the value is contained in T1 (recent), then
+	// promote it to T2 (frequent)
+	if val, ok := c.t1.Peek(key); ok {
+		c.t1.RemoveWithoutEvict(key)
+		c.t2.Add(key, val)
+		return val, ok
+	}
+
+	// Check if the value is contained in T2 (frequent)
+	if val, ok := c.t2.Get(key); ok {
+		return val, ok
+	}
+
+	// No hit
+	return nil, false
+}
+
+// Add adds a value to the cache.
+func (c *ARCCache) Add(key, value interface{}) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+
+	// Check if the value is contained in T1 (recent), and potentially
+	// promote it to frequent T2
+	if c.t1.Contains(key) {
+		c.t1.RemoveWithoutEvict(key)
+		c.t2.Add(key, value)
+		return
+	}
+
+	// Check if the value is already in T2 (frequent) and update it
+	if c.t2.Contains(key) {
+		c.t2.Add(key, value)
+		return
+	}
+
+	// Check if this value was recently evicted as part of the
+	// recently used list
+	if c.b1.Contains(key) {
+		// T1 set is too small, increase P appropriately
+		delta := 1
+		b1Len := c.b1.Len()
+		b2Len := c.b2.Len()
+		if b2Len > b1Len {
+			delta = b2Len / b1Len
+		}
+		if c.p+delta >= c.size {
+			c.p = c.size
+		} else {
+			c.p += delta
+		}
+
+		// Potentially need to make room in the cache
+		if c.t1.Len()+c.t2.Len() >= c.size {
+			c.replace(false)
+		}
+
+		// Remove from B1
+		c.b1.Remove(key)
+
+		// Add the key to the frequently used list
+		c.t2.Add(key, value)
+		return
+	}
+
+	// Check if this value was recently evicted as part of the
+	// frequently used list
+	if c.b2.Contains(key) {
+		// T2 set is too small, decrease P appropriately
+		delta := 1
+		b1Len := c.b1.Len()
+		b2Len := c.b2.Len()
+		if b1Len > b2Len {
+			delta = b1Len / b2Len
+		}
+		if delta >= c.p {
+			c.p = 0
+		} else {
+			c.p -= delta
+		}
+
+		// Potentially need to make room in the cache
+		if c.t1.Len()+c.t2.Len() >= c.size {
+			c.replace(true)
+		}
+
+		// Remove from B2
+		c.b2.Remove(key)
+
+		// Add the key to the frequently used list
+		c.t2.Add(key, value)
+		return
+	}
+
+	// Potentially need to make room in the cache
+	if c.t1.Len()+c.t2.Len() >= c.size {
+		c.replace(false)
+	}
+
+	// Keep the size of the ghost buffers trim
+	if c.b1.Len() > c.size-c.p {
+		c.b1.RemoveOldest()
+	}
+	if c.b2.Len() > c.p {
+		c.b2.RemoveOldest()
+	}
+
+	// Add to the recently seen list
+	c.t1.Add(key, value)
+	return
+}
+
+// replace is used to adaptively evict from either T1 or T2
+// based on the current learned value of P
+func (c *ARCCache) replace(b2ContainsKey bool) {
+	t1Len := c.t1.Len()
+	if t1Len > 0 && (t1Len > c.p || (t1Len == c.p && b2ContainsKey)) {
+		k, _, ok := c.t1.RemoveOldest()
+		if ok {
+			c.b1.Add(k, nil)
+		}
+	} else {
+		k, _, ok := c.t2.RemoveOldest()
+		if ok {
+			c.b2.Add(k, nil)
+		}
+	}
+}
+
+// Len returns the number of cached entries
+func (c *ARCCache) Len() int {
+	c.lock.RLock()
+	defer c.lock.RUnlock()
+	return c.t1.Len() + c.t2.Len()
+}
+
+// Keys returns all the cached keys
+func (c *ARCCache) Keys() []interface{} {
+	c.lock.RLock()
+	defer c.lock.RUnlock()
+	k1 := c.t1.Keys()
+	k2 := c.t2.Keys()
+	return append(k1, k2...)
+}
+
+func (c *ARCCache) subcaches() [4]simplelru.LRUCache {
+	return [4]simplelru.LRUCache{c.t1, c.t2, c.b1, c.b2}
+}
+
+// Remove is used to purge a key from the cache
+func (c *ARCCache) Remove(key interface{}) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+	for _, cache := range c.subcaches() {
+		if cache.Remove(key) {
+			return
+		}
+	}
+}
+
+// Purge is used to clear the cache
+func (c *ARCCache) Purge() {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+	for _, cache := range c.subcaches() {
+		cache.Purge()
+	}
+}
+
+// Contains is used to check if the cache contains a key
+// without updating recency or frequency.
+func (c *ARCCache) Contains(key interface{}) bool {
+	c.lock.RLock()
+	defer c.lock.RUnlock()
+	return c.t1.Contains(key) || c.t2.Contains(key)
+}
+
+// Peek is used to inspect the cache value of a key
+// without updating recency or frequency.
+func (c *ARCCache) Peek(key interface{}) (value interface{}, ok bool) {
+	c.lock.RLock()
+	defer c.lock.RUnlock()
+	if val, ok := c.t1.Peek(key); ok {
+		return val, ok
+	}
+	return c.t2.Peek(key)
+}
diff --git a/arc_test.go b/arc_test.go
new file mode 100644
index 0000000..e597eda
--- /dev/null
+++ b/arc_test.go
@@ -0,0 +1,442 @@
+package lru
+
+import (
+	"math/rand"
+	"testing"
+	"time"
+
+	"google3/third_party/golang/cmp/cmp"
+)
+
+func init() {
+	rand.Seed(time.Now().Unix())
+}
+
+func BenchmarkARC_Rand(b *testing.B) {
+	l, err := NewARC(8192)
+	if err != nil {
+		b.Fatalf("err: %v", err)
+	}
+
+	trace := make([]int64, b.N*2)
+	for i := 0; i < b.N*2; i++ {
+		trace[i] = rand.Int63() % 32768
+	}
+
+	b.ResetTimer()
+
+	var hit, miss int
+	for i := 0; i < 2*b.N; i++ {
+		if i%2 == 0 {
+			l.Add(trace[i], trace[i])
+		} else {
+			_, ok := l.Get(trace[i])
+			if ok {
+				hit++
+			} else {
+				miss++
+			}
+		}
+	}
+	b.Logf("hit: %d miss: %d ratio: %f", hit, miss, float64(hit)/float64(miss))
+}
+
+func BenchmarkARC_Freq(b *testing.B) {
+	l, err := NewARC(8192)
+	if err != nil {
+		b.Fatalf("err: %v", err)
+	}
+
+	trace := make([]int64, b.N*2)
+	for i := 0; i < b.N*2; i++ {
+		if i%2 == 0 {
+			trace[i] = rand.Int63() % 16384
+		} else {
+			trace[i] = rand.Int63() % 32768
+		}
+	}
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		l.Add(trace[i], trace[i])
+	}
+	var hit, miss int
+	for i := 0; i < b.N; i++ {
+		_, ok := l.Get(trace[i])
+		if ok {
+			hit++
+		} else {
+			miss++
+		}
+	}
+	b.Logf("hit: %d miss: %d ratio: %f", hit, miss, float64(hit)/float64(miss))
+}
+
+func TestARC_RandomOps(t *testing.T) {
+	size := 128
+	l, err := NewARC(128)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	n := 200000
+	for i := 0; i < n; i++ {
+		key := rand.Int63() % 512
+		r := rand.Int63()
+		switch r % 3 {
+		case 0:
+			l.Add(key, key)
+		case 1:
+			l.Get(key)
+		case 2:
+			l.Remove(key)
+		}
+
+		if l.t1.Len()+l.t2.Len() > size {
+			t.Fatalf("bad: t1: %d t2: %d b1: %d b2: %d p: %d",
+				l.t1.Len(), l.t2.Len(), l.b1.Len(), l.b2.Len(), l.p)
+		}
+		if l.b1.Len()+l.b2.Len() > size {
+			t.Fatalf("bad: t1: %d t2: %d b1: %d b2: %d p: %d",
+				l.t1.Len(), l.t2.Len(), l.b1.Len(), l.b2.Len(), l.p)
+		}
+	}
+}
+
+func TestARC_Get_RecentToFrequent(t *testing.T) {
+	l, err := NewARC(128)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Touch all the entries, should be in t1
+	for i := 0; i < 128; i++ {
+		l.Add(i, i)
+	}
+	if n := l.t1.Len(); n != 128 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.t2.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Get should upgrade to t2
+	for i := 0; i < 128; i++ {
+		_, ok := l.Get(i)
+		if !ok {
+			t.Fatalf("missing: %d", i)
+		}
+	}
+	if n := l.t1.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.t2.Len(); n != 128 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Get be from t2
+	for i := 0; i < 128; i++ {
+		_, ok := l.Get(i)
+		if !ok {
+			t.Fatalf("missing: %d", i)
+		}
+	}
+	if n := l.t1.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.t2.Len(); n != 128 {
+		t.Fatalf("bad: %d", n)
+	}
+}
+
+func TestARC_Add_RecentToFrequent(t *testing.T) {
+	l, err := NewARC(128)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Add initially to t1
+	l.Add(1, 1)
+	if n := l.t1.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.t2.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Add should upgrade to t2
+	l.Add(1, 1)
+	if n := l.t1.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.t2.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Add should remain in t2
+	l.Add(1, 1)
+	if n := l.t1.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.t2.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+}
+
+func TestARC_Adaptive(t *testing.T) {
+	l, err := NewARC(4)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Fill t1
+	for i := 0; i < 4; i++ {
+		l.Add(i, i)
+	}
+	if n := l.t1.Len(); n != 4 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Move to t2
+	l.Get(0)
+	l.Get(1)
+	if n := l.t2.Len(); n != 2 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Evict from t1
+	l.Add(4, 4)
+	if n := l.b1.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Current state
+	// t1 : (MRU) [4, 3] (LRU)
+	// t2 : (MRU) [1, 0] (LRU)
+	// b1 : (MRU) [2] (LRU)
+	// b2 : (MRU) [] (LRU)
+
+	// Add 2, should cause hit on b1
+	l.Add(2, 2)
+	if n := l.b1.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+	if l.p != 1 {
+		t.Fatalf("bad: %d", l.p)
+	}
+	if n := l.t2.Len(); n != 3 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Current state
+	// t1 : (MRU) [4] (LRU)
+	// t2 : (MRU) [2, 1, 0] (LRU)
+	// b1 : (MRU) [3] (LRU)
+	// b2 : (MRU) [] (LRU)
+
+	// Add 4, should migrate to t2
+	l.Add(4, 4)
+	if n := l.t1.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.t2.Len(); n != 4 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Current state
+	// t1 : (MRU) [] (LRU)
+	// t2 : (MRU) [4, 2, 1, 0] (LRU)
+	// b1 : (MRU) [3] (LRU)
+	// b2 : (MRU) [] (LRU)
+
+	// Add 4, should evict to b2
+	l.Add(5, 5)
+	if n := l.t1.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.t2.Len(); n != 3 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.b2.Len(); n != 1 {
+		t.Fatalf("bad: %d", n)
+	}
+
+	// Current state
+	// t1 : (MRU) [5] (LRU)
+	// t2 : (MRU) [4, 2, 1] (LRU)
+	// b1 : (MRU) [3] (LRU)
+	// b2 : (MRU) [0] (LRU)
+
+	// Add 0, should decrease p
+	l.Add(0, 0)
+	if n := l.t1.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.t2.Len(); n != 4 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.b1.Len(); n != 2 {
+		t.Fatalf("bad: %d", n)
+	}
+	if n := l.b2.Len(); n != 0 {
+		t.Fatalf("bad: %d", n)
+	}
+	if l.p != 0 {
+		t.Fatalf("bad: %d", l.p)
+	}
+
+	// Current state
+	// t1 : (MRU) [] (LRU)
+	// t2 : (MRU) [0, 4, 2, 1] (LRU)
+	// b1 : (MRU) [5, 3] (LRU)
+	// b2 : (MRU) [0] (LRU)
+}
+
+func TestARC(t *testing.T) {
+	l, err := NewARC(128)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	for i := 0; i < 256; i++ {
+		l.Add(i, i)
+	}
+	if l.Len() != 128 {
+		t.Fatalf("bad len: %v", l.Len())
+	}
+
+	for i, k := range l.Keys() {
+		if v, ok := l.Get(k); !ok || v != k || v != i+128 {
+			t.Fatalf("bad key: %v", k)
+		}
+	}
+	for i := 0; i < 128; i++ {
+		_, ok := l.Get(i)
+		if ok {
+			t.Fatalf("should be evicted")
+		}
+	}
+	for i := 128; i < 256; i++ {
+		_, ok := l.Get(i)
+		if !ok {
+			t.Fatalf("should not be evicted")
+		}
+	}
+	for i := 128; i < 192; i++ {
+		l.Remove(i)
+		_, ok := l.Get(i)
+		if ok {
+			t.Fatalf("should be deleted")
+		}
+	}
+
+	l.Purge()
+	if l.Len() != 0 {
+		t.Fatalf("bad len: %v", l.Len())
+	}
+	if _, ok := l.Get(200); ok {
+		t.Fatalf("should contain nothing")
+	}
+}
+
+// Test that Contains doesn't update recent-ness
+func TestARC_Contains(t *testing.T) {
+	l, err := NewARC(2)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	l.Add(1, 1)
+	l.Add(2, 2)
+	if !l.Contains(1) {
+		t.Errorf("1 should be contained")
+	}
+
+	l.Add(3, 3)
+	if l.Contains(1) {
+		t.Errorf("Contains should not have updated recent-ness of 1")
+	}
+}
+
+// Test that Peek doesn't update recent-ness
+func TestARC_Peek(t *testing.T) {
+	l, err := NewARC(2)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	l.Add(1, 1)
+	l.Add(2, 2)
+	if v, ok := l.Peek(1); !ok || v != 1 {
+		t.Errorf("1 should be set to 1: %v, %v", v, ok)
+	}
+
+	l.Add(3, 3)
+	if l.Contains(1) {
+		t.Errorf("should not have updated recent-ness of 1")
+	}
+}
+
+func TestARC_OnEvictedCallback(t *testing.T) {
+	type testCase struct {
+		size    int
+		data    []int
+		evicted []int
+	}
+
+	testCases := map[string]testCase{
+		"basic-lru-like": {
+			size:    2,
+			data:    []int{1, 2, 3, 4},
+			evicted: []int{1, 2},
+		},
+		"t1-to-t2-promotion-simple": {
+			size:    4,
+			data:    []int{1, 2, 1, 2, 3, 4, 5, 6},
+			evicted: []int{3, 4},
+		},
+		"adaptive-replacement-simple": {
+			size:    4,
+			data:    []int{1, 2, 3, 4, 5, 6, 1, 2, 7, 8},
+			evicted: []int{1, 2, 3, 4, 1, 5},
+		},
+		"MFU-not-evicted-by-new-entries": {
+			size:    4,
+			data:    []int{4, 4, 1, 2, 3, 4, 5, 6, 7, 8},
+			evicted: []int{1, 2, 3, 5},
+		},
+		"t2-full-b2-empty-new-items": {
+			size: 4,
+			data: []int{
+				// fill t2, b2 should be empty since its target size is zero
+				1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8,
+				// insert "new" items
+				1, 2, 3, 4,
+			},
+			evicted: []int{1, 2, 3, 4, 5, 1, 2, 3},
+		},
+	}
+
+	runTest := func(t *testing.T, tc testCase) {
+		evicted := []int{}
+		c, err := NewARCWithEvict(tc.size, func(key interface{}, _ interface{}) {
+			evicted = append(evicted, key.(int))
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		for _, val := range tc.data {
+			c.Add(val, val)
+		}
+
+		if !cmp.Equal(tc.evicted, evicted) {
+			t.Errorf("Expectations mismatch.\nWant: %+v\nGot:  %+v", tc.evicted, evicted)
+		}
+	}
+
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) { runTest(t, tc) })
+	}
+}
diff --git a/doc.go b/doc.go
new file mode 100644
index 0000000..2547df9
--- /dev/null
+++ b/doc.go
@@ -0,0 +1,21 @@
+// Package lru provides three different LRU caches of varying sophistication.
+//
+// Cache is a simple LRU cache. It is based on the
+// LRU implementation in groupcache:
+// https://github.com/golang/groupcache/tree/master/lru
+//
+// TwoQueueCache tracks frequently used and recently used entries separately.
+// This avoids a burst of accesses from taking out frequently used entries,
+// at the cost of about 2x computational overhead and some extra bookkeeping.
+//
+// ARCCache is an adaptive replacement cache. It tracks recent evictions as
+// well as recent usage in both the frequent and recent caches. Its
+// computational overhead is comparable to TwoQueueCache, but the memory
+// overhead is linear with the size of the cache.
+//
+// ARC has been patented by IBM, so do not use it if that is problematic for
+// your program.
+//
+// All caches in this package take locks while operating, and are therefore
+// thread-safe for consumers.
+package lru
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..824cb97
--- /dev/null
+++ b/go.mod
@@ -0,0 +1 @@
+module github.com/hashicorp/golang-lru
diff --git a/lru.go b/lru.go
new file mode 100644
index 0000000..e0d5a59
--- /dev/null
+++ b/lru.go
@@ -0,0 +1,116 @@
+package lru
+
+import (
+	"sync"
+
+	"google3/third_party/golang/lru/simplelru/simplelru"
+)
+
+// Cache is a thread-safe fixed size LRU cache.
+type Cache struct {
+	lru  simplelru.LRUCache
+	lock sync.RWMutex
+}
+
+// New creates an LRU of the given size.
+func New(size int) (*Cache, error) {
+	return NewWithEvict(size, nil)
+}
+
+// NewWithEvict constructs a fixed size cache with the given eviction
+// callback.
+func NewWithEvict(size int, onEvicted func(key interface{}, value interface{})) (*Cache, error) {
+	lru, err := simplelru.NewLRU(size, simplelru.EvictCallback(onEvicted))
+	if err != nil {
+		return nil, err
+	}
+	c := &Cache{
+		lru: lru,
+	}
+	return c, nil
+}
+
+// Purge is used to completely clear the cache.
+func (c *Cache) Purge() {
+	c.lock.Lock()
+	c.lru.Purge()
+	c.lock.Unlock()
+}
+
+// Add adds a value to the cache.  Returns true if an eviction occurred.
+func (c *Cache) Add(key, value interface{}) (evicted bool) {
+	c.lock.Lock()
+	evicted = c.lru.Add(key, value)
+	c.lock.Unlock()
+	return evicted
+}
+
+// Get looks up a key's value from the cache.
+func (c *Cache) Get(key interface{}) (value interface{}, ok bool) {
+	c.lock.Lock()
+	value, ok = c.lru.Get(key)
+	c.lock.Unlock()
+	return value, ok
+}
+
+// Contains checks if a key is in the cache, without updating the
+// recent-ness or deleting it for being stale.
+func (c *Cache) Contains(key interface{}) bool {
+	c.lock.RLock()
+	containKey := c.lru.Contains(key)
+	c.lock.RUnlock()
+	return containKey
+}
+
+// Peek returns the key value (or undefined if not found) without updating
+// the "recently used"-ness of the key.
+func (c *Cache) Peek(key interface{}) (value interface{}, ok bool) {
+	c.lock.RLock()
+	value, ok = c.lru.Peek(key)
+	c.lock.RUnlock()
+	return value, ok
+}
+
+// ContainsOrAdd checks if a key is in the cache  without updating the
+// recent-ness or deleting it for being stale,  and if not, adds the value.
+// Returns whether found and whether an eviction occurred.
+func (c *Cache) ContainsOrAdd(key, value interface{}) (ok, evicted bool) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+
+	if c.lru.Contains(key) {
+		return true, false
+	}
+	evicted = c.lru.Add(key, value)
+	return false, evicted
+}
+
+// Remove removes the provided key from the cache.
+func (c *Cache) Remove(key interface{}) {
+	c.lock.Lock()
+	c.lru.Remove(key)
+	c.lock.Unlock()
+}
+
+// RemoveOldest removes the oldest item from the cache.
+func (c *Cache) RemoveOldest() {
+	c.lock.Lock()
+	c.lru.RemoveOldest()
+	c.lock.Unlock()
+}
+
+// Keys returns a slice of the keys in the cache, from oldest to newest.
+func (c *Cache) Keys() []interface{} {
+	c.lock.RLock()
+	keys := c.lru.Keys()
+	c.lock.RUnlock()
+	return keys
+}
+
+// Len returns the number of items in the cache.
+func (c *Cache) Len() int {
+	c.lock.RLock()
+	length := c.lru.Len()
+	c.lock.RUnlock()
+	return length
+}
diff --git a/lru_test.go b/lru_test.go
new file mode 100644
index 0000000..e7e2350
--- /dev/null
+++ b/lru_test.go
@@ -0,0 +1,221 @@
+package lru
+
+import (
+	"math/rand"
+	"testing"
+)
+
+func BenchmarkLRU_Rand(b *testing.B) {
+	l, err := New(8192)
+	if err != nil {
+		b.Fatalf("err: %v", err)
+	}
+
+	trace := make([]int64, b.N*2)
+	for i := 0; i < b.N*2; i++ {
+		trace[i] = rand.Int63() % 32768
+	}
+
+	b.ResetTimer()
+
+	var hit, miss int
+	for i := 0; i < 2*b.N; i++ {
+		if i%2 == 0 {
+			l.Add(trace[i], trace[i])
+		} else {
+			_, ok := l.Get(trace[i])
+			if ok {
+				hit++
+			} else {
+				miss++
+			}
+		}
+	}
+	b.Logf("hit: %d miss: %d ratio: %f", hit, miss, float64(hit)/float64(miss))
+}
+
+func BenchmarkLRU_Freq(b *testing.B) {
+	l, err := New(8192)
+	if err != nil {
+		b.Fatalf("err: %v", err)
+	}
+
+	trace := make([]int64, b.N*2)
+	for i := 0; i < b.N*2; i++ {
+		if i%2 == 0 {
+			trace[i] = rand.Int63() % 16384
+		} else {
+			trace[i] = rand.Int63() % 32768
+		}
+	}
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		l.Add(trace[i], trace[i])
+	}
+	var hit, miss int
+	for i := 0; i < b.N; i++ {
+		_, ok := l.Get(trace[i])
+		if ok {
+			hit++
+		} else {
+			miss++
+		}
+	}
+	b.Logf("hit: %d miss: %d ratio: %f", hit, miss, float64(hit)/float64(miss))
+}
+
+func TestLRU(t *testing.T) {
+	evictCounter := 0
+	onEvicted := func(k interface{}, v interface{}) {
+		if k != v {
+			t.Fatalf("Evict values not equal (%v!=%v)", k, v)
+		}
+		evictCounter++
+	}
+	l, err := NewWithEvict(128, onEvicted)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	for i := 0; i < 256; i++ {
+		l.Add(i, i)
+	}
+	if l.Len() != 128 {
+		t.Fatalf("bad len: %v", l.Len())
+	}
+
+	if evictCounter != 128 {
+		t.Fatalf("bad evict count: %v", evictCounter)
+	}
+
+	for i, k := range l.Keys() {
+		if v, ok := l.Get(k); !ok || v != k || v != i+128 {
+			t.Fatalf("bad key: %v", k)
+		}
+	}
+	for i := 0; i < 128; i++ {
+		_, ok := l.Get(i)
+		if ok {
+			t.Fatalf("should be evicted")
+		}
+	}
+	for i := 128; i < 256; i++ {
+		_, ok := l.Get(i)
+		if !ok {
+			t.Fatalf("should not be evicted")
+		}
+	}
+	for i := 128; i < 192; i++ {
+		l.Remove(i)
+		_, ok := l.Get(i)
+		if ok {
+			t.Fatalf("should be deleted")
+		}
+	}
+
+	l.Get(192) // expect 192 to be last key in l.Keys()
+
+	for i, k := range l.Keys() {
+		if (i < 63 && k != i+193) || (i == 63 && k != 192) {
+			t.Fatalf("out of order key: %v", k)
+		}
+	}
+
+	l.Purge()
+	if l.Len() != 0 {
+		t.Fatalf("bad len: %v", l.Len())
+	}
+	if _, ok := l.Get(200); ok {
+		t.Fatalf("should contain nothing")
+	}
+}
+
+// test that Add returns true/false if an eviction occurred
+func TestLRUAdd(t *testing.T) {
+	evictCounter := 0
+	onEvicted := func(k interface{}, v interface{}) {
+		evictCounter++
+	}
+
+	l, err := NewWithEvict(1, onEvicted)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	if l.Add(1, 1) == true || evictCounter != 0 {
+		t.Errorf("should not have an eviction")
+	}
+	if l.Add(2, 2) == false || evictCounter != 1 {
+		t.Errorf("should have an eviction")
+	}
+}
+
+// test that Contains doesn't update recent-ness
+func TestLRUContains(t *testing.T) {
+	l, err := New(2)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	l.Add(1, 1)
+	l.Add(2, 2)
+	if !l.Contains(1) {
+		t.Errorf("1 should be contained")
+	}
+
+	l.Add(3, 3)
+	if l.Contains(1) {
+		t.Errorf("Contains should not have updated recent-ness of 1")
+	}
+}
+
+// test that Contains doesn't update recent-ness
+func TestLRUContainsOrAdd(t *testing.T) {
+	l, err := New(2)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	l.Add(1, 1)
+	l.Add(2, 2)
+	contains, evict := l.ContainsOrAdd(1, 1)
+	if !contains {
+		t.Errorf("1 should be contained")
+	}
+	if evict {
+		t.Errorf("nothing should be evicted here")
+	}
+
+	l.Add(3, 3)
+	contains, evict = l.ContainsOrAdd(1, 1)
+	if contains {
+		t.Errorf("1 should not have been contained")
+	}
+	if !evict {
+		t.Errorf("an eviction should have occurred")
+	}
+	if !l.Contains(1) {
+		t.Errorf("now 1 should be contained")
+	}
+}
+
+// test that Peek doesn't update recent-ness
+func TestLRUPeek(t *testing.T) {
+	l, err := New(2)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	l.Add(1, 1)
+	l.Add(2, 2)
+	if v, ok := l.Peek(1); !ok || v != 1 {
+		t.Errorf("1 should be set to 1: %v, %v", v, ok)
+	}
+
+	l.Add(3, 3)
+	if l.Contains(1) {
+		t.Errorf("should not have updated recent-ness of 1")
+	}
+}
diff --git a/simplelru/lru.go b/simplelru/lru.go
new file mode 100644
index 0000000..0b40413
--- /dev/null
+++ b/simplelru/lru.go
@@ -0,0 +1,179 @@
+// Package simplelru contains intefrace and implementation of linked-list based LRU cache.
+package simplelru
+
+import (
+	"container/list"
+	"errors"
+)
+
+// EvictCallback is used to get a callback when a cache entry is evicted
+type EvictCallback func(key interface{}, value interface{})
+
+// LRU implements a non-thread safe fixed size LRU cache
+type LRU struct {
+	size      int
+	evictList *list.List
+	items     map[interface{}]*list.Element
+	onEvict   EvictCallback
+}
+
+// entry is used to hold a value in the evictList
+type entry struct {
+	key   interface{}
+	value interface{}
+}
+
+// NewLRU constructs an LRU of the given size
+func NewLRU(size int, onEvict EvictCallback) (*LRU, error) {
+	if size <= 0 {
+		return nil, errors.New("Must provide a positive size")
+	}
+	c := &LRU{
+		size:      size,
+		evictList: list.New(),
+		items:     make(map[interface{}]*list.Element),
+		onEvict:   onEvict,
+	}
+	return c, nil
+}
+
+// Purge is used to completely clear the cache.
+func (c *LRU) Purge() {
+	for k, v := range c.items {
+		if c.onEvict != nil {
+			c.onEvict(k, v.Value.(*entry).value)
+		}
+		delete(c.items, k)
+	}
+	c.evictList.Init()
+}
+
+// Add adds a value to the cache.  Returns true if an eviction occurred.
+func (c *LRU) Add(key, value interface{}) (evicted bool) {
+	// Check for existing item
+	if ent, ok := c.items[key]; ok {
+		c.evictList.MoveToFront(ent)
+		ent.Value.(*entry).value = value
+		return false
+	}
+
+	// Add new item
+	ent := &entry{key, value}
+	entry := c.evictList.PushFront(ent)
+	c.items[key] = entry
+
+	evict := c.evictList.Len() > c.size
+	// Verify size not exceeded
+	if evict {
+		c.removeOldest()
+	}
+	return evict
+}
+
+// Get looks up a key's value from the cache.
+func (c *LRU) Get(key interface{}) (value interface{}, ok bool) {
+	if ent, ok := c.items[key]; ok {
+		c.evictList.MoveToFront(ent)
+		return ent.Value.(*entry).value, true
+	}
+	return
+}
+
+// Contains checks if a key is in the cache, without updating the recent-ness
+// or deleting it for being stale.
+func (c *LRU) Contains(key interface{}) (ok bool) {
+	_, ok = c.items[key]
+	return ok
+}
+
+// Peek returns the key value (or undefined if not found) without updating
+// the "recently used"-ness of the key.
+func (c *LRU) Peek(key interface{}) (value interface{}, ok bool) {
+	var ent *list.Element
+	if ent, ok = c.items[key]; ok {
+		return ent.Value.(*entry).value, true
+	}
+	return nil, ok
+}
+
+// Remove removes the provided key from the cache, returning if the
+// key was contained.
+func (c *LRU) Remove(key interface{}) (present bool) {
+	ent, present := c.items[key]
+	if present {
+		c.removeElement(ent)
+	}
+	return present
+}
+
+// RemoveWithoutEvict removes the provided key from the cache without triggering
+// onEvict callback, returning if the key was contained.
+func (c *LRU) RemoveWithoutEvict(key interface{}) (present bool) {
+	ent, present := c.items[key]
+	if present {
+		c.removeWithoutEvict(ent)
+	}
+	return present
+}
+
+// RemoveOldest removes the oldest item from the cache.
+func (c *LRU) RemoveOldest() (key interface{}, value interface{}, ok bool) {
+	ent := c.evictList.Back()
+	if ent != nil {
+		c.removeElement(ent)
+		kv := ent.Value.(*entry)
+		return kv.key, kv.value, true
+	}
+	return nil, nil, false
+}
+
+// GetOldest returns the oldest entry
+func (c *LRU) GetOldest() (key interface{}, value interface{}, ok bool) {
+	ent := c.evictList.Back()
+	if ent != nil {
+		kv := ent.Value.(*entry)
+		return kv.key, kv.value, true
+	}
+	return nil, nil, false
+}
+
+// Keys returns a slice of the keys in the cache, from oldest to newest.
+func (c *LRU) Keys() []interface{} {
+	keys := make([]interface{}, len(c.items))
+	i := 0
+	for ent := c.evictList.Back(); ent != nil; ent = ent.Prev() {
+		keys[i] = ent.Value.(*entry).key
+		i++
+	}
+	return keys
+}
+
+// Len returns the number of items in the cache.
+func (c *LRU) Len() int {
+	return c.evictList.Len()
+}
+
+// removeOldest removes the oldest item from the cache.
+func (c *LRU) removeOldest() {
+	ent := c.evictList.Back()
+	if ent != nil {
+		c.removeElement(ent)
+	}
+}
+
+// removeElement is used to remove a given list element from the cache
+func (c *LRU) removeElement(e *list.Element) {
+	kv := c.removeWithoutEvict(e)
+	if c.onEvict != nil {
+		c.onEvict(kv.key, kv.value)
+	}
+}
+
+// removeWithoutEvict is used to remove a given list element from the cache
+// without triggering onEvict callback to be called.
+func (c *LRU) removeWithoutEvict(e *list.Element) *entry {
+	c.evictList.Remove(e)
+	kv := e.Value.(*entry)
+	delete(c.items, kv.key)
+	return kv
+}
diff --git a/simplelru/lru_interface.go b/simplelru/lru_interface.go
new file mode 100644
index 0000000..74c7077
--- /dev/null
+++ b/simplelru/lru_interface.go
@@ -0,0 +1,36 @@
+package simplelru
+
+// LRUCache is the interface for simple LRU cache.
+type LRUCache interface {
+	// Adds a value to the cache, returns true if an eviction occurred and
+	// updates the "recently used"-ness of the key.
+	Add(key, value interface{}) bool
+
+	// Returns key's value from the cache and
+	// updates the "recently used"-ness of the key. #value, isFound
+	Get(key interface{}) (value interface{}, ok bool)
+
+	// Check if a key exsists in cache without updating the recent-ness.
+	Contains(key interface{}) (ok bool)
+
+	// Returns key's value without updating the "recently used"-ness of the key.
+	Peek(key interface{}) (value interface{}, ok bool)
+
+	// Removes a key from the cache.
+	Remove(key interface{}) bool
+
+	// Removes the oldest entry from cache.
+	RemoveOldest() (interface{}, interface{}, bool)
+
+	// Returns the oldest entry from the cache. #key, value, isFound
+	GetOldest() (interface{}, interface{}, bool)
+
+	// Returns a slice of the keys in the cache, from oldest to newest.
+	Keys() []interface{}
+
+	// Returns the number of items in the cache.
+	Len() int
+
+	// Clear all cache entries
+	Purge()
+}
diff --git a/simplelru/lru_test.go b/simplelru/lru_test.go
new file mode 100644
index 0000000..ca5676e
--- /dev/null
+++ b/simplelru/lru_test.go
@@ -0,0 +1,167 @@
+package simplelru
+
+import "testing"
+
+func TestLRU(t *testing.T) {
+	evictCounter := 0
+	onEvicted := func(k interface{}, v interface{}) {
+		if k != v {
+			t.Fatalf("Evict values not equal (%v!=%v)", k, v)
+		}
+		evictCounter++
+	}
+	l, err := NewLRU(128, onEvicted)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	for i := 0; i < 256; i++ {
+		l.Add(i, i)
+	}
+	if l.Len() != 128 {
+		t.Fatalf("bad len: %v", l.Len())
+	}
+
+	if evictCounter != 128 {
+		t.Fatalf("bad evict count: %v", evictCounter)
+	}
+
+	for i, k := range l.Keys() {
+		if v, ok := l.Get(k); !ok || v != k || v != i+128 {
+			t.Fatalf("bad key: %v", k)
+		}
+	}
+	for i := 0; i < 128; i++ {
+		_, ok := l.Get(i)
+		if ok {
+			t.Fatalf("should be evicted")
+		}
+	}
+	for i := 128; i < 256; i++ {
+		_, ok := l.Get(i)
+		if !ok {
+			t.Fatalf("should not be evicted")
+		}
+	}
+	for i := 128; i < 192; i++ {
+		ok := l.Remove(i)
+		if !ok {
+			t.Fatalf("should be contained")
+		}
+		ok = l.Remove(i)
+		if ok {
+			t.Fatalf("should not be contained")
+		}
+		_, ok = l.Get(i)
+		if ok {
+			t.Fatalf("should be deleted")
+		}
+	}
+
+	l.Get(192) // expect 192 to be last key in l.Keys()
+
+	for i, k := range l.Keys() {
+		if (i < 63 && k != i+193) || (i == 63 && k != 192) {
+			t.Fatalf("out of order key: %v", k)
+		}
+	}
+
+	l.Purge()
+	if l.Len() != 0 {
+		t.Fatalf("bad len: %v", l.Len())
+	}
+	if _, ok := l.Get(200); ok {
+		t.Fatalf("should contain nothing")
+	}
+}
+
+func TestLRU_GetOldest_RemoveOldest(t *testing.T) {
+	l, err := NewLRU(128, nil)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	for i := 0; i < 256; i++ {
+		l.Add(i, i)
+	}
+	k, _, ok := l.GetOldest()
+	if !ok {
+		t.Fatalf("missing")
+	}
+	if k.(int) != 128 {
+		t.Fatalf("bad: %v", k)
+	}
+
+	k, _, ok = l.RemoveOldest()
+	if !ok {
+		t.Fatalf("missing")
+	}
+	if k.(int) != 128 {
+		t.Fatalf("bad: %v", k)
+	}
+
+	k, _, ok = l.RemoveOldest()
+	if !ok {
+		t.Fatalf("missing")
+	}
+	if k.(int) != 129 {
+		t.Fatalf("bad: %v", k)
+	}
+}
+
+// Test that Add returns true/false if an eviction occurred
+func TestLRU_Add(t *testing.T) {
+	evictCounter := 0
+	onEvicted := func(k interface{}, v interface{}) {
+		evictCounter++
+	}
+
+	l, err := NewLRU(1, onEvicted)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	if l.Add(1, 1) == true || evictCounter != 0 {
+		t.Errorf("should not have an eviction")
+	}
+	if l.Add(2, 2) == false || evictCounter != 1 {
+		t.Errorf("should have an eviction")
+	}
+}
+
+// Test that Contains doesn't update recent-ness
+func TestLRU_Contains(t *testing.T) {
+	l, err := NewLRU(2, nil)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	l.Add(1, 1)
+	l.Add(2, 2)
+	if !l.Contains(1) {
+		t.Errorf("1 should be contained")
+	}
+
+	l.Add(3, 3)
+	if l.Contains(1) {
+		t.Errorf("Contains should not have updated recent-ness of 1")
+	}
+}
+
+// Test that Peek doesn't update recent-ness
+func TestLRU_Peek(t *testing.T) {
+	l, err := NewLRU(2, nil)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	l.Add(1, 1)
+	l.Add(2, 2)
+	if v, ok := l.Peek(1); !ok || v != 1 {
+		t.Errorf("1 should be set to 1: %v, %v", v, ok)
+	}
+
+	l.Add(3, 3)
+	if l.Contains(1) {
+		t.Errorf("should not have updated recent-ness of 1")
+	}
+}