commit 7a03538a662167aaf830ad7f15516ca685f9c0fb
Author: Kyle Isom <kyle@imap.cc>
Date:   Tue Feb 8 20:05:49 2022 -0800

    Initial import.

diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..09098f8
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,5 @@
+module git.sr.ht/~kisom/mru
+
+go 1.17
+
+require github.com/benbjohnson/clock v1.3.0
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..c284fd6
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,2 @@
+github.com/benbjohnson/clock v1.3.0 h1:ip6w0uFQkncKQ979AypyG0ER7mqUSBdKLOgAle/AT8A=
+github.com/benbjohnson/clock v1.3.0/go.mod h1:J11/hYXuz8f4ySSvYwY0FKfm+ezbsZBKZxNJlLklBHA=
diff --git a/mru.go b/mru.go
new file mode 100644
index 0000000..6ef403d
--- /dev/null
+++ b/mru.go
@@ -0,0 +1,128 @@
+package mru
+
+import (
+	"errors"
+	"fmt"
+	"sort"
+
+	"github.com/benbjohnson/clock"
+)
+
+type Item struct {
+	V      interface{}
+	access int64
+}
+
+type Cache struct {
+	store  map[string]*Item
+	access *timestamps
+	cap    int
+	clock  clock.Clock
+}
+
+func New(cap int) *Cache {
+	return &Cache{
+		store:  map[string]*Item{},
+		access: newTimestamps(cap),
+		cap:    cap,
+		clock:  clock.New(),
+	}
+}
+
+func (c *Cache) Len() int {
+	return len(c.store)
+}
+
+// evict should remove the least-recently-used cache item.
+func (c *Cache) evict() {
+	k := c.access.K(0)
+	c.evictKey(k)
+}
+
+// evictKey should remove the entry given by the key item.
+func (c *Cache) evictKey(k string) {
+	delete(c.store, k)
+	i, ok := c.access.Find(k)
+	if !ok {
+		return
+	}
+
+	c.access.Delete(i)
+}
+
+func (c *Cache) sanityCheck() {
+	if len(c.store) != c.access.Len() {
+		panic(fmt.Sprintf("MRU cache is out of sync; store len = %d, access len = %d",
+			len(c.store), c.access.Len()))
+	}
+}
+
+func (c *Cache) ConsistencyCheck() error {
+	if err := c.access.ConsistencyCheck(); err != nil {
+		return err
+	}
+
+	if len(c.store) != c.access.Len() {
+		return fmt.Errorf("mru: cache is out of sync; store len = %d, access len = %d",
+			len(c.store), c.access.Len())
+	}
+
+	for i := range c.access.ts {
+		itm, ok := c.store[c.access.K(i)]
+		if !ok {
+			return errors.New("mru: key in access is not in store")
+		}
+
+		if c.access.T(i) != itm.access {
+			return fmt.Errorf("timestamps are out of sync (%d != %d)",
+				itm.access, c.access.T(i))
+		}
+	}
+
+	if !sort.IsSorted(c.access) {
+		return errors.New("mru: timestamps aren't sorted")
+	}
+
+	return nil
+}
+
+func (c *Cache) Store(k string, v interface{}) {
+	c.sanityCheck()
+
+	if len(c.store) == c.cap {
+		c.evict()
+	}
+
+	if _, ok := c.store[k]; ok {
+		c.evictKey(k)
+	}
+
+	itm := &Item{
+		V:      v,
+		access: c.clock.Now().UnixNano(),
+	}
+
+	c.store[k] = itm
+	c.access.Update(k, itm.access)
+}
+
+func (c *Cache) Get(k string) (interface{}, bool) {
+	c.sanityCheck()
+
+	itm, ok := c.store[k]
+	if !ok {
+		return nil, false
+	}
+
+	c.store[k].access = c.clock.Now().UnixNano()
+	c.access.Update(k, itm.access)
+	return itm.V, true
+}
+
+// Has will not update the timestamp on the item.
+func (c *Cache) Has(k string) bool {
+	c.sanityCheck()
+
+	_, ok := c.store[k]
+	return ok
+}
diff --git a/mru_test.go b/mru_test.go
new file mode 100644
index 0000000..d823b5b
--- /dev/null
+++ b/mru_test.go
@@ -0,0 +1,90 @@
+package mru
+
+import (
+	"testing"
+	"time"
+
+	"github.com/benbjohnson/clock"
+)
+
+func TestBasicCacheEviction(t *testing.T) {
+	mock := clock.NewMock()
+	c := New(2)
+	c.clock = mock
+
+	if err := c.ConsistencyCheck(); err != nil {
+		t.Fatal(err)
+	}
+
+	if c.Len() != 0 {
+		t.Fatal("cache should have size 0")
+	}
+
+	c.Store("raven", 1)
+	if err := c.ConsistencyCheck(); err != nil {
+		t.Fatal(err)
+	}
+
+	if len(c.store) != 1 {
+		t.Fatalf("store should have length=1, have length=%d", len(c.store))
+	}
+
+	mock.Add(time.Second)
+	c.Store("owl", 2)
+	if err := c.ConsistencyCheck(); err != nil {
+		t.Fatal(err)
+	}
+
+	if len(c.store) != 2 {
+		t.Fatalf("store should have length=2, have length=%d", len(c.store))
+	}
+
+	mock.Add(time.Second)
+	c.Store("goat", 3)
+	if err := c.ConsistencyCheck(); err != nil {
+		t.Fatal(err)
+	}
+
+	if len(c.store) != 2 {
+		t.Fatalf("store should have length=2, have length=%d", len(c.store))
+	}
+
+	mock.Add(time.Second)
+	v, ok := c.Get("owl")
+	if !ok {
+		t.Fatal("store should have an entry for owl")
+	}
+	if err := c.ConsistencyCheck(); err != nil {
+		t.Fatal(err)
+	}
+
+	itm, ok := v.(int)
+	if !ok {
+		t.Fatalf("stored item is not an int; have %T", v)
+	}
+	if err := c.ConsistencyCheck(); err != nil {
+		t.Fatal(err)
+	}
+
+	if itm != 2 {
+		t.Fatalf("stored item should be 2, have %d", itm)
+	}
+
+	mock.Add(time.Second)
+	c.Store("elk", 4)
+	if err := c.ConsistencyCheck(); err != nil {
+		t.Fatal(err)
+	}
+
+	if !c.Has("elk") {
+		t.Fatal("store should contain an entry for 'elk'")
+	}
+
+	if !c.Has("owl") {
+		t.Fatal("store should contain an entry for 'owl'")
+	}
+
+	if c.Has("goat") {
+		t.Fatal("store should not contain an entry for 'goat'")
+	}
+}
diff --git a/timestamps.go b/timestamps.go
new file mode 100644
index 0000000..37881ff
--- /dev/null
+++ b/timestamps.go
@@ -0,0 +1,100 @@
+package mru
+
+import (
+	"errors"
+	"fmt"
+	"sort"
+)
+
+// timestamps contains datastructures for maintaining a list of keys sortable
+// by timestamp.
+
+type timestamp struct {
+	t int64
+	k string
+}
+
+type timestamps struct {
+	ts  []timestamp
+	cap int
+}
+
+func newTimestamps(cap int) *timestamps {
+	return &timestamps{
+		ts:  make([]timestamp, 0, cap),
+		cap: cap,
+	}
+}
+
+func (ts *timestamps) K(i int) string {
+	return ts.ts[i].k
+}
+
+func (ts *timestamps) T(i int) int64 {
+	return ts.ts[i].t
+}
+
+func (ts *timestamps) Len() int {
+	return len(ts.ts)
+}
+
+func (ts *timestamps) Less(i, j int) bool {
+	return ts.ts[i].t < ts.ts[j].t
+}
+
+func (ts *timestamps) Swap(i, j int) {
+	ts.ts[i], ts.ts[j] = ts.ts[j], ts.ts[i]
+}
+
+func (ts *timestamps) Find(k string) (int, bool) {
+	for i := 0; i < len(ts.ts); i++ {
+		if ts.ts[i].k == k {
+			return i, true
+		}
+	}
+	return -1, false
+}
+
+func (ts *timestamps) Update(k string, t int64) bool {
+	i, ok := ts.Find(k)
+	if !ok {
+		ts.ts = append(ts.ts, timestamp{t, k})
+		sort.Sort(ts)
+		return false
+	}
+
+	ts.ts[i].t = t
+	sort.Sort(ts)
+	return true
+}
+
+func (ts *timestamps) ConsistencyCheck() error {
+	if !sort.IsSorted(ts) {
+		return errors.New("mru: timestamps are not sorted")
+	}
+
+	keys := map[string]bool{}
+	for i := range ts.ts {
+		if keys[ts.ts[i].k] {
+			return fmt.Errorf("duplicate key %s detected", ts.ts[i].k)
+		}
+		keys[ts.ts[i].k] = true
+	}
+
+	if len(keys) != len(ts.ts) {
+		return fmt.Errorf("mru: timestamp contains %d duplicate keys",
+			len(ts.ts)-len(keys))
+	}
+
+	return nil
+}
+
+func (ts *timestamps) Delete(i int) {
+	ts.ts = append(ts.ts[:i], ts.ts[i+1:]...)
+}
+
+func (ts *timestamps) Dump() {
+	for i := range ts.ts {
+		fmt.Printf("%d: %s, %d\n", i, ts.K(i), ts.T(i))
+	}
+}
diff --git a/timestamps_test.go b/timestamps_test.go
new file mode 100644
index 0000000..13e3bbf
--- /dev/null
+++ b/timestamps_test.go
@@ -0,0 +1,50 @@
+package mru
+
+import (
+	"testing"
+	"time"
+
+	"github.com/benbjohnson/clock"
+)
+
+func TestTimestamps(t *testing.T) {
+	ts := newTimestamps(3)
+	mock := clock.NewMock()
+
+	// raven
+	ts.Update("raven", mock.Now().UnixNano())
+
+	// raven, owl
+	mock.Add(time.Millisecond)
+
+	ts.Update("owl", mock.Now().UnixNano())
+
+	// raven, owl, goat
+	mock.Add(time.Second)
+	ts.Update("goat", mock.Now().UnixNano())
+
+	if err := ts.ConsistencyCheck(); err != nil {
+		t.Fatal(err)
+	}
+	mock.Add(time.Millisecond)
+
+	// raven, goat, owl
+	ts.Update("owl", mock.Now().UnixNano())
+	if err := ts.ConsistencyCheck(); err != nil {
+		t.Fatal(err)
+	}
+
+	// at this point, the keys should be raven, goat, owl.
+	if ts.K(0) != "raven" {
+		t.Fatalf("first key should be raven, have %s", ts.K(0))
+	}
+
+	if ts.K(1) != "goat" {
+		t.Fatalf("second key should be goat, have %s", ts.K(1))
+	}
+
+	if ts.K(2) != "owl" {
+		t.Fatalf("third key should be owl, have %s", ts.K(2))
+	}
+
+}