2 files changed, 118 insertions, 0 deletions
diff --git a/decaymap/decaymap.go b/decaymap/decaymap.go
new file mode 100644
index 0000000..edcbd1a
--- /dev/null
+++ b/decaymap/decaymap.go
@@ -0,0 +1,87 @@
+package decaymap
+
+import (
+	"sync"
+	"time"
+)
+
+func Zilch[T any]() T {
+	var zero T
+	return zero
+}
+
+// Impl is a lazy key->value map. It's a wrapper around a map and a mutex. If values exceed their time-to-live, they are pruned at Get time.
+type Impl[K comparable, V any] struct {
+	data map[K]decayMapEntry[V]
+	lock sync.RWMutex
+}
+
+type decayMapEntry[V any] struct {
+	Value  V
+	expiry time.Time
+}
+
+// New creates a new DecayMap of key type K and value type V.
+//
+// Key types must be comparable to work with maps.
+func New[K comparable, V any]() *Impl[K, V] {
+	return &Impl[K, V]{
+		data: make(map[K]decayMapEntry[V]),
+	}
+}
+
+// expire forcibly expires a key by setting its time-to-live one second in the past.
+func (m *Impl[K, V]) expire(key K) bool {
+	m.lock.RLock()
+	val, ok := m.data[key]
+	m.lock.RUnlock()
+
+	if !ok {
+		return false
+	}
+
+	m.lock.Lock()
+	val.expiry = time.Now().Add(-1 * time.Second)
+	m.data[key] = val
+	m.lock.Unlock()
+
+	return true
+}
+
+// Get gets a value from the DecayMap by key.
+//
+// If a value has expired, forcibly delete it if it was not updated.
+func (m *Impl[K, V]) Get(key K) (V, bool) {
+	m.lock.RLock()
+	value, ok := m.data[key]
+	m.lock.RUnlock()
+
+	if !ok {
+		return Zilch[V](), false
+	}
+
+	if time.Now().After(value.expiry) {
+		m.lock.Lock()
+		// Since previously reading m.data[key], the value may have been updated.
+		// Delete the entry only if the expiry time is still the same.
+		if m.data[key].expiry == value.expiry {
+			delete(m.data, key)
+		}
+		m.lock.Unlock()
+
+		return Zilch[V](), false
+	}
+
+	return value.Value, true
+}
+
+// Set sets a key value pair in the map.
+func (m *Impl[K, V]) Set(key K, value V, ttl time.Duration) {
+	m.lock.Lock()
+	defer m.lock.Unlock()
+
+	m.data[key] = decayMapEntry[V]{
+		Value:  value,
+		expiry: time.Now().Add(ttl),
+	}
+}
diff --git a/decaymap/decaymap_test.go b/decaymap/decaymap_test.go
new file mode 100644
index 0000000..c930e08
--- /dev/null
+++ b/decaymap/decaymap_test.go
@@ -0,0 +1,31 @@
+package decaymap
+
+import (
+	"testing"
+	"time"
+)
+
+func TestImpl(t *testing.T) {
+	dm := New[string, string]()
+
+	dm.Set("test", "hi", 5*time.Minute)
+
+	val, ok := dm.Get("test")
+	if !ok {
+		t.Error("somehow the test key was not set")
+	}
+
+	if val != "hi" {
+		t.Errorf("wanted value %q, got: %q", "hi", val)
+	}
+
+	ok = dm.expire("test")
+	if !ok {
+		t.Error("somehow could not force-expire the test key")
+	}
+
+	_, ok = dm.Get("test")
+	if ok {
+		t.Error("got value even though it was supposed to be expired")
+	}
+}