Support permanent storage

README.md

@@ -30,7 +30,7 @@ The latest version of the library is available at `main` branch of this reposito
 import "github.com/kshard/atom"
 
 // Create new atoms table 
-atoms := atom.New(atom.NewMemMap())
+atoms := atom.New(atom.NewEphemeralMap())
 
 // Convert string to atom
 code := atoms.Atom("String interning")
@@ -39,6 +39,8 @@ code := atoms.Atom("String interning")
 atoms.String(code)
 ```
 
+Use `atom.NewPermanentMap()` to implement atoms backed by either local permanent or external storage.
+
 ## How To Contribute
 
 The library is [MIT](LICENSE) licensed and accepts contributions via GitHub pull requests:

examples/main.go

@@ -20,7 +20,7 @@ const n = 1000000
 func main() {
 	var err error
 
-	atoms := atom.New(atom.NewMemMap())
+	atoms := atom.New(atom.NewEphemeralMap())
 	ids := make([]atom.Atom, n)
 
 	for i := 0; i < n; i++ {

hashmap.go

@@ -8,20 +8,26 @@
 
 package atom
 
-import "sync"
+import (
+	"encoding/binary"
+	"sync"
+	"unsafe"
+)
 
-type hashmap struct {
+//------------------------------------------------------------------------------
+
+type ephemeral struct {
 	sync.RWMutex
 	kv map[Atom]string
 }
 
-func NewMemMap() HashMap {
-	return &hashmap{
+func NewEphemeralMap() HashMap {
+	return &ephemeral{
 		kv: make(map[Atom]string),
 	}
 }
 
-func (m *hashmap) Get(key Atom) (string, error) {
+func (m *ephemeral) Get(key Atom) (string, error) {
 	m.RLock()
 	val, has := m.kv[key]
 	m.RUnlock()
@@ -32,10 +38,54 @@ func (m *hashmap) Get(key Atom) (string, error) {
 	return val, nil
 }
 
-func (m *hashmap) Put(key Atom, val string) error {
+func (m *ephemeral) Put(key Atom, val string) error {
 	m.Lock()
 	m.kv[key] = val
 	m.Unlock()
 
 	return nil
 }
+
+//------------------------------------------------------------------------------
+
+type permanent struct {
+	store Store
+}
+
+func NewPermanentMap(store Store) HashMap {
+	return &permanent{store: store}
+}
+
+func (m *permanent) Get(key Atom) (string, error) {
+	var bkey [5]byte
+	bkey[0] = ':'
+	binary.LittleEndian.PutUint32(bkey[1:], key)
+
+	val, err := m.store.Get(bkey[:])
+	if err != nil {
+		return "", err
+	}
+
+	// This is copied from runtime. It relies on the string
+	// header being a prefix of the slice header!
+	str := *(*string)(unsafe.Pointer(&val))
+
+	return str, nil
+}
+
+func (m *permanent) Put(key Atom, val string) error {
+	var bkey [5]byte
+	bkey[0] = ':'
+	binary.LittleEndian.PutUint32(bkey[1:], key)
+
+	// This is copied from runtime. It relies on the string
+	// header being a prefix of the slice header!
+	bval := *(*[]byte)(unsafe.Pointer(&val))
+
+	err := m.store.Put(bkey[:], bval)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}

symbol_test.go

@@ -1,6 +1,7 @@
 package atom_test
 
 import (
+	"bytes"
 	"strconv"
 	"testing"
 	"time"
@@ -14,8 +15,29 @@ const (
 	sc = "interning"
 )
 
-func TestPut(t *testing.T) {
-	s := atom.New(atom.NewMemMap())
+func TestEphemeralPut(t *testing.T) {
+	s := atom.New(atom.NewEphemeralMap())
+
+	for val, expected := range map[string]uint32{
+		sa: 1247594388,
+		sb: 3572195896,
+		sc: 1304336027,
+	} {
+		sym, err := s.Atom(val)
+		if err != nil {
+			t.Errorf("failed to assign symbol: %s", err)
+		}
+		if sym != expected {
+			t.Errorf("failed to assign symbol: %d, expected %d", sym, expected)
+		}
+		if val != s.String(sym) {
+			t.Errorf("failed to lookup string")
+		}
+	}
+}
+
+func TestPermanentPut(t *testing.T) {
+	s := atom.New(atom.NewPermanentMap(&none{}))
 
 	for val, expected := range map[string]uint32{
 		sa: 1247594388,
@@ -38,8 +60,21 @@ func TestPut(t *testing.T) {
 // ---------------------------------------------------------------
 
 // go test -run=^$ -bench=. -cpu=1 -benchtime=10s -cpuprofile profile.out
-func BenchmarkPut(b *testing.B) {
-	s := atom.New(atom.NewMemMap())
+func BenchmarkEphemeralPut(b *testing.B) {
+	s := atom.New(atom.NewEphemeralMap())
+
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	t := time.Now().Nanosecond()
+
+	for n := 0; n < b.N; n++ {
+		s.Atom("https://pkg.go.dev/hash/fnv@go1.20." + strconv.Itoa(t+n))
+	}
+}
+
+func BenchmarkPermanentPut(b *testing.B) {
+	s := atom.New(atom.NewPermanentMap(&none{}))
 
 	b.ReportAllocs()
 	b.ResetTimer()
@@ -55,7 +90,7 @@ func BenchmarkPut(b *testing.B) {
 
 // go test -fuzz=FuzzSymbolOf
 func FuzzSymbolOf(f *testing.F) {
-	s := atom.New(atom.NewMemMap())
+	s := atom.New(atom.NewEphemeralMap())
 
 	f.Add("abc")
 
@@ -66,3 +101,24 @@ func FuzzSymbolOf(f *testing.F) {
 		}
 	})
 }
+
+// ---------------------------------------------------------------
+
+type none struct {
+	key []byte
+	val []byte
+}
+
+func (n *none) Get(key []byte) ([]byte, error) {
+	if bytes.Equal(n.key, key) {
+		return n.val, nil
+	}
+
+	return nil, nil
+}
+
+func (n *none) Put(key []byte, val []byte) error {
+	n.key = key
+	n.val = val
+	return nil
+}

types.go

@@ -21,3 +21,9 @@ type HashMap interface {
 	Getter
 	Putter
 }
+
+// abstraction of permanent storage
+type Store interface {
+	Get([]byte) ([]byte, error)
+	Put([]byte, []byte) error
+}