Make maps shareable (#47)

When serializing two maps that point to the same data, link them back
together.

internal/serde/reflect.go

@@ -294,13 +294,23 @@ func deserializePointedAt(d *Deserializer, t reflect.Type) reflect.Value {
 }
 
 func serializeMap(s *Serializer, t reflect.Type, p unsafe.Pointer) {
-	size := 0
 	r := reflect.NewAt(t, p).Elem()
+
 	if r.IsNil() {
-		size = -1
-	} else {
-		size = r.Len()
+		serializeVarint(s, 0)
+		return
 	}
+
+	mapptr := r.UnsafePointer()
+
+	id, new := s.assignPointerID(mapptr)
+	serializeVarint(s, int(id))
+	if !new {
+		return
+	}
+
+	size := r.Len()
+
 	serializeVarint(s, size)
 
 	// TODO: allocs
@@ -316,13 +326,27 @@ func serializeMap(s *Serializer, t reflect.Type, p unsafe.Pointer) {
 }
 
 func deserializeMap(d *Deserializer, t reflect.Type, p unsafe.Pointer) {
+	r := reflect.NewAt(t, p)
+
+	ptr, id := d.readPtr()
+	if id == 0 {
+		// nil map
+		return
+	}
+	if ptr != nil {
+		// already deserialized at ptr
+		existing := reflect.NewAt(t, ptr).Elem()
+		r.Elem().Set(existing)
+		return
+	}
+
 	n := deserializeVarint(d)
 	if n < 0 { // nil map
 		return
 	}
 	nv := reflect.MakeMapWithSize(t, n)
-	r := reflect.NewAt(t, p)
 	r.Elem().Set(nv)
+	d.store(id, p)
 	for i := 0; i < n; i++ {
 		k := reflect.New(t.Key())
 		DeserializeAny(d, t.Key(), k.UnsafePointer())

internal/serde/scan.go

@@ -149,6 +149,7 @@ func (c *containers) fixup(i int) {
 			c.remove(i + 1)
 			return
 		}
+		c.remove(i + 1)
 		// Array fully contains next container. Nothing to do
 		return
 	}

serde/serde_test.go

@@ -241,6 +241,32 @@ func TestReflectCustom(t *testing.T) {
 }
 
 func TestReflectSharing(t *testing.T) {
+	testReflect(t, "maps of ints", func(t *testing.T) {
+		m := map[int]int{1: 2, 3: 4}
+
+		type X struct {
+			a map[int]int
+			b map[int]int
+		}
+
+		x := X{
+			a: m,
+			b: m,
+		}
+
+		// make sure map is shared beforehand
+		x.a[5] = 6
+		assertEqual(t, 6, x.b[5])
+
+		serde.RegisterType[X]()
+
+		out := assertRoundTrip(t, x)
+
+		// check map is shared after
+		out.a[7] = 8
+		assertEqual(t, 8, out.b[7])
+	})
+
 	testReflect(t, "slice backing array", func(t *testing.T) {
 		data := make([]int, 10)
 		for i := range data {