vm: add reference counter to slots

pkg/vm/slot.go

@@ -3,18 +3,32 @@ package vm
 // Slot is a fixed-size slice of stack items.
 type Slot struct {
 	storage []StackItem
+	refs    *refCounter
 }
 
 // newSlot returns new slot of n items.
-func newSlot(n int) *Slot {
+func newSlot(n int, refs *refCounter) *Slot {
 	return &Slot{
 		storage: make([]StackItem, n),
+		refs:    refs,
 	}
 }
 
+func (v *VM) newSlot(n int) *Slot {
+	return newSlot(n, v.refs)
+}
+
 // Set sets i-th storage slot.
 func (s *Slot) Set(i int, item StackItem) {
+	if s.storage[i] == item {
+		return
+	}
+	old := s.storage[i]
 	s.storage[i] = item
+	if old != nil {
+		s.refs.Remove(old)
+	}
+	s.refs.Add(item)
 }
 
 // Get returns item contained in i-th slot.

pkg/vm/slot_test.go

@@ -8,7 +8,7 @@ import (
 )
 
 func TestSlot_Get(t *testing.T) {
-	s := newSlot(3)
+	s := newSlot(3, newRefCounter())
 	require.NotNil(t, s)
 	require.Equal(t, 3, s.Size())