vm: implement storage slots

pkg/vm/slot.go

@@ -0,0 +1,29 @@
+package vm
+
+// Slot is a fixed-size slice of stack items.
+type Slot struct {
+	storage []StackItem
+}
+
+// newSlot returns new slot of n items.
+func newSlot(n int) *Slot {
+	return &Slot{
+		storage: make([]StackItem, n),
+	}
+}
+
+// Set sets i-th storage slot.
+func (s *Slot) Set(i int, item StackItem) {
+	s.storage[i] = item
+}
+
+// Get returns item contained in i-th slot.
+func (s *Slot) Get(i int) StackItem {
+	if item := s.storage[i]; item != nil {
+		return item
+	}
+	return NullItem{}
+}
+
+// Size returns slot size.
+func (s *Slot) Size() int { return len(s.storage) }

pkg/vm/slot_test.go

@@ -0,0 +1,21 @@
+package vm
+
+import (
+	"math/big"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestSlot_Get(t *testing.T) {
+	s := newSlot(3)
+	require.NotNil(t, s)
+	require.Equal(t, 3, s.Size())
+
+	// NullItem is the default
+	item := s.Get(2)
+	require.Equal(t, NullItem{}, item)
+
+	s.Set(1, NewBigIntegerItem(big.NewInt(42)))
+	require.Equal(t, NewBigIntegerItem(big.NewInt(42)), s.Get(1))
+}