Add Invocation stack - convenience RAS

pkg/vm/stack/Int.go

@@ -11,8 +11,6 @@ type Int struct {
 // NewInt will convert a big integer into
 // a StackInteger
 func NewInt(val *big.Int) (*Int, error) {
-	// TOODO: check it is 32 bytes
-
 	return &Int{
 		abstractItem: &abstractItem{},
 		val:          val,

pkg/vm/stack/invocation.go

@@ -0,0 +1,53 @@
+package stack
+
+import "errors"
+
+// Invocation embeds a Random Access stack
+// Providing helper methods for the context object
+type Invocation struct{ RandomAccess }
+
+//NewInvocation will return a new
+// Invocation stack
+func NewInvocation() *Invocation {
+	return &Invocation{
+		RandomAccess{
+			vals: make([]Item, 0, StackAverageSize),
+		},
+	}
+}
+
+func (i *Invocation) peekContext(n uint16) (*Context, error) {
+	item, err := i.Peek(n)
+	if err != nil {
+		return nil, err
+	}
+	ctx, err := item.Context()
+	if err != nil {
+		return nil, err
+	}
+	return ctx, nil
+}
+
+// CurrentContext returns the current context on the invocation stack
+func (i *Invocation) CurrentContext() (*Context, error) {
+	return i.peekContext(0)
+}
+
+// CallingContext will return the cntext item
+// that will be called next.
+func (i *Invocation) CallingContext() (*Context, error) {
+	if i.Len() < 1 {
+		return nil, errors.New("Length of invocation stack is < 1, no calling context")
+	}
+	return i.peekContext(1)
+}
+
+// EntryContext will return the context item that
+// started the program
+func (i *Invocation) EntryContext() (*Context, error) {
+
+	// firstItemIndex refers to the first item
+	// that was popped on the stack
+	firstItemIndex := uint16(i.Len() - 1) // N.B. if this overflows because len is zero, then an error will be returned
+	return i.peekContext(firstItemIndex)
+}

pkg/vm/stack/stack.go

@@ -119,3 +119,15 @@ func (ras *RandomAccess) Peek(n uint16) (Item, error) {
 
 	return ras.vals[index], nil
 }
+
+// Convenience Functions
+
+// PopInt will remove the last stack item that was added
+// And cast it to an integer
+func (ras *RandomAccess) PopInt() (*Int, error) {
+	item, err := ras.Pop()
+	if err != nil {
+		return nil, err
+	}
+	return item.Integer()
+}