vm: add a TUCK implementation with tests

Also fix stack's InsertAt for the edge case of inserting into a position that
currently is empty (the first good TUCK test illustrates this).

pkg/vm/stack.go

@@ -132,23 +132,15 @@ func (s *Stack) insert(e, at *Element) *Element {
 	return e
 }
 
-// InsertBefore will insert the element before the mark on the stack.
-func (s *Stack) InsertBefore(e, mark *Element) *Element {
-	if mark == nil {
-		return nil
-	}
-	return s.insert(e, mark.prev)
-}
-
 // InsertAt will insert the given item (n) deep on the stack.
 // Be very careful using it and _always_ check both e and n before invocation
 // as it will silently do wrong things otherwise.
 func (s *Stack) InsertAt(e *Element, n int) *Element {
-	before := s.Peek(n)
+	before := s.Peek(n - 1)
 	if before == nil {
 		return nil
 	}
-	return s.InsertBefore(e, before)
+	return s.insert(e, before)
 }
 
 // Push pushes the given element on the stack.

pkg/vm/vm.go

@@ -290,6 +290,16 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 		v.estack.Push(a)
 		v.estack.Push(b)
 
+	case TUCK:
+		a := v.estack.Dup(0)
+		if a == nil {
+			panic("no top-level element found")
+		}
+		if v.estack.Len() < 2 {
+			panic("can't TUCK with a one-element stack")
+		}
+		v.estack.InsertAt(a, 2)
+
 	case XSWAP:
 		n := int(v.estack.Pop().BigInt().Int64())
 		if n < 0 {

pkg/vm/vm_test.go

@@ -339,6 +339,47 @@ func TestXTUCKgood(t *testing.T) {
 	assert.Equal(t, int64(topelement), vm.estack.Peek(xtuckdepth).BigInt().Int64())
 }
 
+func TestTUCKbadNoitems(t *testing.T) {
+	prog := makeProgram(TUCK)
+	vm := load(prog)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestTUCKbadNoitem(t *testing.T) {
+	prog := makeProgram(TUCK)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestTUCKgood(t *testing.T) {
+	prog := makeProgram(TUCK)
+	vm := load(prog)
+	vm.estack.PushVal(42)
+	vm.estack.PushVal(34)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(34), vm.estack.Peek(0).BigInt().Int64())
+	assert.Equal(t, int64(42), vm.estack.Peek(1).BigInt().Int64())
+	assert.Equal(t, int64(34), vm.estack.Peek(2).BigInt().Int64())
+}
+
+func TestTUCKgood2(t *testing.T) {
+	prog := makeProgram(TUCK)
+	vm := load(prog)
+	vm.estack.PushVal(11)
+	vm.estack.PushVal(42)
+	vm.estack.PushVal(34)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(34), vm.estack.Peek(0).BigInt().Int64())
+	assert.Equal(t, int64(42), vm.estack.Peek(1).BigInt().Int64())
+	assert.Equal(t, int64(34), vm.estack.Peek(2).BigInt().Int64())
+	assert.Equal(t, int64(11), vm.estack.Peek(3).BigInt().Int64())
+}
+
 func TestOVERbadNoitem(t *testing.T) {
 	prog := makeProgram(OVER)
 	vm := load(prog)