vm: fix XTUCK implementation, add tests

The code that we have actually implements XTUCK and not TUCK. And it's a bit
broken, so fix it and add some tests. The most interesting one (that required
to touch stack code) is the one when we have 1 element on the stack and are
trying to tell XTUCK to push 2 elements deep.

pkg/vm/stack.go

@@ -141,6 +141,8 @@ func (s *Stack) InsertBefore(e, mark *Element) *Element {
 }
 
 // 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)
 	if before == nil {

pkg/vm/vm.go

@@ -306,13 +306,19 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 			b.value = aval
 		}
 
-	case TUCK:
+	case XTUCK:
 		n := int(v.estack.Pop().BigInt().Int64())
-		if n <= 0 {
-			panic("OTUCK: invalid length")
+		if n < 0 {
+			panic("XTUCK: invalid length")
 		}
-
-		v.estack.InsertAt(v.estack.Peek(0), n)
+		a := v.estack.Dup(0)
+		if a == nil {
+			panic("no top-level element found")
+		}
+		if n > v.estack.Len() {
+			panic("can't push to the position specified")
+		}
+		v.estack.InsertAt(a, n)
 
 	case ROT:
 		c := v.estack.Pop()

pkg/vm/vm_test.go

@@ -296,6 +296,48 @@ func TestPICKgood(t *testing.T) {
 	assert.Equal(t, int64(result), vm.estack.Pop().BigInt().Int64())
 }
 
+func TestXTUCKbadNoitem(t *testing.T) {
+	prog := makeProgram(XTUCK)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestXTUCKbadNoN(t *testing.T) {
+	prog := makeProgram(XTUCK)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.estack.PushVal(2)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestXTUCKbadNegative(t *testing.T) {
+	prog := makeProgram(XTUCK)
+	vm := load(prog)
+	vm.estack.PushVal(-1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestXTUCKgood(t *testing.T) {
+	prog := makeProgram(XTUCK)
+	topelement := 5
+	xtuckdepth := 3
+	vm := load(prog)
+	vm.estack.PushVal(0)
+	vm.estack.PushVal(1)
+	vm.estack.PushVal(2)
+	vm.estack.PushVal(3)
+	vm.estack.PushVal(4)
+	vm.estack.PushVal(topelement)
+	vm.estack.PushVal(xtuckdepth)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(topelement), vm.estack.Peek(0).BigInt().Int64())
+	assert.Equal(t, int64(topelement), vm.estack.Peek(xtuckdepth).BigInt().Int64())
+}
 
 func makeProgram(opcodes ...Instruction) []byte {
 	prog := make([]byte, len(opcodes)+1) // RET