From 725ee335ec2d0247bf1959bb9e38ca94d572a672 Mon Sep 17 00:00:00 2001
From: lash <dev@holbrook.no>
Date: Fri, 31 Mar 2023 20:05:57 +0100
Subject: [PATCH] Add back handler

---
 go/vm/vm.go      | 10 +++++++++
 go/vm/vm_test.go | 57 +++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 66 insertions(+), 1 deletion(-)

diff --git a/go/vm/vm.go b/go/vm/vm.go
index 19b1779..dff702f 100644
--- a/go/vm/vm.go
+++ b/go/vm/vm.go
@@ -38,6 +38,8 @@ func Apply(input []byte, instruction []byte, st state.State, rs resource.Fetcher
 	} 
 	if sym == "" {
 		instruction = NewLine([]byte{}, MOVE, []string{"_catch"}, nil, nil)
+	} else if sym == "_" {
+		instruction = NewLine([]byte{}, BACK, nil, nil, nil)
 	} else {
 		new_instruction := NewLine([]byte{}, MOVE, []string{sym}, nil, nil)
 		instruction = append(new_instruction, instruction...)
@@ -80,6 +82,9 @@ func Run(instruction []byte, st state.State, rs resource.Fetcher, ctx context.Co
 		case MOVE:
 			st, instruction, err = RunMove(instruction[2:], st, rs, ctx)
 			break
+		case BACK:
+			st, instruction, err = RunBack(instruction[2:], st, rs, ctx)
+			break
 		default:
 			err = fmt.Errorf("Unhandled state: %v", op)
 		}
@@ -169,6 +174,11 @@ func RunMove(instruction []byte, st state.State, rs resource.Fetcher, ctx contex
 	return st, tail, nil
 }
 
+func RunBack(instruction []byte, st state.State, rs resource.Fetcher, ctx context.Context) (state.State, []byte, error) {
+	st.Up()
+	return st, instruction, nil
+}
+
 func refresh(key string, sym []byte, rs resource.Fetcher, ctx context.Context) (string, error) {
 	fn, err := rs.FuncFor(key)
 	if err != nil {
diff --git a/go/vm/vm_test.go b/go/vm/vm_test.go
index 7f5afa3..4dab858 100644
--- a/go/vm/vm_test.go
+++ b/go/vm/vm_test.go
@@ -250,9 +250,10 @@ func TestRunArgInvalid(t *testing.T) {
 
 func TestRunArgInstructions(t *testing.T) {
 	st := state.NewState(5)
+	rs := TestResource{}
+
 	rt := router.NewRouter()
 	rt.Add("foo", "bar")
-	rs := TestResource{}
 	b := []byte{0x03}
 	b = append(b, []byte("foo")...)
 	b = append(b, rt.ToBytes()...)
@@ -284,3 +285,57 @@ func TestRunArgInstructions(t *testing.T) {
 	}
 	_ = r
 }
+
+func TestRunMoveAndBack(t *testing.T) {
+	st := state.NewState(5)
+	rs := TestResource{}
+	rt := router.NewRouter()
+	rt.Add("foo", "bar")
+	b := []byte{0x03}
+	b = append(b, []byte("foo")...)
+	b = append(b, rt.ToBytes()...)
+	bi := NewLine([]byte{}, LOAD, []string{"one"}, nil, []uint8{0})
+
+	var err error
+	st, b, err = Apply(b, bi, st, &rs, context.TODO())
+	if err != nil {
+		t.Error(err)
+	}
+	l := len(b)
+	if l != 0 {
+		t.Errorf("expected empty remainder, got length %v: %v", l, b)
+	}
+
+	rt = router.NewRouter()
+	rt.Add("foo", "baz")
+	b = []byte{0x03}
+	b = append(b, []byte("foo")...)
+	b = append(b, rt.ToBytes()...)
+	bi = NewLine([]byte{}, LOAD, []string{"two"}, nil, []uint8{0})
+	st, b, err = Apply(b, bi, st, &rs, context.TODO())
+	if err != nil {
+		t.Error(err)
+	}
+	l = len(b)
+	if l != 0 {
+		t.Errorf("expected empty remainder, got length %v: %v", l, b)
+	}
+
+	rt = router.NewRouter()
+	rt.Add("foo", "_")
+	b = []byte{0x03}
+	b = append(b, []byte("foo")...)
+	b = append(b, rt.ToBytes()...)
+	st, b, err = Apply(b, []byte{}, st, &rs, context.TODO())
+	if err != nil {
+		t.Error(err)
+	}
+	l = len(b)
+	if l != 0 {
+		t.Errorf("expected empty remainder, got length %v: %v", l, b)
+	}
+	loc := st.Where()
+	if loc != "bar" {
+		t.Errorf("expected where-string 'bar', got %v", loc)
+	}
+}