Fix ast.Walk to respect WalkStop

Fixes #97

ast/ast.go

@@ -468,20 +468,25 @@ type Walker func(n Node, entering bool) (WalkStatus, error)
 
 // Walk walks a AST tree by the depth first search algorithm.
 func Walk(n Node, walker Walker) error {
+	_, err := walkHelper(n, walker)
+	return err
+}
+
+func walkHelper(n Node, walker Walker) (WalkStatus, error) {
 	status, err := walker(n, true)
 	if err != nil || status == WalkStop {
-		return err
+		return status, err
 	}
 	if status != WalkSkipChildren {
 		for c := n.FirstChild(); c != nil; c = c.NextSibling() {
-			if err = Walk(c, walker); err != nil {
+			if st, err := walkHelper(c, walker); err != nil || st == WalkStop {
-				return err
+				return WalkStop, err
 			}
 		}
 	}
 	status, err = walker(n, false)
 	if err != nil || status == WalkStop {
-		return err
+		return WalkStop, err
 	}
-	return nil
+	return WalkContinue, nil
 }

ast/ast_test.go

@@ -1,6 +1,9 @@
 package ast
 
-import "testing"
+import (
+	"reflect"
+	"testing"
+)
 
 func TestRemoveChildren(t *testing.T) {
 	root := NewDocument()
@@ -16,3 +19,57 @@ func TestRemoveChildren(t *testing.T) {
 
 	t.Logf("%+v", node2.PreviousSibling())
 }
+
+func TestWalk(t *testing.T) {
+	tests := []struct {
+		name   string
+		node   Node
+		want   []NodeKind
+		action map[NodeKind]WalkStatus
+	}{
+		{
+			"visits all in depth first order",
+			node(NewDocument(), node(NewHeading(1), NewText()), NewLink()),
+			[]NodeKind{KindDocument, KindHeading, KindText, KindLink},
+			map[NodeKind]WalkStatus{},
+		},
+		{
+			"stops after heading",
+			node(NewDocument(), node(NewHeading(1), NewText()), NewLink()),
+			[]NodeKind{KindDocument, KindHeading},
+			map[NodeKind]WalkStatus{KindHeading: WalkStop},
+		},
+		{
+			"skip children",
+			node(NewDocument(), node(NewHeading(1), NewText()), NewLink()),
+			[]NodeKind{KindDocument, KindHeading, KindLink},
+			map[NodeKind]WalkStatus{KindHeading: WalkSkipChildren},
+		},
+	}
+	for _, tt := range tests {
+		var kinds []NodeKind
+		collectKinds := func(n Node, entering bool) (WalkStatus, error) {
+			if entering {
+				kinds = append(kinds, n.Kind())
+			}
+			if status, ok := tt.action[n.Kind()]; ok {
+				return status, nil
+			}
+			return WalkContinue, nil
+		}
+		t.Run(tt.name, func(t *testing.T) {
+			if err := Walk(tt.node, collectKinds); err != nil {
+				t.Errorf("Walk() error = %v", err)
+			} else if !reflect.DeepEqual(kinds, tt.want) {
+				t.Errorf("Walk() expected = %v, got = %v", tt.want, kinds)
+			}
+		})
+	}
+}
+
+func node(n Node, children ...Node) Node {
+	for _, c := range children {
+		n.AppendChild(n, c)
+	}
+	return n
+}