Fix positional predicate for the "ancestor" axis

query.go

@@ -147,6 +147,7 @@ type ancestorQuery struct {
 	name     string
 	iterator func() NodeNavigator
 	table    map[uint64]bool
+	pos      int
 
 	Self      bool
 	Input     query
@@ -164,6 +165,8 @@ func (a *ancestorQuery) Select(t iterator) NodeNavigator {
 			if node == nil {
 				return nil
 			}
+			// Reset position for a new input context node
+			a.pos = 0
 			first := true
 			node = node.Copy()
 			a.iterator = func() NodeNavigator {
@@ -186,6 +189,8 @@ func (a *ancestorQuery) Select(t iterator) NodeNavigator {
 			node_id := getHashCode(node.Copy())
 			if _, ok := a.table[node_id]; !ok {
 				a.table[node_id] = true
+				// Increase position for each matched node in current input context
+				a.pos++
 				return node
 			}
 		}
@@ -215,6 +220,13 @@ func (a *ancestorQuery) Properties() queryProp {
 	return queryProps.Position | queryProps.Count | queryProps.Cached | queryProps.Merge | queryProps.Reverse
 }
 
+// position returns the ordinal of the current matched node within the axis
+// traversal for the current input context node. This is required so numeric
+// predicates like [1] or [2] on the ancestor axis resolve in axis order.
+func (a *ancestorQuery) position() int {
+	return a.pos
+}
+
 // attributeQuery is an XPath attribute node query.(@*)
 type attributeQuery struct {
 	name     string

xpath_axes_test.go

@@ -32,6 +32,39 @@ func Test_ancestor(t *testing.T) {
 	//test_xpath_elements(t, employee_example, `//ancestor::name`, 4, 9, 14)
 }
 
+func Test_ancestor_predicate(t *testing.T) {
+	doc := createElement(0, "",
+		createElement(1, "html",
+			createElement(2, "body",
+				createElement(3, "h1"),
+				createElement(4, "section",
+					createElement(5, "div",
+						createElement(6, "section",
+							createElement(7, "div",
+								createElement(8, "span"),
+							),
+						),
+					),
+				),
+				createElement(9, "section",
+					createElement(10, "div",
+						createElement(11, "section",
+							createElement(12, "div",
+								createElement(13, "span"),
+							),
+						),
+					),
+				),
+			),
+		),
+	)
+
+	test_xpath_elements(t, doc, `//span/ancestor::*`, 7, 6, 5, 4, 2, 1, 12, 11, 10, 9)
+	test_xpath_elements(t, doc, `//span/ancestor::section`, 6, 4, 11, 9)
+	test_xpath_elements(t, doc, `//span/ancestor::section[1]`, 6, 11)
+	test_xpath_elements(t, doc, `//span/ancestor::section[2]`, 4, 9)
+}
+
 func Test_ancestor_or_self(t *testing.T) {
 	// Expected the value is [2, 3, 8, 13], but got [3, 2, 8, 13]
 	test_xpath_elements(t, employee_example, `//employee/ancestor-or-self::*`, 3, 2, 8, 13)

xpath_test.go

@@ -587,6 +587,26 @@ func (n *TNode) getAttribute(key string) string {
 	return ""
 }
 
+func createElement(line int, name string, children ...*TNode) *TNode {
+	nodeType := ElementNode
+	if name == "" {
+		nodeType = RootNode
+	}
+	n := createNode(name, nodeType)
+	n.lines = line
+	for _, c := range children {
+		c.Parent = n
+		c.PrevSibling = n.LastChild
+		if c.PrevSibling == nil {
+			n.FirstChild = c
+		} else {
+			c.PrevSibling.NextSibling = c
+		}
+		n.LastChild = c
+	}
+	return n
+}
+
 func createBookExample() *TNode {
 	/*
 	   <?xml version="1.0" encoding="UTF-8"?>