Fix positional predicate for the "ancestor" axis

The expression `/ancestor::div[1]` should select only the first "div" node
while traversing up the document tree, but this wasn't the case.

This fixes positional predicates by having ancestorQuery implement the Position
interface.

The test was written by hand by me and the implementation was helped by GitHub
Copilot w/ GPT-5.

Author: mislav

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"?>