add support for absolute path queries.

Paths that are prefixed with '/' or '//' are now handled.  If a
path is prefixed with '//', it will perform an all-descendant
search starting from the root.  If a path is prefixed with a
single '/', it will perform a search starting from the root.

THIS IS A BREAKING CHANGE FOR '//' QUERIES.

Previously, a path that was prefixed with '//' would begin from
the current element, not from the root. To retain the old
behavior, the path must be prefixed with './/' instead of '//'.

path.go

@@ -31,8 +31,11 @@ only the following limited syntax is supported:
 
 Examples:
 
-Select the title elements of all descendant book elements having a
-'category' attribute of 'WEB':
+Starting from the root element, select the bookstore child element:
+	/bookstore
+
+Starting from the root element, select the title elements of all descendant
+book elements having a 'category' attribute of 'WEB':
     //book[@category='WEB']/title
 
 Select the first book element with a title child containing the text
@@ -49,7 +52,7 @@ containing the text 'special':
 
 Select all descendant book elements whose title element has an attribute
 'language' set to 'french':
-    //book/title[@language='french']/..
+    .//book/title[@language='french']/..
 
 */
 type Path struct {
@@ -180,22 +183,20 @@ type compiler struct {
 // through an element tree and returns a slice of segment
 // descriptors.
 func (c *compiler) parsePath(path string) []segment {
-	// If path starts or ends with //, fix it
-	if strings.HasPrefix(path, "//") {
-		path = "." + path
-	}
+	// If path ends with //, fix it
 	if strings.HasSuffix(path, "//") {
 		path = path + "*"
 	}
 
-	// Paths cannot be absolute
+	var segments []segment
+
+	// Check for an absolute path
 	if strings.HasPrefix(path, "/") {
-		c.err = ErrPath("paths cannot be absolute.")
-		return nil
+		segments = append(segments, segment{new(selectRoot), []filter{}})
+		path = path[1:]
 	}
 
-	// Split path into segment objects
-	var segments []segment
+	// Split path into segments
 	for _, s := range splitPath(path) {
 		segments = append(segments, c.parseSegment(s))
 		if c.err != ErrPath("") {
@@ -225,7 +226,7 @@ func (c *compiler) parseSegment(path string) segment {
 	pieces := strings.Split(path, "[")
 	seg := segment{
 		sel:     c.parseSelector(pieces[0]),
-		filters: make([]filter, 0),
+		filters: []filter{},
 	}
 	for i := 1; i < len(pieces); i++ {
 		fpath := pieces[i]
@@ -305,6 +306,17 @@ func (s *selectSelf) apply(e *Element, p *pather) {
 	p.candidates = append(p.candidates, e)
 }
 
+// selectRoot selects the element's root node.
+type selectRoot struct{}
+
+func (s *selectRoot) apply(e *Element, p *pather) {
+	root := e
+	for root.parent != nil {
+		root = root.parent
+	}
+	p.candidates = append(p.candidates, root)
+}
+
 // selectParent selects the element's parent into the candidate list.
 type selectParent struct{}
 

path_test.go

@@ -112,8 +112,19 @@ var tests = []test{
 	// parent queries
 	{"./bookstore/book[@category='COOKING']/title/../../book[4]/title", "Learning XML"},
 
+	// root queries
+	{"/bookstore/book[1]/title", "Everyday Italian"},
+	{"/bookstore/book[4]/title", "Learning XML"},
+	{"/bookstore/book[5]/title", nil},
+	{"/bookstore/book[3]/author[0]", "James McGovern"},
+	{"/bookstore/book[3]/author[1]", "James McGovern"},
+	{"/bookstore/book[3]/author[3]/./.", "Kurt Cagle"},
+	{"/bookstore/book[3]/author[6]", nil},
+	{"/bookstore/book[-1]/title", "Learning XML"},
+	{"/bookstore/book[-4]/title", "Everyday Italian"},
+	{"/bookstore/book[-5]/title", nil},
+
 	// bad paths
-	{"/bookstore", errorResult("etree: paths cannot be absolute.")},
 	{"./bookstore/book[]", errorResult("etree: path contains an empty filter expression.")},
 	{"./bookstore/book[@category='WEB'", errorResult("etree: path has invalid filter [brackets].")},
 	{"./bookstore/book[@category='WEB]", errorResult("etree: path has mismatched filter quotes.")},
@@ -171,3 +182,24 @@ func TestPath(t *testing.T) {
 func fail(t *testing.T, test test) {
 	t.Errorf("etree: failed test '%s'\n", test.path)
 }
+
+func TestAbsolutePath(t *testing.T) {
+	doc := NewDocument()
+	err := doc.ReadFromString(testXML)
+	if err != nil {
+		t.Error(err)
+	}
+
+	elements := doc.FindElements("//book/author")
+	for _, e := range elements {
+		title := e.FindElement("/bookstore/book[1]/title")
+		if title == nil || title.Text() != "Everyday Italian" {
+			t.Errorf("etree: absolute path test failed")
+		}
+
+		title = e.FindElement("//book[p:price='29.99']/title")
+		if title == nil || title.Text() != "Harry Potter" {
+			t.Errorf("etree: absolute path test failed")
+		}
+	}
+}