Include line numbers from the parser

Neither libxml2 nor Nokogiri contain an API for setting the line numbers
for a node. When the libxml2 headers are available, the line numbers can
be set directly in the node structure.

Closes: #53

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Added option `:max_errors` to control the maximum number of errors reported
   by `#errors`
 - Added option `:max_tree_depth` to control the maximum parse tree depth.
+- Line number support via `Nokogiri::XML::Node#line` as long as Nokogumbo has
+  been compiled with libxml2 support.
 
 ### Changed
 - Integrated [Gumbo parser](https://github.com/google/gumbo-parser) into

README.md

@@ -202,6 +202,36 @@ rules defined in the HTML5 specification for doing so.
 * Instead of returning `unknown` as the element name for unknown tags, the
 original tag name is returned verbatim.
 
+# Flavors of Nokogumbo
+Nokogumbo uses libxml2, the XML library underlying Nokogiri, to speed up
+parsing. If the libxml2 headers are not available, then Nokogumbo resorts to
+using Nokogiri's Ruby API to construct the DOM tree.
+
+Nokogiri can be configured to either use the system library version of libxml2
+or use a bundled version. By default (as of Nokogiri version 1.8.4), Nokogiri
+will use a bundled version.
+
+To prevent differences between versions of libxml2, Nokogumbo will only use
+libxml2 if the build process can find the exact same version used by Nokogiri.
+This leads to three possibilities
+
+1. Nokogiri is compiled with the bundled libxml2. In this case, Nokogumbo will
+   (by default) use the same version of libxml2.
+2. Nokogiri is compiled with the system libxml2. In this case, if the libxml2
+   headers are available, then Nokogumbo will (by default) use the system
+   version and headers.
+3. Nokogiri is compiled with the system libxml2 but its headers aren't
+   available at build time for Nokogumbo. In this case, Nokogumbo will use the
+   slower Ruby API.
+
+Using libxml2 can be required by passing `-- --with-libxml2` to `bundle exec
+rake` or to `gem install`. Using libxml2 can be prohibited by instead passing
+`-- --without-libxml2`.
+
+Functionally, the only difference between using libxml2 or not is in the
+behavior of `Nokogiri::XML::Node#line`. If it is used, then `#line` will
+return the line number of the corresponding node. Otherwise, it will return 0.
+
 # Installation
 
     git clone https://github.com/rubys/nokogumbo.git

ext/nokogumbo/nokogumbo.c

@@ -290,6 +290,17 @@ static xmlNsPtr lookup_or_add_ns (
 #endif
 }
 
+static void set_line(xmlNodePtr node, size_t line) {
+#if NGLIB
+  // libxml2 uses 65535 to mean look elsewhere for the line number on some
+  // nodes.
+  if (line < 65535)
+    node->line = (unsigned short)line;
+#else
+  // XXX: If Nokogiri gets a `#line=` method, we'll use that.
+#endif
+}
+
 // Construct an XML tree rooted at xml_output_node from the Gumbo tree rooted
 // at gumbo_node.
 static void build_tree (
@@ -323,31 +334,38 @@ static void build_tree (
       continue;
     }
     const GumboNode *gumbo_child = children->data[child_index++];
+    xmlNodePtr xml_child;
 
     switch (gumbo_child->type) {
       case GUMBO_NODE_DOCUMENT:
         abort(); // Bug in Gumbo.
 
       case GUMBO_NODE_TEXT:
       case GUMBO_NODE_WHITESPACE:
-        xmlAddChild(xml_node, xmlNewDocText(doc, BAD_CAST gumbo_child->v.text.text));
+        xml_child = xmlNewDocText(doc, BAD_CAST gumbo_child->v.text.text);
+        set_line(xml_child, gumbo_child->v.text.start_pos.line);
+        xmlAddChild(xml_node, xml_child);
         break;
 
       case GUMBO_NODE_CDATA:
-        xmlAddChild(xml_node,
-                    xmlNewCDataBlock(doc, BAD_CAST gumbo_child->v.text.text,
-                                     (int) strlen(gumbo_child->v.text.text)));
+        xml_child = xmlNewCDataBlock(doc, BAD_CAST gumbo_child->v.text.text,
+                                     (int) strlen(gumbo_child->v.text.text));
+        set_line(xml_child, gumbo_child->v.text.start_pos.line);
+        xmlAddChild(xml_node, xml_child);
         break;
 
       case GUMBO_NODE_COMMENT:
-        xmlAddChild(xml_node, xmlNewDocComment(doc, BAD_CAST gumbo_child->v.text.text));
+        xml_child = xmlNewDocComment(doc, BAD_CAST gumbo_child->v.text.text);
+        set_line(xml_child, gumbo_child->v.text.start_pos.line);
+        xmlAddChild(xml_node, xml_child);
         break;
 
       case GUMBO_NODE_TEMPLATE:
         // XXX: Should create a template element and a new DocumentFragment
       case GUMBO_NODE_ELEMENT:
       {
-        xmlNodePtr xml_child = xmlNewDocNode(doc, NIL, BAD_CAST gumbo_child->v.element.name, NULL);
+        xml_child = xmlNewDocNode(doc, NIL, BAD_CAST gumbo_child->v.element.name, NULL);
+        set_line(xml_child, gumbo_child->v.text.start_pos.line);
         if (xml_root == NIL)
           xml_root = xml_child;
         xmlNsPtr ns = NIL;

test/test_nokogumbo.rb

@@ -220,6 +220,11 @@ def test_document_encoding
     assert_equal "Кирилические символы", doc.at('body').text.gsub(/\n\s+/,'')
   end
 
+  def test_line_numbers
+    doc = Nokogiri::HTML5(buffer)
+    assert_includes [0, 8], doc.at('h1').line
+    assert_includes [0, 10], doc.at('span').line
+  end
 
 private