Support for the XPath lang() function.

lib/oga/xpath/evaluator.rb

@@ -1103,6 +1103,35 @@ module Oga
         return false
       end
 
+      ##
+      # Processes the `lang()` function call.
+      #
+      # This function returns `true` if the current context node is in the given
+      # language, `false` otherwise.
+      #
+      # The language is based on the value of the "xml:lang" attribute of either
+      # the context node or an ancestor node (in case the context node has no
+      # such attribute).
+      #
+      # @param [Oga::XML::NodeSet] context
+      # @param [Oga::XPath::Node] language
+      # @return [TrueClass|FalseClass]
+      #
+      def on_call_lang(context, language)
+        lang_str = on_call_string(context, language)
+        node     = current_node
+
+        while node.respond_to?(:attribute)
+          found = node.attribute('xml:lang')
+
+          return found.value == lang_str if found
+
+          node = node.parent
+        end
+
+        return false
+      end
+
       ##
       # Processes an `(int)` node.
       #

spec/oga/xpath/evaluator/calls/lang_spec.rb

@@ -0,0 +1,46 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'lang() function' do
+    before do
+      @document  = parse('<root xml:lang="en"><a></a><a xml:lang="nl"></a></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    context 'selecting nodes with lang="en"' do
+      before do
+        @set = @evaluator.evaluate('root[lang("en")]')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the <root> node' do
+        @set[0].should == @document.children[0]
+      end
+    end
+
+    context 'selecting nodes with lang="nl"' do
+      before do
+        @set = @evaluator.evaluate('root/a[lang("nl")]')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the second <a> node' do
+        @set[0].should == @document.children[0].children[1]
+      end
+    end
+
+    context 'inheriting the language from ancestor nodes' do
+      before do
+        @set = @evaluator.evaluate('root/a[lang("en")]')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the first <a> node' do
+        @set[0].should == @document.children[0].children[0]
+      end
+    end
+  end
+end