Basic support for the preceding-sibling xpath axis

lib/oga/xpath/evaluator.rb

@@ -354,6 +354,33 @@ module Oga
         return nodes
       end
 
+      ##
+      # Evaluates the `preceding-sibling` axis.
+      #
+      # @param [Oga::XPath::Node] ast_node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_axis_preceding_sibling(ast_node, context)
+        nodes = XML::NodeSet.new
+
+        context.each do |context_node|
+          check  = true
+          parent = has_parent?(context_node) ? context_node.parent : nil
+
+          @document.each_node do |doc_node|
+            # Test everything *until* we hit the current context node.
+            if doc_node == context_node
+              break
+            elsif doc_node.parent == parent and node_matches?(doc_node, ast_node)
+              nodes << doc_node
+            end
+          end
+        end
+
+        return nodes
+      end
+
       ##
       # Returns a node set containing all the child nodes of the given set of
       # nodes.

spec/oga/xpath/evaluator/axes/preceding_sibling_spec.rb

@@ -0,0 +1,33 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'preceding-sibling axis' do
+    before do
+      @document = parse(<<-EOF.strip.gsub(/\s+/m, ''))
+<root>
+  <foo>
+  </foo>
+  <bar>
+    <foo></foo>
+    <baz></baz>
+  </bar>
+</root>
+      EOF
+
+      @second_foo = @document.children[0].children[1].children[0]
+      @evaluator  = described_class.new(@document)
+    end
+
+    context 'matching nodes in the current context' do
+      before do
+        @set = @evaluator.evaluate('root/bar/baz/preceding-sibling::foo')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the second <foo> node' do
+        @set[0].should == @second_foo
+      end
+    end
+  end
+end