From dd37b028a0709e946b1394b0276a61c944703b37 Mon Sep 17 00:00:00 2001
From: Yorick Peterse <yorickpeterse@gmail.com>
Date: Thu, 24 Jul 2014 09:49:05 +0200
Subject: [PATCH] Support for the XPath descendant axis.

---
 lib/oga/xpath/evaluator.rb            | 19 +++++++++
 spec/oga/xpath/evaluator/axes_spec.rb | 59 ++++++++++++++++++++++++++-
 2 files changed, 77 insertions(+), 1 deletion(-)

diff --git a/lib/oga/xpath/evaluator.rb b/lib/oga/xpath/evaluator.rb
index bdf12cf..3cb68c9 100644
--- a/lib/oga/xpath/evaluator.rb
+++ b/lib/oga/xpath/evaluator.rb
@@ -217,6 +217,25 @@ module Oga
         return on_test(node, context)
       end
 
+      ##
+      # Evaluator the `descendant` axis. This method processes child nodes until
+      # the very end of the tree, no "short-circuiting" mechanism is used.
+      #
+      # @param [Oga::XPath::Node] node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_axis_descendant(node, context)
+        nodes    = on_test(node, context)
+        children = child_nodes(context)
+
+        unless children.empty?
+          nodes += on_axis_descendant(node, children)
+        end
+
+        return nodes
+      end
+
       ##
       # Returns a node set containing all the child nodes of the given set of
       # nodes.
diff --git a/spec/oga/xpath/evaluator/axes_spec.rb b/spec/oga/xpath/evaluator/axes_spec.rb
index f313ebf..1611998 100644
--- a/spec/oga/xpath/evaluator/axes_spec.rb
+++ b/spec/oga/xpath/evaluator/axes_spec.rb
@@ -2,7 +2,7 @@ require 'spec_helper'
 
 describe Oga::XPath::Evaluator do
   before do
-    @document = parse('<a><b><c></c></b><d></d></a>')
+    @document = parse('<a><b><c></c></b><d></d><a></a></a>')
     @c_node   = @document.children[0].children[0].children[0]
   end
 
@@ -170,4 +170,61 @@ describe Oga::XPath::Evaluator do
       it_behaves_like :empty_node_set
     end
   end
+
+  context 'descendant axis' do
+    before do
+      @evaluator = described_class.new(@document)
+
+      @first_a  = @document.children[0]
+      @second_a = @first_a.children[-1]
+    end
+
+    context 'direct descendants' do
+      before do
+        @set = @evaluator.evaluate('descendant::a')
+      end
+
+      it_behaves_like :node_set, :length => 2
+
+      example 'return the first <a> node' do
+        @set[0].should == @first_a
+      end
+
+      example 'return the second <a> node' do
+        @set[1].should == @second_a
+      end
+    end
+
+    context 'nested descendants' do
+      before do
+        @set = @evaluator.evaluate('descendant::c')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the <c> node' do
+        @set[0].name.should == 'c'
+      end
+    end
+
+    context 'descendants of a specific node' do
+      before do
+        @set = @evaluator.evaluate('a/descendant::a')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the second <a> node' do
+        @set[0].should == @second_a
+      end
+    end
+
+    context 'invalid descendants' do
+      before do
+        @set = @evaluator.evaluate('descendant::foobar')
+      end
+
+      it_behaves_like :empty_node_set
+    end
+  end
 end