From 423af37422f5f53109a591607ce7369494c061ee Mon Sep 17 00:00:00 2001
From: Yorick Peterse <yorickpeterse@gmail.com>
Date: Mon, 18 Aug 2014 19:00:32 +0200
Subject: [PATCH] Basic support for the XPath last() function.

---
 lib/oga/xpath/evaluator.rb                  | 42 ++++++++++++++++++++-
 spec/oga/xpath/evaluator/calls/last_spec.rb | 17 +++++++++
 2 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 spec/oga/xpath/evaluator/calls/last_spec.rb

diff --git a/lib/oga/xpath/evaluator.rb b/lib/oga/xpath/evaluator.rb
index 9892610..73d234c 100644
--- a/lib/oga/xpath/evaluator.rb
+++ b/lib/oga/xpath/evaluator.rb
@@ -98,12 +98,16 @@ module Oga
       # @return [Oga::XML::NodeSet]
       #
       def on_test(ast_node, context)
-        nodes = XML::NodeSet.new
+        nodes     = XML::NodeSet.new
+        predicate = ast_node.children[2]
 
         context.each do |xml_node|
           nodes << xml_node if node_matches?(xml_node, ast_node)
         end
 
+        # Filter the nodes based on the predicate.
+        nodes = process(predicate, nodes) if predicate
+
         return nodes
       end
 
@@ -523,6 +527,42 @@ module Oga
         return process(left, context) + process(right, context)
       end
 
+      ##
+      # Delegates function calls to specific handlers.
+      #
+      # Handler functions take two arguments:
+      #
+      # 1. The context node set
+      # 2. A variable list of XPath function arguments, passed as individual
+      #    Ruby method arguments.
+      #
+      # @param [Oga::XPath::Node] ast_node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_call(ast_node, context)
+        name, args = *ast_node
+
+        handler = name.gsub('-', '_')
+
+        return send("on_call_#{handler}", context, *args)
+      end
+
+      ##
+      # Processes the `last()` function call. This function call returns a node
+      # set containing the last node of the context set.
+      #
+      # @param [Oga::XML::NodeSet] context
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_call_last(context)
+        if context.empty?
+          return context
+        else
+          return XML::NodeSet.new([context.last])
+        end
+      end
+
       ##
       # Returns a node set containing all the child nodes of the given set of
       # nodes.
diff --git a/spec/oga/xpath/evaluator/calls/last_spec.rb b/spec/oga/xpath/evaluator/calls/last_spec.rb
new file mode 100644
index 0000000..7714ff3
--- /dev/null
+++ b/spec/oga/xpath/evaluator/calls/last_spec.rb
@@ -0,0 +1,17 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'last() function' do
+    before do
+      @document = parse('<root><a>foo</a><a>bar</a></root>')
+      @second_a = @document.children[0].children[1]
+      @set      = described_class.new(@document).evaluate('root/a[last()]')
+    end
+
+    it_behaves_like :node_set, :length => 1
+
+    example 'return the second <a> node' do
+      @set[0].should == @second_a
+    end
+  end
+end