Support for the XPath string() function.

lib/oga/xpath/evaluator.rb

@@ -756,6 +756,35 @@ module Oga
         end
       end
 
+      ##
+      # Evaluates the `string()` function call.
+      #
+      # This function call converts the given argument *or* the current context
+      # node to a string. If a node set is given then only the first node is
+      # converted to a string.
+      #
+      # @param [Oga::XML::NodeSet] context
+      # @param [Oga::XPath::Node] expression
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_call_string(context, expression = nil)
+        if expression
+          convert = process(expression, context)
+
+          if convert.is_a?(XML::NodeSet)
+            convert = convert.first
+          end
+        else
+          convert = current_node
+        end
+
+        if convert.respond_to?(:text)
+          return convert.text
+        else
+          return convert.to_s
+        end
+      end
+
       ##
       # Processes an `(int)` node. This method simply returns the value as a
       # Float.

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

@@ -0,0 +1,76 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'string() function' do
+    before do
+      @document = parse(<<-EOF)
+<root>
+  <a>a1</a>
+  <b num="10">b1</b>
+  <c><!--foo--></c>
+  <d><![CDATA[foobar]]></d>
+</root>
+      EOF
+      @evaluator = described_class.new(@document)
+    end
+
+    context 'outside predicates' do
+      example 'convert the <root> node to a string' do
+        @evaluator.evaluate('string(root)')
+          .should == "\n  a1\n  b1\n  \n  foobar\n"
+      end
+
+      example 'convert the <a> node to a string' do
+        @evaluator.evaluate('string(root/a)').should == 'a1'
+      end
+
+      example 'convert the <b> node to a string' do
+        @evaluator.evaluate('string(root/b)').should == 'b1'
+      end
+
+      example 'convert the "num" attribute to a string' do
+        @evaluator.evaluate('string(root/b/@num)').should == '10'
+      end
+
+      example 'convert the first node in a set to a string' do
+        @evaluator.evaluate('string(root/*)').should == 'a1'
+      end
+
+      example 'convert an integer to a string' do
+        @evaluator.evaluate('string(10)').should == '10.0'
+      end
+
+      example 'convert a float to a string' do
+        @evaluator.evaluate('string(10.5)').should == '10.5'
+      end
+
+      example 'convert a string to a string' do
+        @evaluator.evaluate('string("foo")').should == 'foo'
+      end
+
+      example 'convert a comment to a string' do
+        @evaluator.evaluate('string(root/c/comment())').should == 'foo'
+      end
+
+      example 'convert a CDATA to a string' do
+        @evaluator.evaluate('string(root/d/node())').should == 'foobar'
+      end
+
+      example 'return an empty string by default' do
+        @evaluator.evaluate('string(foobar)').should == ''
+      end
+    end
+
+    context 'inside predicates' do
+      before do
+        @set = @evaluator.evaluate('root/b[string()]')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the <b> node' do
+        @set[0].name.should == 'b'
+      end
+    end
+  end
+end