Support for the XPath "mod" operator.

lib/oga/xpath/evaluator.rb

@@ -658,6 +658,22 @@ module Oga
         return on_call_number(context, left) / on_call_number(context, right)
       end
 
+      ##
+      # Processes the `mod` operator.
+      #
+      # This operator converts the left and right expressions to numbers and
+      # returns the modulo of the two numbers.
+      #
+      # @param [Oga::XPath::Node] ast_node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Float]
+      #
+      def on_mod(ast_node, context)
+        left, right = *ast_node
+
+        return on_call_number(context, left) % on_call_number(context, right)
+      end
+
       ##
       # Delegates function calls to specific handlers.
       #

spec/oga/xpath/evaluator/operators/mod_operator.rb

@@ -0,0 +1,34 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'mod operator' do
+    before do
+      @document  = parse('<root><a>2</a><b>3</b></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    example 'return the modulo of two numbers' do
+      @evaluator.evaluate('2 mod 3').should == 2.0
+    end
+
+    example 'return the modulo of a number and a string' do
+      @evaluator.evaluate('2 mod "3"').should == 2.0
+    end
+
+    example 'return the modulo of two strings' do
+      @evaluator.evaluate('"2" mod "3"').should == 2.0
+    end
+
+    example 'return the modulo of a node set and a number' do
+      @evaluator.evaluate('root/a mod 3').should == 2.0
+    end
+
+    example 'return the modulo of two node sets' do
+      @evaluator.evaluate('root/a mod root/b').should == 2.0
+    end
+
+    example 'return NaN when trying to get the modulo of invalid values' do
+      @evaluator.evaluate('"" mod 1').should be_nan
+    end
+  end
+end