Support for the XPath sub/- operator.

lib/oga/xpath/evaluator.rb

@@ -690,6 +690,22 @@ module Oga
         return on_call_number(context, left) * on_call_number(context, right)
       end
 
+      ##
+      # Processes the `-` operator.
+      #
+      # This operator converts the left and right expressions to numbers and
+      # subtracts the right number of the left number.
+      #
+      # @param [Oga::XPath::Node] ast_node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Float]
+      #
+      def on_sub(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/sub_operator.rb

@@ -0,0 +1,34 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'subtraction operator' do
+    before do
+      @document  = parse('<root><a>2</a><b>3</b></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    example 'subtract two numbers' do
+      @evaluator.evaluate('2 - 3').should == -1.0
+    end
+
+    example 'subtract a number and a string' do
+      @evaluator.evaluate('2 - "3"').should == -1.0
+    end
+
+    example 'subtract two strings' do
+      @evaluator.evaluate('"2" - "3"').should == -1.0
+    end
+
+    example 'subtract a node set and a number' do
+      @evaluator.evaluate('root/a - 3').should == -1.0
+    end
+
+    example 'subtract two node sets' do
+      @evaluator.evaluate('root/a - root/b').should == -1.0
+    end
+
+    example 'return NaN when trying to subtract invalid values' do
+      @evaluator.evaluate('"" - 1').should be_nan
+    end
+  end
+end