Support for the XPath number() function.

lib/oga/xpath/evaluator.rb

@@ -788,6 +788,26 @@ module Oga
         end
       end
 
+      ##
+      # Evaluates the `number()` function call.
+      #
+      # This function call converts its first argument *or* the current context
+      # node to a number, similar to the `string()` function.
+      #
+      # @example
+      #  number("10") # => 10.0
+      #
+      # @see [#on_call_string]
+      # @param [Oga::XML::NodeSet] context
+      # @param [Oga::XPath::Node] expression
+      # @return [Float]
+      #
+      def on_call_number(context, expression = nil)
+        str_val = on_call_string(context, expression)
+
+        return Float(str_val) rescue Float::NAN
+      end
+
       ##
       # Processes the `concat()` function call.
       #

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

@@ -0,0 +1,34 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'number() function' do
+    before do
+      @document  = parse('<root><a>10</a><b>10.5</b><!--10--></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    example 'convert a literal string to a number' do
+      @evaluator.evaluate('number("10")').should == 10.0
+    end
+
+    example 'convert a literal string with deciamsl to a number' do
+      @evaluator.evaluate('number("10.5")').should == 10.5
+    end
+
+    example 'convert a node set to a number' do
+      @evaluator.evaluate('number(root/a)').should == 10.0
+    end
+
+    example 'convert a node set with decimals to a number' do
+      @evaluator.evaluate('number(root/b)').should == 10.5
+    end
+
+    example 'convert a comment to a number' do
+      @evaluator.evaluate('number(root/comment())').should == 10.0
+    end
+
+    example 'return NaN for values that can not be converted to floats' do
+      @evaluator.evaluate('number("a")').should be_nan
+    end
+  end
+end