Support for the XPath boolean() function.

lib/oga/xpath/evaluator.rb

@@ -1031,6 +1031,29 @@ module Oga
         return replaced
       end
 
+      ##
+      # Processes the `boolean()` function call.
+      #
+      # This function converts the 1st argument to a boolean.
+      #
+      # The boolean `true` is returned for the following:
+      #
+      # * A non empty string
+      # * A non empty node set
+      # * A non zero number, either positive or negative
+      #
+      # The boolean `false` is returned for all other cases.
+      #
+      def on_call_boolean(context, expression)
+        retval = process(expression, context)
+
+        if retval.is_a?(Numeric)
+          return !retval.nan? && !retval.zero?
+        else
+          return retval && !retval.empty?
+        end
+      end
+
       ##
       # Processes an `(int)` node.
       #

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

@@ -0,0 +1,50 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'boolean() function' do
+    before do
+      @document  = parse('<root><a>foo</a></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    example 'return true for a non empty string literal' do
+      @evaluator.evaluate('boolean("foo")').should == true
+    end
+
+    example 'return false for an empty string' do
+      @evaluator.evaluate('boolean("")').should == false
+    end
+
+    example 'return true for a positive integer' do
+      @evaluator.evaluate('boolean(10)').should == true
+    end
+
+    example 'return true for a positive float' do
+      @evaluator.evaluate('boolean(10.5)').should == true
+    end
+
+    example 'return true for a negative integer' do
+      @evaluator.evaluate('boolean(-5)').should == true
+    end
+
+    example 'return true for a negative float' do
+      @evaluator.evaluate('boolean(-5.2)').should == true
+    end
+
+    example 'return false for a zero integer' do
+      @evaluator.evaluate('boolean(0)').should == false
+    end
+
+    example 'return false for a zero float' do
+      @evaluator.evaluate('boolean(0.0)').should == false
+    end
+
+    example 'return true for a non empty node set' do
+      @evaluator.evaluate('boolean(root/a)').should == true
+    end
+
+    example 'return false for an empty node set' do
+      @evaluator.evaluate('boolean(root/b)').should == false
+    end
+  end
+end