Support for the xpath starts-with() function.

lib/oga/xpath/evaluator.rb

@@ -807,6 +807,27 @@ module Oga
         return retval
       end
 
+      ##
+      # Processes the `starts-with()` function call.
+      #
+      # This function call returns `true` if the string in the 1st argument
+      # starts with the string in the 2nd argument.
+      #
+      # @example
+      #  starts-with("hello world", "hello") # => true
+      #
+      # @param [Oga::XML::NodeSet] context
+      # @param [Oga::XPath::Node] haystack The string to search.
+      # @param [Oga::XPath::Node] needle The string to search for.
+      # @return [TrueClass|FalseClass]
+      #
+      def on_call_starts_with(context, haystack, needle)
+        haystack_str = on_call_string(context, haystack)
+        needle_str   = on_call_string(context, needle)
+
+        return haystack_str.start_with?(needle_str)
+      end
+
       ##
       # Processes an `(int)` node.
       #

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

@@ -0,0 +1,38 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'starts-with() function' do
+    before do
+      @document  = parse('<root><a>foo</a><b>foobar</b></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    example 'return true if the 1st string starts with the 2nd string' do
+      @evaluator.evaluate('starts-with("foobar", "foo")').should == true
+    end
+
+    example "return false if the 1st string doesn't start with the 2nd string" do
+      @evaluator.evaluate('starts-with("foobar", "baz")').should == false
+    end
+
+    example 'return true if the 1st node set starts with the 2nd string' do
+      @evaluator.evaluate('starts-with(root/a, "foo")').should == true
+    end
+
+    example 'return true if the 1st node set starts with the 2nd node set' do
+      @evaluator.evaluate('starts-with(root/b, root/a)').should == true
+    end
+
+    example "return false if the 1st node set doesn't start with the 2nd string" do
+      @evaluator.evaluate('starts-with(root/a, "baz")').should == false
+    end
+
+    example 'return true if the 1st string starts with the 2nd node set' do
+      @evaluator.evaluate('starts-with("foobar", root/a)').should == true
+    end
+
+    example 'return true when using two empty strings' do
+      @evaluator.evaluate('starts-with("", "")').should == true
+    end
+  end
+end