Support for the XPath substring() function.

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

@@ -0,0 +1,30 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'substring() function' do
+    before do
+      @document  = parse('<root><a>foobar</a><b>3</b></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    example 'return the substring of a string' do
+      @evaluator.evaluate('substring("foo", 2)').should == 'oo'
+    end
+
+    example 'return the substring of a string using a custom length' do
+      @evaluator.evaluate('substring("foo", 2, 1)').should == 'o'
+    end
+
+    example 'return the substring of a node set' do
+      @evaluator.evaluate('substring(root/a, 2)').should == 'oobar'
+    end
+
+    example 'return the substring of a node set with a node set as the length' do
+      @evaluator.evaluate('substring(root/a, 1, root/b)').should == 'foo'
+    end
+
+    example 'return an empty string when the source string is empty' do
+      @evaluator.evaluate('substring("", 1, 3)').should == ''
+    end
+  end
+end