Support for the XPath substring-before() function.

lib/oga/xpath/evaluator.rb

@@ -837,6 +837,11 @@ module Oga
       # @example
       #  contains("hello world", "o w") # => 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 [String]
+      #
       def on_call_contains(context, haystack, needle)
         haystack_str = on_call_string(context, haystack)
         needle_str   = on_call_string(context, needle)
@@ -844,6 +849,30 @@ module Oga
         return haystack_str.include?(needle_str)
       end
 
+      ##
+      # Processes the `substring-before()` function call.
+      #
+      # This function call returns the substring of the 1st argument that occurs
+      # before the string given in the 2nd argument. For example:
+      #
+      #     substring-before("2014-08-25", "-")
+      #
+      # This would return "2014" as its the first string that occurs before "-".
+      #
+      # @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 [String]
+      #
+      def on_call_substring_before(context, haystack, needle)
+        haystack_str = on_call_string(context, haystack)
+        needle_str   = on_call_string(context, needle)
+
+        before, sep, after = haystack_str.partition(needle_str)
+
+        return sep.empty? ? sep : before
+      end
+
       ##
       # Processes an `(int)` node.
       #

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

@@ -0,0 +1,30 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'substring-before() function' do
+    before do
+      @document  = parse('<root><a>-</a><b>a-b-c</b></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    example 'return the substring of the 1st string before the 2nd string' do
+      @evaluator.evaluate('substring-before("a-b-c", "-")').should == 'a'
+    end
+
+    example 'return an empty string if the 2nd string is not present' do
+      @evaluator.evaluate('substring-before("a-b-c", "x")').should == ''
+    end
+
+    example 'return the substring of the 1st node set before the 2nd string' do
+      @evaluator.evaluate('substring-before(root/b, "-")').should == 'a'
+    end
+
+    example 'return the substring of the 1st node set before the 2nd node set' do
+      @evaluator.evaluate('substring-before(root/b, root/a)').should == 'a'
+    end
+
+    example 'return an empty string when using two empty strings' do
+      @evaluator.evaluate('substring-before("", "")').should == ''
+    end
+  end
+end