Fixed CSS evaluation of :first-of-type

The old XPath "position() = 1" would work in Nokogiri due to the way they
retrieve descendants. In Oga however this would simply always return the first
node.

To fix this Oga now counts the amount of preceding siblings that match the same
full name.

lib/oga/css/parser.y

@@ -515,7 +515,11 @@ end
   # @return [AST::Node]
   #
   def on_pseudo_class_first_of_type
-    return s(:eq, s(:call, 'position'), s(:int, 1))
+    return s(
+      :eq,
+      s(:call, 'count', s(:axis, 'preceding-sibling', s(:call, 'name'))),
+      s(:int, 0)
+    )
   end
 
   ##

spec/oga/css/evaluator/pseudo_classes/first_of_type_spec.rb

@@ -3,22 +3,25 @@ require 'spec_helper'
 describe 'CSS selector evaluation' do
   context ':first-of-type pseudo class' do
     before do
-      @document = parse('<root><a /><b /></root>')
+      @document = parse(<<-EOF)
+<dl>
+  <dt>foo</dt>
+  <dd>
+    <dl>
+      <dt>bar</dt>
+      <dd>baz</dd>
+    </dl>
+  </dd>
+</dl>
+      EOF
 
-      @a1 = @document.children[0].children[0]
-      @b1 = @document.children[0].children[1]
+      @dt1 = @document.at_xpath('dl/dt')
+      @dt2 = @document.at_xpath('dl/dd/dl/dt')
     end
 
-    example 'return a node set containing the first node' do
-      evaluate_css(@document, 'root :first-of-type').should == node_set(@a1)
-    end
-
-    example 'return a node set containing the first node with a node test' do
-      evaluate_css(@document, 'root a:first-of-type').should == node_set(@a1)
-    end
-
-    example 'return a node set containing the first <b> node' do
-      evaluate_css(@document, 'root b:first-of-type').should == node_set(@b1)
+    example 'return a node set containing all <dt> nodes' do
+      evaluate_css(@document, 'dl dt:first-of-type')
+        .should == node_set(@dt1, @dt2)
     end
   end
 end

spec/oga/css/parser/pseudo_classes/first_of_type_spec.rb

@@ -4,7 +4,13 @@ describe Oga::CSS::Parser do
   context ':first-of-type pseudo class' do
     example 'parse the :first-of-type pseudo class' do
       parse_css(':first-of-type').should == parse_xpath(
-        'descendant::*[position() = 1]'
+        'descendant::*[count(preceding-sibling::name()) = 0]'
+      )
+    end
+
+    example 'parse the a:first-of-type pseudo class' do
+      parse_css('a:first-of-type').should == parse_xpath(
+        'descendant::a[count(preceding-sibling::name()) = 0]'
       )
     end
   end