Use correct modulo for nth-child and negatives.

lib/oga/css/parser.y

@@ -401,7 +401,7 @@ end
     count_call = s(:call, 'count', s(:axis, count_axis, s(:test, nil, '*')))
 
    # literal 2, 4, etc
-    if arg.type == :int
+    if int_node?(arg)
       node = s(:eq, count_call, s(:int, arg.children[0] - 1))
     else
       step, offset = *arg
@@ -414,12 +414,22 @@ end
       end
 
       # -n-6, -n-4, etc
-      if !step and offset.children[0] <= 0
+      if !step and non_positive_number?(offset)
         node = s(:eq, count_call, s(:int, -1))
 
       # 2n+2, 2n-4, etc
       elsif offset
-        mod_val = step ? s(:int, 2) : s(:int, 1)
+        # -2n
+        if step and non_positive_number?(step)
+          mod_val = s(:int, -step.children[0])
+
+        # 2n
+        elsif step
+          mod_val = step
+
+        else
+          mod_val = s(:int, 1)
+        end
 
         node = s(
           :and,
@@ -436,4 +446,22 @@ end
     return node
   end
 
+  private
+
+  ##
+  # @param [AST::Node] node
+  # @return [TrueClass|FalseClass]
+  #
+  def int_node?(node)
+    return node.type == :int
+  end
+
+  ##
+  # @param [AST::Node] node
+  # @return [TrueClass|FalseClass]
+  #
+  def non_positive_number?(node)
+    return node.children[0] <= 0
+  end
+
 # vim: set ft=racc:

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

@@ -69,6 +69,13 @@ describe Oga::CSS::Parser do
       )
     end
 
+    example 'parse the x:nth-child(3n+1) pseudo class' do
+      parse_css('x:nth-child(3n+1)').should == parse_xpath(
+        'descendant-or-self::x[(count(preceding-sibling::*) + 1) >= 1 ' \
+          'and (((count(preceding-sibling::*) + 1) - 1) mod 3) = 0]'
+      )
+    end
+
     example 'parse the x:nth-child(2n-6) pseudo class' do
       parse_css('x:nth-child(2n-6)').should == parse_xpath(
         'descendant-or-self::x[(count(preceding-sibling::*) + 1) >= 2 ' \