From 0faceffacb7dd0df097aa628c26060de518d91ff Mon Sep 17 00:00:00 2001
From: Yorick Peterse <yorickpeterse@gmail.com>
Date: Sun, 2 Nov 2014 19:29:09 +0100
Subject: [PATCH] Parsing support for nth-child(n+X)

---
 lib/oga/css/parser.y                          | 53 ++++++++++++-------
 .../parser/pseudo_classes/nth_child_spec.rb   |  7 +++
 2 files changed, 40 insertions(+), 20 deletions(-)

diff --git a/lib/oga/css/parser.y b/lib/oga/css/parser.y
index 7b03892..a841752 100644
--- a/lib/oga/css/parser.y
+++ b/lib/oga/css/parser.y
@@ -274,11 +274,14 @@ rule
     # n
     : T_NTH { s(:nth, s(:int, 1)) }
 
+    # n+2
+    | T_NTH integer { s(:nth, s(:int, 1), val[1]) }
+
     # -n
     | T_MINUS T_NTH { s(:nth, s(:int, 1)) }
 
     # -n+2, -n-2
-    | T_MINUS T_NTH integer { s(:nth, nil, val[2]) }
+    | T_MINUS T_NTH integer { s(:nth, s(:int, -1), val[2]) }
 
     # 2n
     | integer T_NTH { s(:nth, val[0]) }
@@ -406,28 +409,12 @@ end
     else
       step, offset = *arg
       before_count = s(:add, count_call, s(:int, 1))
-
-      if step and step.children[0] >= 0
-        compare = :gte
-      else
-        compare = :lte
-      end
+      compare      = step_comparison(step)
 
       # 2n+2, 2n-4, etc
       if offset
-        # -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(
+        mod_val = step_modulo_value(step)
+        node    = s(
           :and,
           s(compare, before_count, offset),
           s(:eq, s(:mod, s(:sub, before_count, offset), mod_val), s(:int, 0))
@@ -460,4 +447,30 @@ end
     return node.children[0] <= 0
   end
 
+  ##
+  # @param [AST::Node] node
+  # @return [Symbol]
+  #
+  def step_comparison(node)
+    return node.children[0] >= 0 ? :gte : :lte
+  end
+
+  ##
+  # @param [AST::Node] node
+  # @return [AST::Node]
+  #
+  def step_modulo_value(step)
+    # -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
+  end
+
 # vim: set ft=racc:
diff --git a/spec/oga/css/parser/pseudo_classes/nth_child_spec.rb b/spec/oga/css/parser/pseudo_classes/nth_child_spec.rb
index c62345b..3fe9217 100644
--- a/spec/oga/css/parser/pseudo_classes/nth_child_spec.rb
+++ b/spec/oga/css/parser/pseudo_classes/nth_child_spec.rb
@@ -52,6 +52,13 @@ describe Oga::CSS::Parser do
       )
     end
 
+    example 'parse the x:nth-child(n+5) pseudo class' do
+      parse_css('x:nth-child(n+5)').should == parse_xpath(
+        'descendant-or-self::x[(count(preceding-sibling::*) + 1) >= 5 ' \
+          'and (((count(preceding-sibling::*) + 1) - 5) mod 1) = 0]'
+      )
+    end
+
     example 'parse the x:nth-child(2n) pseudo class' do
       parse_css('x:nth-child(2n)').should == parse_css('x:nth-child(even)')
     end