[gtk+] cssmatcher: Speed up common matching

[Benjamin Otte <otte src gnome org>]

commit 03312371daafe3a94d888474eb53d7361d5ed432
Author: Benjamin Otte <otte redhat com>
Date:   Thu May 28 17:40:55 2015 +0200

    cssmatcher: Speed up common matching
    
    first-child and last-child are the most common usages of the nth-child
    machinery, so special-casing them makes sense.

 gtk/gtkcssmatcher.c |   15 ++++++++++++---
 1 files changed, 12 insertions(+), 3 deletions(-)
---
diff --git a/gtk/gtkcssmatcher.c b/gtk/gtkcssmatcher.c
index 14597cc..a152327 100644
--- a/gtk/gtkcssmatcher.c
+++ b/gtk/gtkcssmatcher.c
@@ -358,6 +358,18 @@ gtk_css_matcher_node_nth_child (GtkCssNode *node,
 {
   int pos, x;
 
+  /* special-case the common "first-child" and "last-child" */
+  if (a == 0)
+    {
+      while (b > 0 && node != NULL)
+        {
+          b--;
+          node = prev_node_func (node);
+        }
+
+      return b == 0 && node == NULL;
+    }
+
   /* count nodes */
   for (pos = 0; node != NULL; pos++)
     node = prev_node_func (node);
@@ -366,9 +378,6 @@ gtk_css_matcher_node_nth_child (GtkCssNode *node,
    * and return TRUE if X is integer >= 0 */
   x = pos - b;
 
-  if (a == 0)
-    return x == 0;
-
   if (x % a)
     return FALSE;