change Layout linked list to by cycling

config.c

@@ -395,6 +395,7 @@ config_parse_screen(cfg_t *cfg, int screen)
     /* Layouts */
     virtscreen->layouts = layout = p_new(Layout, 1);
     if(cfg_size(cfg_layouts, "layout"))
+    {
         for(i = 0; i < cfg_size(cfg_layouts, "layout"); i++)
         {
             cfgsectmp = cfg_getnsec(cfg_layouts, "layout", i);
@@ -408,7 +409,14 @@ config_parse_screen(cfg_t *cfg, int screen)
             layout->image = a_strdup(cfg_getstr(cfgsectmp, "image"));
 
             if(i < cfg_size(cfg_layouts, "layout") - 1)
-                layout = layout->next = p_new(Layout, 1);
+            {
+                layout->next = p_new(Layout, 1);
+                layout->next->prev = layout;
+                layout = layout->next;
+            }
+        }
+        /* do cycling */
+        virtscreen->layouts->prev = layout;
     }
     else
     {

layout.c

@@ -243,11 +243,19 @@ uicb_tag_setlayout(int screen, char *arg)
         curtags = get_current_tags(screen);
         for(i = 0; l && l != curtags[0]->layout; i++, l = l->next);
         p_delete(&curtags);
+
         if(!l)
             i = 0;
-        for(i = compute_new_value_from_arg(arg, (double) i),
+
-            l = globalconf.screens[screen].layouts; l && i > 0; i--)
+        i = compute_new_value_from_arg(arg, (double) i);
+
+        if(i >= 0)
+            for(l = globalconf.screens[screen].layouts; l && i > 0; i--)
                  l = l->next;
+        else
+            for(l = globalconf.screens[screen].layouts; l && i < 0; i++)
+                 l = l->prev;
+
         if(!l)
             l = globalconf.screens[screen].layouts;
     }

layout.h

@@ -31,6 +31,7 @@ struct Layout
 {
     char *image;
     LayoutArrange *arrange;
+    Layout *prev;
     Layout *next;
 };