listlistmodel: Add a cache

Cache the last looked up item and use it for looking up the next item if
it's closest. This massively speeds up iteration over the model, because
each call to get_item() will be adjacent to the previous one.

Improves performance of the inspector quite a bit.

gtk/gtklistlistmodel.c

@@ -41,6 +41,9 @@ struct _GtkListListModel
   gpointer (* get_item) (gpointer, gpointer);
   gpointer data;
   GDestroyNotify notify;
+
+  guint cache_pos;
+  gpointer cache_item;
 };
 
 struct _GtkListListModelClass
@@ -72,6 +75,18 @@ gtk_list_list_model_get_n_items (GListModel *list)
   return self->n_items;
 }
 
+static gboolean
+gtk_list_list_model_cache_is_valid (GtkListListModel *self)
+{
+  return self->cache_item != NULL;
+}
+
+static void
+gtk_list_list_model_invalidate_cache (GtkListListModel *self)
+{
+  self->cache_item = NULL;
+}
+
 static gpointer
 gtk_list_list_model_get_item (GListModel *list,
                               guint       position)
@@ -79,31 +94,50 @@ gtk_list_list_model_get_item (GListModel *list,
   GtkListListModel *self = GTK_LIST_LIST_MODEL (list);
   gpointer result;
   guint i;
+  guint start, end;
 
   if (position >= self->n_items)
-    {
-      return NULL;
-    }
-  else if (self->get_last &&
-           position >= self->n_items / 2)
-    {
-      result = self->get_last (self->data);
+    return NULL;
 
-      for (i = self->n_items - 1; i > position; i--)
+  start = 0;
+  end = self->n_items;
+  if (gtk_list_list_model_cache_is_valid (self))
+    {
+      if (self->cache_pos <= position)
+        start = self->cache_pos;
+      else
+        end = self->cache_pos;
+    }
+
+  if (self->get_last &&
+      position > (start + end) / 2)
+    {
+      if (end == self->cache_pos && gtk_list_list_model_cache_is_valid (self))
+        result = self->get_previous (self->cache_item, self->data);
+      else
+        result = self->get_last (self->data);
+
+      for (i = end - 1; i > position; i--)
         {
           result = self->get_previous (result, self->data);
         }
     }
   else
     {
-      result = self->get_first (self->data);
+      if (start == self->cache_pos && gtk_list_list_model_cache_is_valid (self))
+        result = self->cache_item;
+      else
+        result = self->get_first (self->data);
 
-      for (i = 0; i < position; i++)
+      for (i = start; i < position; i++)
         {
           result = self->get_next (result, self->data);
         }
     }
 
+  self->cache_item = result;
+  self->cache_pos = position;
+
   return self->get_item (result, self->data);
 }
 
@@ -275,7 +309,9 @@ gtk_list_list_model_item_added_at (GtkListListModel *self,
   g_return_if_fail (GTK_IS_LIST_LIST_MODEL (self));
   g_return_if_fail (position <= self->n_items);
 
-  self->n_items += 1;
+  self->n_items++;
+  if (position <= self->cache_pos)
+    self->cache_pos++;
 
   g_list_model_items_changed (G_LIST_MODEL (self), position, 0, 1);
   g_object_notify_by_pspec (G_OBJECT (self), properties[PROP_N_ITEMS]);
@@ -327,6 +363,12 @@ gtk_list_list_model_item_moved (GtkListListModel *self,
 
   min = MIN (position, previous_position);
   max = MAX (position, previous_position) + 1;
+
+  if (self->cache_item == item)
+    self->cache_pos = position;
+  else if (self->cache_pos >= min && self->cache_pos < max)
+    self->cache_pos += (self->cache_pos > position ? 1 : -1);
+
   g_list_model_items_changed (G_LIST_MODEL (self), min, max - min, max - min);
 }
 
@@ -338,6 +380,10 @@ gtk_list_list_model_item_removed_at (GtkListListModel *self,
   g_return_if_fail (position < self->n_items);
 
   self->n_items -= 1;
+  if (position == self->cache_pos)
+    gtk_list_list_model_invalidate_cache (self);
+  else if (position < self->cache_pos)
+    self->cache_pos--;
 
   g_list_model_items_changed (G_LIST_MODEL (self), position, 1, 0);
   g_object_notify_by_pspec (G_OBJECT (self), properties[PROP_N_ITEMS]);
@@ -358,6 +404,8 @@ gtk_list_list_model_clear (GtkListListModel *self)
   self->n_items = 0;
   self->notify = NULL;
 
+  gtk_list_list_model_invalidate_cache (self);
+
   if (n_items > 0)
     {
       g_list_model_items_changed (G_LIST_MODEL (self), 0, n_items, 0);