gridview: Add some helpers

Add helper functions to get the section of
an item and to computes the column for an
item, based on sections and the current number
of columns.

gtk/gtkgridview.c

@@ -30,6 +30,7 @@
 #include "gtkmultiselection.h"
 #include "gtktypebuiltins.h"
 #include "gtkwidgetprivate.h"
+#include "gtksectionmodel.h"
 
 /* Maximum number of list items created by the gridview.
  * For debugging, you can set this to G_MAXUINT to ensure
@@ -434,6 +435,51 @@ gtk_grid_view_get_allocation (GtkListBase  *base,
   return TRUE;
 }
 
+/* Returns the section that position falls into
+ */
+static void
+get_section_for_position (GtkGridView  *self,
+                          unsigned int  position,
+                          unsigned int *section_start,
+                          unsigned int *section_end)
+{
+  GListModel *model;
+  unsigned int start, end;
+
+  model = G_LIST_MODEL (gtk_list_item_manager_get_model (self->item_manager));
+
+  if (!gtk_list_item_manager_get_has_sections (self->item_manager))
+    {
+      start = 0;
+      end = g_list_model_get_n_items (model);
+    }
+  else
+    {
+      gtk_section_model_get_section (GTK_SECTION_MODEL (model), position, &start, &end);
+    }
+
+  if (section_start)
+    *section_start = start;
+  if (section_end)
+    *section_end = end;
+}
+
+/* Returns the column that the given item will fall in, taking
+ * sections into account. Note that this depends on whether
+ * we are currently showing sections, and on the number of
+ * columns that the grid is allocating.
+ */
+static unsigned int
+get_column_for_position (GtkGridView  *self,
+                         unsigned int  position)
+{
+  unsigned int start;
+
+  get_section_for_position (self, position, &start, NULL);
+
+  return (position - start) % self->n_columns;
+}
+
 static gboolean
 gtk_grid_view_get_position_from_allocation (GtkListBase           *base,
                                             int                    x,