Extract reweighting logic out of Organize_Controller into

item::reweight_all_children as an API and write a test for it.  Work
in progress on #1914.

modules/gallery/helpers/item.php

@@ -437,4 +437,16 @@ class item_Core {
     }
     return call_user_func_array($callback, $args);
   }
+
+  /**
+   * Reset all child weights of a given album to a monotonically increasing sequence based on the
+   * current sort order of the album.
+   */
+  static function resequence_child_weights($album) {
+    $weight = 0;
+    foreach ($album->children() as $child) {
+      $child->weight = ++$weight;
+      $child->save();
+    }
+  }
 }

modules/gallery/tests/Item_Helper_Test.php

@@ -235,4 +235,19 @@ class Item_Helper_Test extends Gallery_Unit_Test_Case {
       $level3b->id,
       item::find_by_relative_url("{$level1->slug}/{$level2b->slug}/{$level3b->slug}")->id);
   }
+
+  public function resequence_child_weights_test() {
+    $album = test::random_album();
+    $photo1 = test::random_photo($album);
+    $photo2 = test::random_photo($album);
+    $this->assert_true($photo2->weight > $photo1->weight);
+
+    $album->reload();
+    $album->sort_order = "DESC";
+    $album->save();
+    item::resequence_child_weights($album);
+
+    $this->assert_equal(2, $photo1->reload()->weight);
+    $this->assert_equal(1, $photo2->reload()->weight);
+  }
 }

modules/organize/controllers/organize.php

@@ -128,13 +128,7 @@ class Organize_Controller extends Controller {
     access::required("edit", $album);
 
     if ($album->sort_column != "weight") {
-      // Force all the weights into the current order before changing the order to manual
-      $weight = 0;
-      foreach ($album->children() as $child) {
-        $child->weight = ++$weight;
-        $child->save();
-      }
-
+      item::resequence_child_weights($album);
       $album->sort_column = "weight";
       $album->sort_order = "ASC";
       $album->save();