Fix weight redistributor sometimes dropping items

CHANGELOG.md

@@ -1,7 +1,9 @@
 # Changelog
 
 ## [4.x - Unreleased] - 2026-xx-xx
-
+### Fixed
+- In some rare cases `WeightRedistributor` would silently drop items
+
 ## [4.2.0] - 2026-07-05
 ### Added
 - A mechanism to allow forcing certain items to be packed together [RubenKluft]

src/WeightRedistributor.php

@@ -169,7 +169,12 @@ private function doVolumeRepack(iterable $items, Box $currentBox): PackedBoxList
         $packer->setBoxQuantity($currentBox, $this->boxQuantitiesAvailable[$currentBox] + 1);
         $packer->setItems($items);
 
-        return $packer->doBasicPacking(true);
+        $packedBoxes = $packer->doBasicPacking(true);
+        if ($packedBoxes->count() !== 1 || $packer->getUnpackedItems()->count() !== 0) {
+            return new PackedBoxList($this->packedBoxSorter);
+        }
+
+        return $packedBoxes;
     }
 
     /**

tests/WeightRedistributorTest.php

@@ -10,6 +10,7 @@
 namespace DVDoug\BoxPacker;
 
 use DVDoug\BoxPacker\Test\ConstrainedPlacementNoStackingTestItem;
+use DVDoug\BoxPacker\Test\LimitedSupplyTestBox;
 use DVDoug\BoxPacker\Test\TestBox;
 use DVDoug\BoxPacker\Test\TestItem;
 use PHPUnit\Framework\Attributes\CoversClass;
@@ -67,4 +68,35 @@ public function testWeightDistributionWorks(): void
 
         self::assertEquals(0, $packedBoxes->getWeightVariance());
     }
+
+    /**
+     * Test to ensure no items are silently dropped during weight redistribution.
+     */
+    public function testWeightRedistributionDoesNotSilentlyDropItems(): void
+    {
+        $packer = new Packer();
+        $packer->addBox(new LimitedSupplyTestBox('Box', 29, 29, 29, 0, 29, 29, 29, 68, 2));
+
+        $packer->addItem(new TestItem('Item 0', 10, 10, 10, 2, Rotation::BestFit), 2);
+        $packer->addItem(new TestItem('Item 1', 10, 10, 10, 3, Rotation::BestFit));
+        $packer->addItem(new TestItem('Item 2', 10, 10, 10, 4, Rotation::BestFit));
+        $packer->addItem(new TestItem('Item 3', 10, 10, 10, 8, Rotation::BestFit), 5);
+        $packer->addItem(new TestItem('Item 4', 10, 10, 10, 18, Rotation::BestFit), 3);
+
+        // packer initially packs 6 items into each box,
+        // the imbalance in weights will be attempted to be corrected by the WeightRedistributor
+        $packedBoxes = $packer->pack();
+        self::assertCount(2, $packedBoxes);
+
+        $packedItemCount = 0;
+        foreach ($packedBoxes as $packedBox) {
+            $packedItemCount += $packedBox->items->count();
+        }
+
+        self::assertSame(
+            12,
+            $packedItemCount + $packer->getUnpackedItems()->count(),
+            'No items should be lost during weight redistribution'
+        );
+    }
 }