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<?php |
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/** |
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* Box packing (3D bin packing, knapsack problem). |
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* |
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* @author Doug Wright |
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*/ |
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namespace DVDoug\BoxPacker; |
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use Psr\Log\LoggerAwareInterface; |
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use Psr\Log\LoggerAwareTrait; |
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use Psr\Log\LogLevel; |
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use Psr\Log\NullLogger; |
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/** |
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* Actual packer. |
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* |
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* @author Doug Wright |
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* @internal |
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*/ |
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class WeightRedistributor implements LoggerAwareInterface |
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{ |
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use LoggerAwareTrait; |
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/** |
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* List of box sizes available to pack items into. |
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* |
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* @var BoxList |
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*/ |
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private $boxes; |
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/** |
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* Constructor. |
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* @param BoxList $boxList |
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*/ |
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public function __construct(BoxList $boxList) |
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{ |
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$this->boxes = clone $boxList; |
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$this->logger = new NullLogger(); |
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} |
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/** |
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* Given a solution set of packed boxes, repack them to achieve optimum weight distribution. |
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* |
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* @param PackedBoxList $originalBoxes |
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* |
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* @return PackedBoxList |
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*/ |
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public function redistributeWeight(PackedBoxList $originalBoxes) |
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{ |
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$targetWeight = $originalBoxes->getMeanItemWeight(); |
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$this->logger->log(LogLevel::DEBUG, "repacking for weight distribution, weight variance {$originalBoxes->getWeightVariance()}, target weight {$targetWeight}"); |
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/** @var PackedBox[] $boxes */ |
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$boxes = iterator_to_array($originalBoxes); |
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usort($boxes, static function (PackedBox $boxA, PackedBox $boxB) { |
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return $boxB->getWeight() - $boxA->getWeight(); |
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}); |
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do { |
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$iterationSuccessful = false; |
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foreach ($boxes as $a => &$boxA) { |
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foreach ($boxes as $b => &$boxB) { |
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if ($b <= $a || $boxA->getWeight() === $boxB->getWeight()) { |
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continue; //no need to evaluate |
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} |
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$iterationSuccessful = $this->equaliseWeight($boxA, $boxB, $targetWeight); |
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if ($iterationSuccessful) { |
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$boxes = array_filter($boxes, static function ($box) { //remove any now-empty boxes from the list |
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return $box instanceof PackedBox; |
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}); |
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break 2; |
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} |
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} |
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} |
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} while ($iterationSuccessful); |
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//Combine back into a single list |
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$packedBoxes = new PackedBoxList(); |
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$packedBoxes->insertFromArray($boxes); |
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return $packedBoxes; |
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} |
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/** |
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* Attempt to equalise weight distribution between 2 boxes. |
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* |
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* @param PackedBox $boxA |
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* @param PackedBox $boxB |
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* @param float $targetWeight |
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* |
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* @return bool was the weight rebalanced? |
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*/ |
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private function equaliseWeight(PackedBox &$boxA, PackedBox &$boxB, $targetWeight) |
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{ |
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$anyIterationSuccessful = false; |
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if ($boxA->getWeight() > $boxB->getWeight()) { |
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$overWeightBox = $boxA; |
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$underWeightBox = $boxB; |
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} else { |
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$overWeightBox = $boxB; |
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$underWeightBox = $boxA; |
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} |
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$overWeightBoxItems = $overWeightBox->getItems()->asArray(); |
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$underWeightBoxItems = $underWeightBox->getItems()->asArray(); |
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foreach ($overWeightBoxItems as $key => $overWeightItem) { |
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if (!static::wouldRepackActuallyHelp($overWeightBoxItems, $overWeightItem, $underWeightBoxItems, $targetWeight)) { |
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continue; // moving this item would harm more than help |
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} |
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$newLighterBoxes = $this->doVolumeRepack(array_merge($underWeightBoxItems, [$overWeightItem])); |
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if ($newLighterBoxes->count() !== 1) { |
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continue; //only want to move this item if it still fits in a single box |
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} |
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$underWeightBoxItems[] = $overWeightItem; |
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if (count($overWeightBoxItems) === 1) { //sometimes a repack can be efficient enough to eliminate a box |
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$boxB = $newLighterBoxes->top(); |
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$boxA = null; |
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return true; |
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} |
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unset($overWeightBoxItems[$key]); |
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$newHeavierBoxes = $this->doVolumeRepack($overWeightBoxItems); |
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if (count($newHeavierBoxes) !== 1) { |
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continue; //this should never happen, if we can pack n+1 into the box, we should be able to pack n |
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} |
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$underWeightBox = $boxB = $newLighterBoxes->top(); |
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$boxA = $newHeavierBoxes->top(); |
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$anyIterationSuccessful = true; |
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} |
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return $anyIterationSuccessful; |
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} |
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/** |
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* Do a volume repack of a set of items. |
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* |
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* @param array $items |
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* |
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* @return PackedBoxList |
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*/ |
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private function doVolumeRepack($items) |
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{ |
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$packer = new Packer(); |
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$packer->setBoxes($this->boxes); // use the full set of boxes to allow smaller/larger for full efficiency |
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$packer->setItems($items); |
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return $packer->doVolumePacking(true, true); |
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} |
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/** |
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* Not every attempted repack is actually helpful - sometimes moving an item between two otherwise identical |
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* boxes, or sometimes the box used for the now lighter set of items actually weighs more when empty causing |
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* an increase in total weight. |
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*/ |
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private static function wouldRepackActuallyHelp(array $overWeightBoxItems, Item $overWeightItem, array $underWeightBoxItems, $targetWeight) |
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{ |
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$overWeightItemsWeight = array_sum(array_map(static function (Item $item) {return $item->getWeight(); }, $overWeightBoxItems)); |
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$underWeightItemsWeight = array_sum(array_map(static function (Item $item) {return $item->getWeight(); }, $underWeightBoxItems)); |
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if ($overWeightItem->getWeight() + $underWeightItemsWeight > $targetWeight) { |
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return false; |
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} |
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$oldVariance = static::calculateVariance($overWeightItemsWeight, $underWeightItemsWeight); |
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$newVariance = static::calculateVariance($overWeightItemsWeight - $overWeightItem->getWeight(), $underWeightItemsWeight + $overWeightItem->getWeight()); |
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return $newVariance < $oldVariance; |
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} |
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private static function calculateVariance($boxAWeight, $boxBWeight) |
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{ |
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return pow($boxAWeight - (($boxAWeight + $boxBWeight) / 2), 2); //don't need to calculate B and ÷ 2, for a 2-item population the difference from mean is the same for each box |
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} |
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} |
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dvdoug /
BoxPacker
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