/app/code/core/Mage/Checkout/Model/Cart.php

<?php
/**
 * Magento
 *
 * NOTICE OF LICENSE
 *
 * This source file is subject to the Open Software License (OSL 3.0)
 * that is bundled with this package in the file LICENSE.txt.
 * It is also available through the world-wide-web at this URL:
 * http://opensource.org/licenses/osl-3.0.php
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to license@magentocommerce.com so we can send you a copy immediately.
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade Magento to newer
 * versions in the future. If you wish to customize Magento for your
 * needs please refer to http://www.magentocommerce.com for more information.
 *
 * @category    Mage
 * @package     Mage_Checkout
 * @copyright   Copyright (c) 2010 Magento Inc. (http://www.magentocommerce.com)
 * @license     http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 */

/**
 * Shoping cart model
 *
 * @category    Mage
 * @package     Mage_Checkout
 * @author      Magento Core Team <core@magentocommerce.com>
 */
class Mage_Checkout_Model_Cart extends Varien_Object
{
    protected $_summaryQty = null;
    protected $_productIds = null;

    /**
     * Get shopping cart resource model
     */
    protected function _getResource()
    {
        return Mage::getResourceSingleton('checkout/cart');
    }

    /**
     * Retrieve checkout session model
     *
     * @return Mage_Checkout_Model_Session
     */
    public function getCheckoutSession()
    {
        return Mage::getSingleton('checkout/session');
    }

    /**
     * Retrieve custome session model
     *
     * @return Mage_Customer_Model_Customer
     */
    public function getCustomerSession()
    {
        return Mage::getSingleton('customer/session');
    }

    public function getItems()
    {
        if (!$this->getQuote()->getId()) {
            return array();
        }
        return $this->getQuote()->getItemsCollection();
    }

    /**
     * Retrieve array of cart product ids
     *
     * @return array
     */
    public function getQuoteProductIds()
    {
        $products = $this->getData('product_ids');
        if (is_null($products)) {
            $products = array();
            foreach ($this->getQuote()->getAllItems() as $item) {
                $products[$item->getProductId()] = $item->getProductId();
            }
            $this->setData('product_ids', $products);
        }
        return $products;
    }

    /**
     * Get quote object associated with cart. By default it is current customer session quote
     *
     * @return Mage_Sales_Model_Quote
     */
    public function getQuote()
    {
        if (!$this->hasData('quote')) {
            $this->setData('quote', $this->getCheckoutSession()->getQuote());
        }
        return $this->_getData('quote');
    }

    /**
     * Initialize cart quote state to be able use it on cart page
     */
    public function init()
    {
        $this->getQuote()->setCheckoutMethod('');

        /**
         * If user try do checkout, reset shipiing and payment data
         */
        if ($this->getCheckoutSession()->getCheckoutState() !== Mage_Checkout_Model_Session::CHECKOUT_STATE_BEGIN) {
            $this->getQuote()
                ->removeAllAddresses()
                ->removePayment();
            $this->getCheckoutSession()->resetCheckout();
        }

        if (!$this->getQuote()->hasItems()) {
            $this->getQuote()->getShippingAddress()
                ->setCollectShippingRates(false)
                ->removeAllShippingRates();
        }

        return $this;
    }

    /**
     * Convert order item to quote item
     *
     * @param Mage_Sales_Model_Order_Item $orderItem
     * @param mixed $qtyFlag if is null set product qty like in order
     * @return Mage_Checkout_Model_Cart
     */
    public function addOrderItem($orderItem, $qtyFlag=null)
    {
        /* @var $orderItem Mage_Sales_Model_Order_Item */
        if (is_null($orderItem->getParentItem())) {
            $product = Mage::getModel('catalog/product')
                ->setStoreId(Mage::app()->getStore()->getId())
                ->load($orderItem->getProductId());
            if (!$product->getId()) {
                return $this;
            }

            $info = $orderItem->getProductOptionByCode('info_buyRequest');
            $info = new Varien_Object($info);
            if (is_null($qtyFlag)) {
                $info->setQty($orderItem->getQtyOrdered());
            } else {
                $info->setQty(1);
            }

            $this->addProduct($product, $info);
        }
        return $this;
    }

    /**
     * Get product object based on requested product information
     *
     * @param   mixed $productInfo
     * @return  Mage_Catalog_Model_Product
     */
    protected function _getProduct($productInfo)
    {
        if ($productInfo instanceof Mage_Catalog_Model_Product) {
            $product = $productInfo;
        } elseif (is_int($productInfo)) {
            $product = Mage::getModel('catalog/product')
                ->setStoreId(Mage::app()->getStore()->getId())
                ->load($productInfo);
        } else {
            Mage::throwException(Mage::helper('checkout')->__('The product could not be found.'));
        }
        return $product;
    }

    /**
     * Get request for product add to cart procedure
     *
     * @param   mixed $requestInfo
     * @return  Varien_Object
     */
    protected function _getProductRequest($requestInfo)
    {
        if ($requestInfo instanceof Varien_Object) {
            $request = $requestInfo;
        } elseif (is_numeric($requestInfo)) {
            $request = new Varien_Object();
            $request->setQty($requestInfo);
        } else {
            $request = new Varien_Object($requestInfo);
        }

        if (!$request->hasQty()) {
            $request->setQty(1);
        }
        return $request;
    }

    /**
     * Add product to shopping cart (quote)
     *
     * @param   int $productId
     * @param   int $qty
     * @return  Mage_Checkout_Model_Cart
     */
    public function addProduct($product, $info=null)
    {
        $product = $this->_getProduct($product);
        $request = $this->_getProductRequest($info);

        //Check if current product already exists in cart
        $productId = $product->getId();
        $items = $this->getQuote()->getAllItems();
        $quoteProduct = null;
        foreach ($items as $item) {
            if ($item->getProductId() == $productId) {
                $quoteProduct = $item;
                break;
            }
        }

        if ($product->getStockItem()) {
            $minimumQty = $product->getStockItem()->getMinSaleQty();
            //If product was not found in cart and there is set minimal qty for it
            if($minimumQty && $minimumQty > 0 && $request->getQty() < $minimumQty && $quoteProduct === null){
                $request->setQty($minimumQty);
            }
        }

        if ($product->getId()) {
            $result = $this->getQuote()->addProduct($product, $request);
            /**
             * String we can get if prepare process has error
             */
            if (is_string($result)) {
                $this->getCheckoutSession()->setRedirectUrl($product->getProductUrl());
                if ($this->getCheckoutSession()->getUseNotice() === null) {
                    $this->getCheckoutSession()->setUseNotice(true);
                }
                Mage::throwException($result);
            }
        }
        else {
            Mage::throwException(Mage::helper('checkout')->__('The product does not exist.'));
        }

        Mage::dispatchEvent('checkout_cart_product_add_after', array('quote_item'=>$result, 'product'=>$product));
        $this->getCheckoutSession()->setLastAddedProductId($product->getId());
        return $this;
    }

    /**
     * Adding products to cart by ids
     *
     * @param   array $productIds
     * @return  Mage_Checkout_Model_Cart
     */
    public function addProductsByIds($productIds)
    {
        $allAvailable = true;
        $allAdded     = true;

        if (!empty($productIds)) {
            foreach ($productIds as $productId) {
                $productId = (int) $productId;
                if (!$productId) {
                    continue;
                }
                $product = Mage::getModel('catalog/product')
                    ->setStoreId(Mage::app()->getStore()->getId())
                    ->load($productId);
                if ($product->getId() && $product->isVisibleInCatalog()) {
                    try {
                        $this->getQuote()->addProduct($product);
                    } catch (Exception $e){
                        $allAdded = false;
                    }
                } else {
                    $allAvailable = false;
                }
            }

            if (!$allAvailable) {
                $this->getCheckoutSession()->addError(
                    Mage::helper('checkout')->__('Some of the requested products are unavailable.')
                );
            }
            if (!$allAdded) {
                $this->getCheckoutSession()->addError(
                    Mage::helper('checkout')->__('Some of the requested products are not available in the desired quantity.')
                );
            }
        }
        return $this;
    }

    /**
     * Update cart items information
     *
     * @param   array $data
     * @return  Mage_Checkout_Model_Cart
     */
    public function updateItems($data)
    {
        Mage::dispatchEvent('checkout_cart_update_items_before', array('cart'=>$this, 'info'=>$data));

        foreach ($data as $itemId => $itemInfo) {
            $item = $this->getQuote()->getItemById($itemId);
            if (!$item) {
                continue;
            }

            if (!empty($itemInfo['remove']) || (isset($itemInfo['qty']) && $itemInfo['qty']=='0')) {
                $this->removeItem($itemId);
                continue;
            }

            $qty = isset($itemInfo['qty']) ? (float) $itemInfo['qty'] : false;
            if ($qty > 0) {
                $item->setQty($qty);
            }
        }

        Mage::dispatchEvent('checkout_cart_update_items_after', array('cart'=>$this, 'info'=>$data));
        return $this;
    }

    /**
     * Remove item from cart
     *
     * @param   int $itemId
     * @return  Mage_Checkout_Model_Cart
     */
    public function removeItem($itemId)
    {
        $this->getQuote()->removeItem($itemId);
        return $this;
    }

    /**
     * Save cart
     *
     * @return Mage_Checkout_Model_Cart
     */
    public function save()
    {
        $this->getQuote()->getBillingAddress();
        $this->getQuote()->getShippingAddress()->setCollectShippingRates(true);
        $this->getQuote()->collectTotals();
        $this->getQuote()->save();
        $this->getCheckoutSession()->setQuoteId($this->getQuote()->getId());
        /**
         * Cart save usually called after chenges with cart items.
         */
        Mage::dispatchEvent('checkout_cart_save_after', array('cart'=>$this));
        return $this;
    }

    public function truncate()
    {
        foreach ($this->getQuote()->getItemsCollection() as $item) {
            $item->isDeleted(true);
        }
    }

    public function getProductIds()
    {
        $quoteId = Mage::getSingleton('checkout/session')->getQuoteId();
        if (null === $this->_productIds) {
            $this->_productIds = array();
            if ($this->getSummaryQty()>0) {
               foreach ($this->getQuote()->getAllItems() as $item) {
                   $this->_productIds[] = $item->getProductId();
               }
            }
            $this->_productIds = array_unique($this->_productIds);
        }
        return $this->_productIds;
    }

    /**
     * Get shopping cart items summary (inchlude config settings)
     *
     * @return decimal
     */
    public function getSummaryQty()
    {
        $quoteId = Mage::getSingleton('checkout/session')->getQuoteId();

        //If there is no quote id in session trying to load quote
        //and get new quote id. This is done for cases when quote was created
        //not by customer (from backend for example).
        if (!$quoteId && Mage::getSingleton('customer/session')->isLoggedIn()) {
            $quote = Mage::getSingleton('checkout/session')->getQuote();
            $quoteId = Mage::getSingleton('checkout/session')->getQuoteId();
        }

        if ($quoteId && $this->_summaryQty === null) {
            if (Mage::getStoreConfig('checkout/cart_link/use_qty')) {
                $this->_summaryQty = $this->getItemsQty();
            }
            else {
                $this->_summaryQty = $this->getItemsCount();
            }
        }
        return $this->_summaryQty;
    }

    /**
     * Get shopping cart items count
     *
     * @return int
     */
    public function getItemsCount()
    {
        return $this->getQuote()->getItemsCount()*1;
    }

    /**
     * Get shopping cart summary qty
     *
     * @return decimal
     */
    public function getItemsQty()
    {
        return $this->getQuote()->getItemsQty()*1;
    }
}