<?php
/* app/code/community/Inchoo/Sale/Block/Catalog/Layer/Filter/Sale.php */
 
class Inchoo_Sale_Block_Catalog_Layer_Filter_Sale 
extends Mage_Catalog_Block_Layer_Filter_Abstract
{
 
    public function __construct()
    {
        parent::__construct();
        $this->_filterModelName = 'inchoo_sale/catalog_layer_filter_sale';
    }
 
}

<?php
/* app/code/community/Inchoo/Sale/Model/Catalog/Layer/Filter/Sale.php */
 
class Inchoo_Sale_Model_Catalog_Layer_Filter_Sale 
extends Mage_Catalog_Model_Layer_Filter_Abstract
{
 
    const FILTER_ON_SALE = 1;
    const FILTER_NOT_ON_SALE = 2;
 
    /**
     * Class constructor
     */
    public function __construct()
    {
        parent::__construct();
        $this->_requestVar = 'sale';
    }
 
    /**
     * Apply sale filter to layer
     *
     * @param   Zend_Controller_Request_Abstract $request
     * @param   Mage_Core_Block_Abstract $filterBlock
     * @return  Mage_Catalog_Model_Layer_Filter_Sale
     */
    public function apply(Zend_Controller_Request_Abstract $request, $filterBlock)
    {
        $filter = (int) $request->getParam($this->getRequestVar());
        if (!$filter || Mage::registry('inchoo_sale_filter')) {
            return $this;
        }
 
        $select = $this->getLayer()->getProductCollection()->getSelect();
        /* @var $select Zend_Db_Select */
 
        if ($filter == self::FILTER_ON_SALE) {
            $select->where('price_index.final_price < price_index.price');
            $stateLabel = Mage::helper('inchoo_sale')->__('On Sale');
        } else {
            $select->where('price_index.final_price >= price_index.price');
            $stateLabel = Mage::helper('inchoo_sale')->__('Not On Sale');
        }
 
        $state = $this->_createItem(
            $stateLabel, $filter
        )->setVar($this->_requestVar);
        /* @var $state Mage_Catalog_Model_Layer_Filter_Item */
 
        $this->getLayer()->getState()->addFilter($state);
 
        Mage::register('inchoo_sale_filter', true);
 
        return $this;
    }
 
    /**
     * Get filter name
     *
     * @return string
     */
    public function getName()
    {
        return Mage::helper('inchoo_sale')->__('Sale');
    }
 
    /**
     * Get data array for building sale filter items
     *
     * @return array
     */
    protected function _getItemsData()
    {
        $data = array();
        $status = $this->_getCount();
 
        $data[] = array(
            'label' => Mage::helper('inchoo_sale')->__('On Sale'),
            'value' => self::FILTER_ON_SALE,
            'count' => $status['yes'],
        );
 
        $data[] = array(
            'label' => Mage::helper('inchoo_sale')->__('Not On Sale'),
            'value' => self::FILTER_NOT_ON_SALE,
            'count' => $status['no'],
        );
        return $data;
    }
 
    protected function _getCount()
    {
        // Clone the select
        $select = clone $this->getLayer()->getProductCollection()->getSelect();
        /* @var $select Zend_Db_Select */
 
        $select->reset(Zend_Db_Select::ORDER);
        $select->reset(Zend_Db_Select::LIMIT_COUNT);
        $select->reset(Zend_Db_Select::LIMIT_OFFSET);
        $select->reset(Zend_Db_Select::WHERE);
 
        // Count the on sale and not on sale
        $sql = 'SELECT IF(final_price >= price, "no", "yes") as on_sale, COUNT(*) as count from ('
                .$select->__toString().') AS q GROUP BY on_sale';
 
        $connection = Mage::getSingleton('core/resource')->getConnection('core_read');
        /* @var $connection Zend_Db_Adapter_Abstract */
 
        return $connection->fetchPairs($sql);
    }
 
}

<?php
/* app/code/community/Inchoo/Sale/Block/Catalog/Layer/View.php */
 
class Inchoo_Sale_Block_Catalog_Layer_View 
extends Mage_Catalog_Block_Layer_View
{
    const SALE_FILTER_POSITION = 2;
 
    /**
     * State block name
     *
     * @var string
     */
    protected $_saleBlockName;
 
    /**
     * Initialize blocks names
     */
    protected function _initBlocks()
    {
        parent::_initBlocks();
 
        $this->_saleBlockName = 'inchoo_sale/catalog_layer_filter_sale';
    }
 
    /**
     * Prepare child blocks
     *
     * @return Mage_Catalog_Block_Layer_View
     */
    protected function _prepareLayout()
    {
        $saleBlock = $this->getLayout()->createBlock($this->_saleBlockName)
                ->setLayer($this->getLayer())
                ->init();
 
        $this->setChild('sale_filter', $saleBlock);
 
        return parent::_prepareLayout();
    }
 
    /**
     * Get all layer filters
     *
     * @return array
     */
    public function getFilters()
    {
        $filters = parent::getFilters();
 
        if (($saleFilter = $this->_getSaleFilter())) {
            // Insert sale filter to the self::SALE_FILTER_POSITION position
            $filters = array_merge(
                array_slice(
                    $filters,
                    0,
                    self::SALE_FILTER_POSITION - 1
                ),
                array($saleFilter),
                array_slice(
                    $filters,
                    self::SALE_FILTER_POSITION - 1,
                    count($filters) - 1
                )
            );
        }
 
        return $filters;
    }
 
    /**
     * Get sale filter block
     *
     * @return Mage_Catalog_Block_Layer_Filter_Sale
     */
    protected function _getSaleFilter()
    {
        return $this->getChild('sale_filter');
    }
 
}