Bringing in value of Coupon Code into "tax" calculation.php in magento

Im trying to pull in the name of the discount code currently applied to the cart into the calculation.php file. The name of the discount code is optionalTax but Im having trouble passing it through or retrieving it directly. Its appear开发者_如何学运维s to be referenced as $quote->getCouponCode() in mage/sales/model/quote.php and I want to use it in mage/tax/model/calculation.php

Anyone have any idea on how to call it in as I've tried using the model as per (which I think is correct)

public function calcTaxAmount($price, $taxRate, $priceIncludeTax=false, $round=true)
    {
        $taxRate = $taxRate/100;

        if ($priceIncludeTax) {
            $amount = $price*(1-1/(1+$taxRate));
        } else {
   $cModel = Mage::getModel('catalog/sales');
   $thisDiscountCode = $cModel->$quote->getCouponCode();
   die($thisDiscountCode);
   $amount = $price*$taxRate; 
        }

        if ($round) {
            return $this->round($amount);
        } else {
            return $amount;
        }
    }

Chris

you need to get the quote from right model i guess:

Mage::getSingleton('checkout/session')->getQuote();