Discount information for order in Magento

I need information which promotion/coupon changed which item in order and what was the price of this item before discount and the final price of this item. I know, it's complicated so here's an example:

<order>
<items>
<item> <!-- This item have discount in order -->
<sku>1234</sku>
<promotion>
<promo_id>456</promo_id>
<discount_value>10</discount_value><!-- In % or $ -->
</promotion>
<final_price>25</final_price>
</item>
<item><!-- This item don't have discount in order -->
<sku>1234</sku>
<promotion/>
<final_price>35</final_price>
</item>
</items>
</order>

I hope it's comprehensible explanation. Thanks for all help.

EDIT: I forgot about one thing. All i have it's information from sales/order model. It's an interface, so i don't have access to session data, I get validated orders from database.

You can easily retrieve applied shopping cart rule ids from order item model.

1. You need to walk-though order items to collect all applied rule ids to items:

   $collectedRuleIds = array();
   $itemRules = array();
   foreach ($order->getAllVisibleItems() as $orderItem) {
       if ($orderItem->getAppliedRuleIds()) {
           $itemRules[$orderItem->getId()] = explode(',', $orderItem->getAppliedRuleIds());
           $collectedRuleIds = array_merge($collectedRuleIds, $itemRules[$orderItem->getId()]);
       }
   }
   

2. Then load rules collection, to retrieve rule information and assign it to order items.

   $rules = false;
   if ($collectedRuleIds) {
       $rules = Mage::getModel('salesrule/rule')->getCollection()->addFieldToFilter('rule_id', array('in' => $collectedRuleIds));
   }
   
   if ($rules) {
       foreach ($itemRules as $itemId => $ruleIds) {
           $orderItem = $order->getItemById($itemId);
           $appliedRules = array();
           foreach ($ruleIds as $ruleId) {
               if ($rules->getItemById($ruleId)) {
                   $appliedRules[] = $rules->getItemById($ruleId);
               }
           }
           $orderItem->setAppliedRules($appliedRules);
       }
   }
   

3. During exporting data, just check getAppliedRules property of order item to get information about type of promotion for building proper export file.