Magento – How magento split the discount amount on each ordered item in case of shopping cart rule having fixed amount of discount on whole cart

Welcome to the rabbit hole. You are going to have to deal with Mage/SalesRule and most likely the generic Mage/Rule.

However, as I've mentioned before, shopping cart rule is a very bad name. They're not shopping cart rules, they are checkout, aka Cash Register rules, as they're not evaluated in the cart but in the checkout.

If you really want to do this, you will have to duplicate the rule evaluation code from the checkout and probably alter it, because some rules (like free shipping) depend on the availability of an address object or customer group, which you can't count on. Start with Mage::getModel('salesrule/rule')->getCollection().

Personally, I would hardcode this and just check for the products in question in the cart, if this is a one time deal, but if this is common functionality for the shop, I wish you the best of luck ;-)

Edit: Assuming you have at least a rule ID, you can get conditions using Mage::getModel('salesrule/rule')->load($id)->getConditions(). The implementation is in Mage_Rule_Model_Abstract.

The individual sales rules are possible, so you can create a cart rule that will give a percentage for any order containing items bought from a given category. You can then create other rules for other categories. So each of them could cumulatively give e.g. 2% off the order.

However, those rules are not aware of each other. So they do not know what the current discount is (from the other rules) and cannot then have the 'if' to decide whether a further discount is possible.

As for the product specific discount, you can do that one with a rule or a just a special price on the product.

If you want to do this with a module then the easy bit is coding the rules that you want. Quite a few other changes are needed. There may be a need to update templates (to show your extra line item), some xml for 'totals_sort' order - it goes on.

I also feel that your original sales idea might actually be confusing for customers. Sometimes the spend X get Y rules need to be easy for people to understand.