How to change discount policy

[ezakimak]

Hi everyone,

here in my store, discount policy has changed again,
the default in prestashop 1.2.5 is based on reduction_percent per product or customer group , but
this time I need to apply a discount based on the total of the purchase ,
for example

* if total of order >= 1000 & < 5000 , apply 10% discount

* if total of order >= 5000 & < 10000, apply 13% discount

* if total of order >= 10000 & < 20000, apply 15% discount

* if total of order >= 20000, apply 20% discount


anyone has idea where to start , I guess I will have to change a lot of code in ps

thanks

regards