The thing is we need would like to count how many results of each type are displayed to user and it's accumulative. For example, if User1 gets "Cat1:13,Cat2:3,Cat3:4" and User2 gets "Cat1:10,Cat2:5,Cat3:0". After those visits, we will have:
Cat1 = 23 (13 + 10)Cat2 = 8 (3 + 5)Cat3 = 4 (4 + 0)