warehouse putaway management application

I want to create a software which gets active as it receives the message of invert(items) and then my software must suggest the user the proper place to store it on another software called 'Swim'. Swim is the basically the software that runs all the putaway operations There are plenty of cases and I need to cover them all inside my program. I want to kickstart my programming with one of the cases that is mentioned below Suppose there are two rows R1 and R2. In R1 the last stack is empty whereas in the R2 the second stack is empty. So my software must suggest the user running the Swim software that this new SKU has to be kept in the R2 second stack as it will be the nearest. So there are more cases like this and I have to include all of them to improve the putaway process efficient. So could someone help me with the code. so that i could understand the flow of the program as right now i am totally cluless.