Sunday, March 13, 2022

Wolf, Goat and Cabbage Part II

This continues my previous post about the "Wolf, Goat and Cabbage" logic puzzle. Using a MIP model to solve it strikes me as somewhat analogous to hammering in a nail with the butt end of a screwdriver handle: it works, but maybe it's not the ideal tool. To me, a constraint solver would make more sense, so I coded up a model using IBM's CP Optimizer (henceforth "CPO", part of the CPLEX Optimization Suite). Unsurprisingly, it worked (after some debugging) and produced the same solution.

MIP solvers largely share the same "vocabulary" for building models (real / integer / binary variables, linear and maybe quadratic expressions, equality and inequality constraints). From my limited exposure to constraint solvers, there is significant variation in both what things the solver does and does not understand. Some of that is variable types. A very basic solver may understand logical (true/false) and integer variables, and maybe real-valued variables (although I'm not sure handling real variables is anywhere near universal). CPO (and presumably some other solvers) understand "interval variables", which as the name suggests represent intervals of discrete values (usually time intervals, though possibly location or some other aspect). Similarly, different solvers will understand different types of global constraints. I suspect that every CP solver worth mentioning understands "all different" (no two variables in a bunch of integer variables can take the same value), but some solvers will implement "no overlap" constraints (the time interval during which I am eating and the time interval during which I am showering cannot overlap) or precedence constraints (this job has to end before this other job can start). Those kinds of constraints make certain scheduling problems easier to solve with CP than with a MIP model.

Anyway, I'm not entirely new to CPO, though far from proficient, and I tripped over a few "features" while coding the puzzle. I wanted to use boolean (true/false) variables for certain things, such as whether an item had made it to the far side of the river (true) or was stuck on the near side (false). CPO lets you declare a boolean variable but then treats it as an integer variable, meaning that you need to think in terms of 0 and 1 rather than false and true. So you can't say "if $x$ then ..."; instead, you need to say "if $x = 1$ then ..." (and trust me, the syntax is clunkier than what I'm typing here). When you go to get the value of your boolean variable $x$ after solving the model, CPO returns a double precision value. CPLEX users will be used to this, because in a MIP model even integer variables are treated as double-precision during matrix computations. CP solvers, though, like to do integer arithmetic (as far as I know), so it's a bit unclear why my boolean variable has to be converted from double precision to integer (or boolean). Even odder is that, at least in the Java API, there is a method that returns the value of an integer variable as an integer if you pass the name of the variable as the argument, but if you pass the actual variable you are going to get a double. (Did a federal government panel design this?)

In any event, logic of the CPO model is moderately straightforward, with constraints like "you can't carry something in the boat if it isn't on the bank the boat departs from" and "if the wolf and goat end up in the same place at the end of a period, the farmer better end up there too". Some things are bit less clunky with CPO than with CPLEX. For instance, to figure out what (if anything) is in the boat at a given time, the MIP model requires binary variables subscripted by time and item index (1 if that item is in the boat on that trip 0 if not). The CPO model just needs an integer variable for each time period whose value is either the index of the thing in the boat or a dummy value if the boat is empty. Furthermore, the nature of the variable automatically takes care of a capacity constraint. Since there is only one variable for what's in the boat, at most one thing (whatever that variable indicates) can be in the boat.

Some (most?) constraint solvers, including CPO, provide a way to use a variable as an index to another variable. In my code, the integer variable indicating what's in the boat at time $t$ is used to look up the location variable (near or far bank) for that item at time $t$ from a vector of location variables for all items at time $t$.

Anyway, the code in my repository has been updated to include the CPO model, and it's heavily commented in case you wanted to compare it to the MIP model.