Friday, March 11, 2022

Wolf, Goat and Cabbage

On Operations Research Stack Exchange, someone asked about a possible connection between the "wolf, goat and cabbage" logic puzzle and Monge's optimal transport problem. In the logic puzzle, a farmer has to get a wolf, a goat and a cabbage across a river using a boat that can only accommodate one of them (plus the farmer) at a time. If you leave the wolf and goat alone together at any point, the wolf eats the goat. If you leave the goat and cabbage alone together at any point, the goat eats the cabbage. Fortunately, the wolf has no appetite for cabbage and the cabbage does not seem to want to eat anything, else the problem would be infeasible.

Neither Monge's transport problem nor the more commonly taught (in my opinion) Hitchcock transportation problem directly apply, although you can (almost) treat the puzzle as a multiperiod commodity flow with an "inventory" of each item (wolf, goat, cabbage) on each side of the river. The "almost" part is that you need some of the variables to be integer, for two reasons. One is that the LP relaxation of the logic constraints (e.g., inventory of wolf + inventory of goat $\le 1$ on this side of the river at this time) will result in fractional values (we'll leave half a wolf and half a goat here and carry the other halves across the river) (which would greatly diminish the values of both wolf and goat). The other is that the objective is to minimize the number of trips made. It would be tempting to just assign a cost of 1 to each movement of an object in either direction, but the catch is that you will occasionally cross with nothing in the boat (besides the farmer). Those "deadheading" trips count toward the objective, but it's tricky to assign a cost to a zero flow.

To fill in some idle time, I coded up a MIP model. Mind you, I am not advocating MIP as a way to solve problems like this; I just wanted to confirm my thinking (in particular, that an LP commodity flow model would have fractional solutions). Assume that the farmer arrives at the left bank at time 0 with all three items, and that each trip (in either direction) counts as one time unit, with the first trip occurring at time 1. We need to set an upper bound $T$ on the number of trips. Since the state of the system is described by the location of four things (counting the farmer), with each have two possible locations (left bank, right bank), $T=2^4 =16$ works. The set of items will be denoted $I=\lbrace W, G, C\rbrace.$ My formulation uses the following variables.

  • $L_{i,t}\in [0,1]$ and $R_{i,t}\in [0,1]$ are the inventories of item $i\in I$ at time $t\in \lbrace 0,\dots,T$ on the left and right banks respectively, after any trip occurring in that period.
  • $x_{i,t}\in \lbrace 0,1 \rbrace$ and $y_{i,t}\in \lbrace 0,1 \rbrace$ are the amount of item $i$ crossing the river at time $t$ from left to right or right to left respectively.
  • $z_t\in \lbrace 0,1 \rbrace$ is 1 if transport is ongoing and 0 if we are done (the farmer and all three items are on the right bank).

It would be perfectly fine (but unnecessary) to make the inventory variables integer-valued, and we could also make the inventory variables integer-valued and drop the integrality restrictions on the cartage variables ($x$ and $y$).

Some of the variables can be fixed at the outset.

  • We start with all inventory on the left bank, so $L_{i,0}=1$ and $R_{i,0}=0$ for all $i\in I.$ 
  • There is no trip at time 0, so $z_0=0$ and $x_{i,0}=y_{i,0}$ for all $i\in I$.
  • Trips alternate left-to-right (odd time periods) and right-to-left (even time periods), so $x_{i,t}=0$ for all $i\in I$ and for all even $t$ and $y_{i,t}=0$ for all $i\in I$ and for all odd $t$.

The objective function is to minimize the number of trips required. $$\min \sum_{t=1}^T z_t.$$

The constraints are rather straightforward.

  • The inventory on either bank in any period is the inventory on that bank from the preceding period, plus any arriving inventory and minus any departing inventory. So for $t\ge 1$ $$L_{i,t} = L_{i, t-1} - x_{i,t} + y_{i,t}\quad \forall i\in I$$ and $$R_{i,t} = R_{i,t-1} + x_{i,t} - y_{i,t}\quad \forall i\in I.$$
  • In an odd numbered period (where the farmer ends up on the right bank), neither wolf and goat nor goat and cabbage can be on the left bank. So for odd $t$ $$L_{W,t} + L_{G,t} \le 1$$ and $$L_{G,t} + L_{C,t}\le 1.$$
  • In an even numbered period (where the farmer ends up on the left bank), neither wolf and goat nor goat and cabbage can be on the right bank unless the problem is completed ($z_t = 0$), in which case the farmer remains on the right bank. So for even $t$ $$R_{W,t}+R_{G_t} + z_t \le 2$$ and $$R_{G,t} + R_{C,t} + z_t \le 2.$$
  • Transport continues until the left bank is empty. $$3z_t \ge \sum_{i\in I} L_{i,t - 1}\quad \forall t\ge 1.$$

It does indeed produce a correct solution, using seven trips (see the Wikipedia page for the solution) ... and with integrality conditions dropped it does indeed produce a nonsense solution with fractions of items being transported.

Java code for this model (using CPLEX) is in my Git repository.

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