Marine Biology Simulation Case Study

Supplemental Example:
Fast Fish

Problem Specification

The marine biologists decided that they would also like faster-moving fish in the simulation. In particular, the biologists decided that: 

• A fast fish can move one or two cells away in a single timestep, in any of the four directions. It can only move to an empty location two cells away if the first cell in the two-step sequence is empty also. In other words, if an immediately adjacent location is empty, the fast fish looks at the cell beyond it in the same direction to see if it is empty also. The left diagram below shows all the possible neighboring locations (~) of a fast fish (F). The diagram on the right shows how neighboring fish (N) can block the fast fish from moving to some of its empty neighboring locations (the ones marked X). Fast fish, like other fish, move only if they do not breed.

             ~                   X
             ~                   N  
         ~ ~ F ~ ~           X N F ~ ~
             ~                   ~  
             ~                   ~
  

The key to the movement of fast fish is the way they choose the empty neighboring locations to which they could move.  Fast fish move within a wider neighborhood than just their immediately adjacent neighbors. You may want to create a new method, findMoveLocs, that would find the possible move locations in the wider neighborhood, and then redefine the nextLocation method to call findMoveLocs rather than calling emptyNeighbors.  

As you design your implementation of the fast fish subclass, be sure to consider fast fish in different locations and with neighbors in different locations, as in the two examples above or the example below.  For each configuration, identify the locations to which the fast fish might move.  Then develop test cases and expected results to test your new fast fish subclass.

       F  ~  ~  ~
       ~  ~  ~  ~
       ~  ~  ~  ~
       ~  ~  ~  ~

You might want to make all fast fish one color (such as cyan) to help them show up as you run the simulation, just as Pat did for darters and slow fish.  Then develop an initial configuration file that contains fast fish (or a mixture of fast fish and your other Fish subclasses).  Test your new subclass using the test cases you developed earlier.

Refactoring Variant

Sometimes when adding functionality to a program, you realize that you could have designed it differently in the first place in a way that would make it easier to modify now. You might even decide that it's worth going back and changing the original before making your new changes. This is called refactoring.

Modify the Fish class to break up the nextLocation method and create a findMoveLocs method there. Which pieces of the current nextLocation method in Fish should stay in nextLocation and which should move to findMoveLocs? If you design this correctly, neither DarterFish nor SlowFish will have to change at all, and your new fast fish subclass will only have to redefine findMoveLocs, not nextLocation.