Solutions for the Missionaries and Cannibals Problem.
In this problem, three missionaries and three cannibals must cross a river using a boat which can carry at most two people, under the constraint that, for both banks, if there are missionaries present on the bank, they cannot be outnumbered by cannibals (if they were, the cannibals would eat the missionaries). The boat cannot cross the river by itself with no people on board. And, in some variations, one of the cannibals has only one arm and cannot row. [source: Wikipedia]
The problem was solved using three different languages: Java, Python and Prolog:
A State class saves the current state of the problem, that is, how many missionaries and cannibals are in each side of the river and where is the boat (left or right).
The generateSuccessors method checks the actions (e.g. cross a two missionaries from left to right) that can be applied to a particular state and returns the valid successor states.
The initial state of the problem is passed as input to the bread first search algorithm (class BreadthFirstSearch) that returns the solution to the problem.
The Java solution can be found in the java folder. You can use the jar executable file to run it:
java -jar missionaries_and_cannibals.jar
First was defined the 10 rules that determine which are the possible successor states for each possible action. Then a recursive rule path is responsible for find the solution of the problem.
Running the Prolog code in SWI-Prolog:
?- ['missionaries_and_cannibals.pl'].
?- find.
The Python solution is similar to the Java one. A State class saves the current state of the problem. The successors method checks the actions that can be applied to each state. And the breadth_first_search method returns the solution to the problem.
Running the Python solution:
python missionaries_and_cannibals.py