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MissionariesAndCannibals.pl
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MissionariesAndCannibals.pl
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/*
https://en.wikipedia.org/wiki/Missionaries_and_cannibals_problem
http://www.geeksforgeeks.org/missionaries-and-cannibals/
http://artificialintelligence-notes.blogspot.in/2011/04/missionaries-and-cannibals-problem.html
http://learnfrommike.blogspot.in/2012/09/solving-missionaries-and-cannibals.html
http://www.dailyfreecode.com/Code/production-system-missionary-cannibal-3046.aspx
http://www.dailyfreecode.com/Code/prolog-missionary-cannibal-problem-3052.aspx
*/
% A Prolog program for the missionaries and cannibals problem (Problem 3.9, page 115).
% To run this problem, please start gprolog on a CS Linux
% workstation by typing gprolog. At the prompt ?- type in "[mission]." as
% follows:
% | ?- [mission].
% and then type in "solve." as follows:
% | ?- solve.
% To quit Prolog, type "halt." at ?-
% You may interrupt the execution by typing Ctrl-C and then "a" for abort.
solve :-
initial( Start),
breadthfirst( [ [Start] ], Solution),
% Solution is a path (in reverse order) from initial to a goal
write(Solution), nl,
printsol(Solution).
% safe(NumOfMissionaries, NumOfCannibals) is true if NumOfMissionaries is 0 or 3 or equal to NumOfCannibals
safe(0, _).
safe(3, _).
safe(X, X).
% A state is represented by a term:
% state( NumOfMissionaries, NumOfCannibals, BoatAtEast)
initial(state(3,3,1)).
goal(state(0,0,0)).
goalpath([Node | _]) :- goal(Node).
% move( State1, State2): making a move in State1 results in State2;
move( state( M1, C1, 1), % Before move
state( M2, C1, 0) ) % After move
:- M1 > 1, M2 is M1-2, safe(M2, C1). % Two missionaries from east to west
move( state( M1, C1, 0), % Before move
state( M2, C1, 1) ) % After move
:- M1 < 2, M2 is M1+2, safe(M2, C1). % Two missionaries from west to east
move( state( M1, C1, 1), % Before move
state( M1, C2, 0) ) % After move
:- C1 > 1, C2 is C1-2, safe(M1, C2). % Two cannibals from east to west
move( state( M1, C1, 0), % Before move
state( M1, C2, 1) ) % After move
:- C1 < 2, C2 is C1+2, safe(M1, C2). % Two cannibals from west to east
move( state( M1, C1, 1), % Before move
state( M1, C2, 0) ) % After move
:- C1 > 0, C2 is C1-1, safe(M1, C2). % One cannibal from east to west
move( state( M1, C1, 0), % Before move
state( M1, C2, 1) ) % After move
:- C1 < 3, C2 is C1+1, safe(M1, C2). % One cannibal from west to east
move( state( M1, C1, 1), % Before move
state( M2, C2, 0) ) % After move
:- M1 > 0, M2 is M1-1, % One missionary and one
C1 > 0, C2 is C1-1, safe(M2, C2). % cannibal from east to west
move( state( M1, C1, 0), % Before move
state( M2, C2, 1) ) % After move
:- M1 < 3, M2 is M1+1, % One missionary and one
C1 < 3, C2 is C1+1, safe(M2, C2). % cannibal from west to east
printsol([X]) :- write(X), write(': initial state'), nl.
printsol([X,Y|Z]) :- printsol([Y | Z]), write(X), explain(Y, X), nl.
explain(state(M1, C1, 1), state(M2, C2, _)) :-
X is M1-M2, Y is C1-C2,
write(': '), write(X), write(' missionaries and '),
write(Y), write(' cannibals moved from East to West').
explain(state(M1, C1, 0), state(M2, C2, _)) :-
X is M2-M1, Y is C2-C1,
write(': '), write(X), write(' missionaries and '),
write(Y), write(' cannibals moved from West to East').
% An implementation of breadth-first search.
% breadthfirst( [ Path1, Path2, ...], Solution):
% each Pathi represents [Node | Ancestors ], where Node is in the open list and
% Ancestors is a path from the parent of Node to the initial node in the search tree.
% Solution is a path (in reverse order) from initial to a goal.
breadthfirst( [ Path | _], Path) :-
goalpath( Path ). % if Path is a goal-path, then it is a solution.
breadthfirst( [Path | Paths], Solution) :-
extend( Path, NewPaths),
append( Paths, NewPaths, Paths1),
breadthfirst( Paths1, Solution).
% setof(X, Condition, Set) is a built-in function: it collects all X satisfying Condition into Set.
extend( [Node | Path], NewPaths) :-
setof( [NewNode, Node | Path],
( move( Node, NewNode), not(member( NewNode, [Node | Path] )) ),
NewPaths),
!.
extend( _, [] ). % setof failed: Node has no successor
not(P) :- P, !, fail.
not(_).