/* Simple random number generator.
   Good for numbers in some range less than 100,000 long.
   Called thus:
	random(X, Lower, Upper)
   where Lower and Upper must be instantiated to integers,
   and X must be uninstantiated. Instantiates X to an integer
   in the range    Lower ... Upper-1.

   Cobbled up by Peter Ross, 3 May 1983
*/
seed(27134, 9213, 17773).

getrand(X) :-   seed(A,B,C),
                A1 is (A*171) mod 30269,
                B1 is (B*170) mod 30307,
                C1 is (C*172) mod 30323,
		retract(seed(A,B,C)),
		asserta(seed(A1,B1,C1)),
		X is (7*A + 11*B + 13*C) mod 100000.

random(X,Lower,Upper) :-  (nonvar(X); var(Lower); var(Upper); Upper =< Lower),
			  !,
			  write('*** error in random(_,_,_) ***'), nl.
random(X,Lower,Upper) :-  Range is Upper-Lower,
			  getrand(X1),
			  !,
			  X is (X1 mod Range) + Lower.

/* why the cut in random, you may ask? So that your program can backtrack
   past random(_,_,_), and efficiently ...
*/