/* Random number generator */

random_pick(List,El):-                  /* to pick random element from a list*/
      length(List,L),
      random(L,N),
      nth(N,List,El).

random(Range,Num):-                     /* to choose random number in range */
      seed(Seed),                       /* get seed from database */
      Num is (Seed mod Range) + 1,      /* fit seed into range */
      retract(seed(Seed)),              /* delete old value of seed */
      NewSeed is (125*Seed+1) mod 4093, /* calculate new value*/
      assert(seed(NewSeed)).            /* and assert it into database */

seed(13).                               /* random number seed database entry */

/* to get the nth element of list */

nth(1,[Hd|T],Hd):- !.                   /* if n is i then return first element*/

nth(N,[Hd|T],El):-                      /* otherwise*/
      N1 is N-1,                        /* decrement n by 1 */
      nth(N1,T,El).                     /* and recurse down list */

/* to find the length of a list */

length(List,L):-lengthl(0,List,L).

lengthl(Ans,[],Ans).
lengthl(L1,[H|T],L2):-
      L is L1+1,
      lengthl(L,T,L2).