% From the book
% PROLOG PROGRAMMING IN DEPTH
% by Michael A. Covington, Donald Nute, and Andre Vellino
% (Prentice Hall, 1997).
% Copyright 1997 Prentice-Hall, Inc.
% For educational use only

% File REALSQR1.PL
% From Chapter 2

% close_enough/2 and early version of real_square_root/2


close_enough(X,X) :- !.

close_enough(X,Y) :- X < Y,
                     Diff is Y-X,
                     Diff < 0.0001.

close_enough(X,Y) :- Y < X,
                     close_enough(Y,X).



real_square_root(X,nonexistent) :- X < 0.0.         % Clause 1

real_square_root(X,Y) :- X >= 0.0,                  % Clause 2
                         R is sqrt(X),
                         close_enough(R,Y).

real_square_root(X,Y) :- X > 0.0,                   % Clause 3
                         R is -sqrt(X),
                         close_enough(R,Y).