:- object(presort,
extends(incremental)).
:- info([
author is 'Paulo Moura',
version is 1.0,
date is 2000/4/22,
comment is '.',
source is 'Example adopted from the Francis G. McCabe L&O documentation.']).
:- uses(quick(_)).
route(Towns, Route) :-
arrange(Towns, Towns2),
^^route(Towns2, Route).
arrange(Towns, Sorted) :-
centre(Towns, X, Y),
quick(geographic(X, Y))::sort(Towns, Sorted).
centre(Towns, X, Y) :-
average(Towns, 0, 0, U, V, 0, L),
X is U/L,
Y is V/L.
average([], U, V, U, V, L, L).
average([Town| Towns], UX, VX, U, V, I, L):-
Town::at(UT, VT),
UX2 is UX+UT,
VX2 is VX+VT,
I2 is I + 1,
average(Towns, UX2, VX2, U, V, I2, L).
:- end_object.
