% Figure 9.2  Quicksort.


% quicksort( List, SortedList): sort List by the quicksort algorithm

quicksort( [], []).

quicksort( [X|Tail], Sorted)  :-
   split( X, Tail, Small, Big),
   quicksort( Small, SortedSmall),
   quicksort( Big, SortedBig),
   conc( SortedSmall, [X|SortedBig], Sorted).

split( X, [], [], []).

split( X, [Y|Tail], [Y|Small], Big)  :-
   gt( X, Y), !,
   split( X, Tail, Small, Big).

split( X, [Y|Tail], Small, [Y|Big])  :-
   split( X, Tail, Small, Big).

gt(A,B):-A > B.

%  conc(L1,L2,L3): list L3 is th econcatenation of lists L1 and L2

conc( [], L, L).

conc( [X | L1], L2, [X | L3])  :-
  conc( L1, L2, L3).