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yap-6.3/pl/sort.yap

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/*************************************************************************
* *
* YAP Prolog *
* *
* Yap Prolog was developed at NCCUP - Universidade do Porto *
* *
* Copyright L.Damas, V.S.Costa and Universidade do Porto 1985-1997 *
* *
**************************************************************************
* *
* File: sort.pl *
* Last rev: *
* mods: *
* comments: sorting in Prolog *
* *
*************************************************************************/
/* The three sorting routines are all variations of merge-sort, done by
bisecting the list, sorting the nearly equal halves, and merging the
results. The half-lists aren't actually constructed, the number of
elements is counted instead (which is why 'length' is in this file).
*/
% length of a list.
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length(L, M) :-
'$skip_list'(M0, L, R),
( R == [] -> M = M0 ;
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var(R) -> '$$_length'(R, M, M0) ;
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'$do_error'(type_error(list,L),length(L,M))
).
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%
% in case A1 is unbound or a difference list, things get tricky
%
'$$_length'(R, M, M0) :-
( var(M) -> '$$_length1'(R,M,M0)
; M >= M0 -> '$$_length2'(R,M,M0) ).
%
% Size is unbound, generate lists
%
'$$_length1'([], M, M).
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'$$_length1'([_|L], O, N) :-
M is N + 1,
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'$$_length1'(L, O, M).
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%
% Size is bound, generate single list
%
'$$_length2'(NL, O, N) :-
( N =:= O -> NL = [];
M is N + 1, NL = [_|L], '$$_length2'(L, O, M) ).
sort(L,O) :-
'$sort'(L,O).
msort(L,O) :-
'$msort'(L,O).
keysort(L,O) :-
'$keysort'(L,O).
:- meta_predicate prolog:predsort(3,+,-).
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%% predsort(:Compare, +List, -Sorted) is det.
%
% Sorts similar to sort/2, but determines the order of two terms
% by calling Compare(-Delta, +E1, +E2). This call must unify
% Delta with one of <, > or =. If built-in predicate compare/3 is
% used, the result is the same as sort/2. See also keysort/2.
predsort(P, L, R) :-
length(L, N),
predsort(P, N, L, _, R1), !,
R = R1.
predsort(P, 2, [X1, X2|L], L, R) :- !,
call(P, Delta, X1, X2),
sort2(Delta, X1, X2, R).
predsort(_, 1, [X|L], L, [X]) :- !.
predsort(_, 0, L, L, []) :- !.
predsort(P, N, L1, L3, R) :-
N1 is N // 2,
plus(N1, N2, N),
predsort(P, N1, L1, L2, R1),
predsort(P, N2, L2, L3, R2),
predmerge(P, R1, R2, R).
sort2(<, X1, X2, [X1, X2]).
sort2(=, X1, _, [X1]).
sort2(>, X1, X2, [X2, X1]).
predmerge(_, [], R, R) :- !.
predmerge(_, R, [], R) :- !.
predmerge(P, [H1|T1], [H2|T2], Result) :-
call(P, Delta, H1, H2),
predmerge(Delta, P, H1, H2, T1, T2, Result).
predmerge(>, P, H1, H2, T1, T2, [H2|R]) :-
predmerge(P, [H1|T1], T2, R).
predmerge(=, P, H1, _, T1, T2, [H1|R]) :-
predmerge(P, T1, T2, R).
predmerge(<, P, H1, H2, T1, T2, [H1|R]) :-
predmerge(P, T1, [H2|T2], R).