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yap-6.3/packages/xml/xml_example/misc.pl
2012-02-14 12:41:12 +00:00

96 lines
2.7 KiB
Prolog

% Utility Predicates
% The following predicates are used in the puzzle solutions.
% unique_solution( +Goal ) holds when Goal has one ground solution. Operationally,
% Goal may produce several solutions, ("don't care" non-deterministically), but they
% must all be identical (==).
unique_solution( Goal ) :-
findall( Goal, Goal, [Solution|Solutions] ),
same_solution( Solutions, Solution ),
Solution = Goal.
same_solution( [], _Solution ).
same_solution( [Solution0|Solutions], Solution ) :-
Solution0 == Solution,
same_solution( Solutions, Solution ).
% forall( +Enumerator, +Test ) is true if Enumerator and Test are goals and Test holds everywhere
% that Enumerator does. NB: does not further instantiate arguments.
%% forall( Enumerator, Test ) :-
%% \+ (call(Enumerator), \+ call(Test)).
% member( ?Element, ?List ) holds when Element is a member of List.
member( H, [H|_] ).
member( H, [_|T] ):-
member( H, T ).
% select( ?Element, ?List0, ?List1 ) is true if List1 is equal to List1 with Element removed.
select( H, [H|T], T ).
select( Element, [H|T0], [H|T1] ):-
select( Element, T0, T1 ).
% memberchk( +Element, +List ) succeeds (once) if Element is a member of List.
memberchk( Element, List ):-
member( Element, List ),
!.
% between( +Lower, +Upper, ?Index ) is true if Lower =< Index =< Upper. Two valid cases are
% possible:
% - Index is already instantiated to an integer so the checks on order are applied (test).
% - Index is a logical variable so a series of alternative solutions is generated as the
% monotonic sequence of values between Lower and Upper (non-deterministic generator).
%% between( Lower, Upper, Index ):-
%% integer( Lower ),
%% integer( Upper ),
%% Lower =< Upper,
%% ( integer( Index ) -> % Case 1: "test"
%% Index >= Lower,
%% Index =< Upper
%% ; var( Index ) -> % Case 2: "generate".
%% generate_between( Lower, Upper, Index )
%% ).
generate_between( Lower, Upper, Index ) :-
( Lower =:= Upper ->
Index = Lower
; Index = Lower
; Next is Lower + 1,
Next =< Upper,
generate_between( Next, Upper, Index )
).
% sum( +List, ?Sum ) holds when the List of numbers sum to Sum.
sum( [H|T], Sum ) :-
sum1( T, H, Sum ).
sum1( [], Sum, Sum ).
sum1( [H|T], Sum0, Sum ):-
Sum1 is Sum0 + H,
sum1( T, Sum1, Sum ).
% put_chars( +Chars ) if Chars is a (possibly empty) list of character codes and the
% corresponding characters are written to the current output stream.
put_chars( [] ).
put_chars( [Char|Chars] ) :-
put( Char ),
put_chars( Chars ).
% get_chars( ?Chars ) if Chars is a (possibly empty) list of character codes read
% from the current input stream.
get_chars( Input ) :-
get0( Char ),
( Char > -1 ->
Input = [Char|Chars],
get_chars( Chars )
; otherwise ->
Input = []
).