Puzzle Utilities

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The following predicates are used in the puzzle solutions.

Higher-order Predicates

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: forall/2 does not instantiate arguments further.

forall( Enumerator, Test ) :-
    \+ (call(Enumerator), \+ call(Test)).

count_solutions( +Goal, ?Count )

is true if Count is the number of solutions for Goal. The solutions might not be distinct.

count_solutions/2 enumerates the possible solutions to Goal but does not instantiate Goal's arguments further.

count_solutions( Goal, Count ) :-
    findall( x, Goal, Xs ),
    length( Xs, Count ).

Lists

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 List0 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 ),
    !.

append( ?Front, ?Back, ?List )

succeeds if Front, Back and List are all lists and List is the concatenation of Front and Back.

append( [], L, L ).
append( [H|T], L, [H|L1] ) :-
    append( T, L, L1 ).

length( ?List, ?N )

succeeds if N is the length of List.

length( List, N ) :-
    len1( List, 0, N ).

len1( [], N, N ).
len1( [_H|T], N0, N ) :-
    N1 is N0+1,
    len1( T, N1, N ).

Arithmetic

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: a series of alternative solutions may be 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 ).

Character Input/Output

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 = []
    ).

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