Re: FD constraints - strange behavior

Hi, Fred

Hello,

Using Gnu-Prolog up to 1.3.0, I'm facing something strange (might be a
bug ?) illustrated in the predicates below. Why on earth doesn't v1(_)
find the solution ? Defining a new variable for the subexpression
(C+30) seems to help Gnu-Prolog...

It could be related to an overflow problem, but It would be surprising
: we are far below max_integer. It does not seem to be related to the
"FD sparse representation" pitfall (setting fd_set_vector_max to a big
value has no effect).

Thanks in advance.

-----------

Execution :

-----------

| ?- v1(Res).

no

| ?- v2(Res).

Res = [9,7]

yes

| ?- v3(Res).

Res = [9,7]

yes

-----------

Listing :

-----------

v1(Ls) :-

Ls = [C,G],

fd_domain(C, 8, 9), fd_domain(G, 7, 8),

780 + 52*(C+30) + 12*G*(C+30) #= 156*(C+30),

fd_labelingff(Ls).

v2(Ls) :-

Ls = [C,G],

fd_domain(C, 8, 9), fd_domain(G, 7, 8),

C30#=30+C, % replacing expression in v1

780 + 52*(C30) + 12*G*(C30) #= 156*(C30),

fd_labelingff(Ls).

v3(Ls) :-

Ls = [C,G],

fd_domain(C, 8, 9), fd_domain(G, 7, 8),

C#=9, % adding a constraint to v1

780 + 52*(C+30) + 12*G*(C+30) #= 156*(C+30),

fd_labelingff(Ls).

--------------------------------------------------

thank you for report.

it is due to how big expressions are split into simpler ones. This is
done with new brand intermediate variables whose initial domain is
0..max_int. When 2 of these variables are multiplied (non-linearity) an
overflow occurs and resulting in an empty domain (max < min). This
is clearly a behavior we don"t want. I add it to my TODO list. A
workaround consists in splitting by hand the expression (as you did).

By the way, my original code (for a CLP problem taken from a book)
showing the "bug" was :

fractions1(Ls) :-

Ls = [A,B,C,D,E,F,G,H,I],

fd_domain(Ls, 1, 9),

fd_all_different(Ls),

BC #= 10*B+C, EF #= 10*E+F, HI #= 10*H+I,

A*EF*HI + D*BC*HI + G*BC*EF #= BC*EF*HI,

fd_labelingff(Ls).

Strange, I have no problem with this one !

| ?- fractions1(Ls).

Ls = [5,3,4,7,6,8,9,1,2] ? ;

Ls = [5,3,4,9,1,2,7,6,8] ? ;

Ls = [7,6,8,5,3,4,9,1,2] ? ;

Ls = [7,6,8,9,1,2,5,3,4] ? ;

Ls = [9,1,2,5,3,4,7,6,8] ? ;

Ls = [9,1,2,7,6,8,5,3,4] ? ;

(140 ms) no

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