Minizinc Lazyfd解决方案忽略约束

Question

The problem is to find a schedule for some people to play golf (or whatever) in groups of a fixed size. We have to guarantee that every player is only in one group at a time.

Here is my code:

int: gr;             % number of groups
int: sz;             % size of groups
int: we;             % number of weeks

int: n=gr*sz;        % number of players

set of int: G=1..gr; % set of group indices
set of int: P=1..n;  % set of players
set of int: W=1..we; % set of weeks

% test instance
gr = 2;
sz = 2;
we = 2;

array[G,W] of var set of P: X;
   %X[g,w] is the set of people that form group with index g in week w


% forall group x, |x| = sz
constraint forall (g in G, w in W) 
  (card (X[g,w]) = sz);

% one person cannot play in two groups simultaneously
constraint forall (g in G, w in W, p in X[g,w], g2 in (g+1..gr))
  (not(p in X[g2,w]));

solve satisfy;

My problem now is that if I use the G12 lazyfd solver, ie

$ minizinc -b lazy this.mzn

I get

X = array2d(1..2 ,1..2 ,[1..2, 1..2, 1..2, 1..2]);
----------

which seems to ignore my second constraint. On the other hand, using G12 without the lazy option, ie

$ minizinc this.mzn

yields

X = array2d(1..2 ,1..2 ,[1..2, 1..2, 3..4, 3..4]);
----------

which is correct. G12 MIP and Gecode also give a correct result.

How is this possible? And how can I use the lazy solver such that I can rely on it? Or is it just my installation that is messed up somehow?

Answer 1

G12/lazyFD is known to be broken in various places. The problem is that the G12 solvers are no longer being developed and will most likely be removed from the distributions soon.

I would offer Chuffed as an alternative. Chuffed is FD solver written in C++ with lazy clause generation. It should be correct and will perform better then the G12 solver (at least when the solutions are correct).

Chuffed and other MiniZinc solvers can be found on the software page of the MiniZinc website .