[Coin-discuss] some information about Cbc

[Benoît MERLET]

Hello,

First of all, let me present myself : i'm a French student in training 
period at France Telecom R&D. I work on a branch-and-cut algorithm for a 
specific capacitated network design problem.

For the implementation of the method, i use Cbc with Cgl. It works fine, 
but takes time, and i think i can improve it.

Indeed, in the generateCuts() method of the CglCutGenerator i develop, i 
make a copy of the OsiSolverInterface constant reference `si' :

         OsiClpSolverInterface solver;
         solver.loadProblem(
                       * (si.getMatrixByRow()),
                       si.getColLower(),
                       si.getColUpper(),
                       si.getObjCoefficients(),
                       si.getRowLower(),
                       si.getRowUpper()
               );

The reason is i have to modify it (add cuts) and re-solve the LP 
relaxation of the node until the cutGenerationProcess() can't generate 
cuts anymore :

         do
         {
                 solver.initialSolve();

                 if( solver->isProvenPrimalInfeasible() )
                         return;

                 const double * sol = solver.getColSolution();
                 const int    nVars = solver.getNumCols();

                 // cs is the OsiCuts cutset
                 // all the cuts added to cs are also added to solver
                 nCuts = cutGenerationProcess(solver, sol, cs);

         }
         while( nCuts != 0 );

This generator have to be run only once, so, after creating the 
CbcModel, i do :

         model.setMaximumCutPasses(1);

The problem is that on every node of the branch-and-cut, the solver is 
copied, just because i can't modify it.

For information, i've already tried to "hack" the philosophy of Cgl 
adding a parameter to the constructor of my CglCutGenerator :

	OsiSolverInterface * cbcSolver

which is saved as an attribute. I instanciate my generator as follow :

	MyCglCutGenerator generator(model.solver());

and, in the generator, the cuts are directly added to `cbcSolver' and 
not to the cutset `cs' in order to avoid their duplication. But sadly, 
it takes more time having a solution than the first way ...
Why ? I don't know. :)

Have you ideas why my "hack" doesn't work ?
Have you proposition to avoid this copy at every node ?

Thank you by advance,
Benoît.