Exemple #1
0
/** problem writing method of reader */
static
SCIP_DECL_READERWRITE(readerWriteCip)
{  /*lint --e{715}*/
   SCIP_HASHTABLE* varhash = NULL;
   SCIP_READERDATA* readerdata;
   int i;

   assert(reader != NULL);
   assert(strcmp(SCIPreaderGetName(reader), READER_NAME) == 0);

   SCIPinfoMessage(scip, file, "STATISTICS\n");
   SCIPinfoMessage(scip, file, "  Problem name     : %s\n", name);
   SCIPinfoMessage(scip, file, "  Variables        : %d (%d binary, %d integer, %d implicit integer, %d continuous)\n",
      nvars, nbinvars, nintvars, nimplvars, ncontvars);
   SCIPinfoMessage(scip, file, "  Constraints      : %d initial, %d maximal\n", startnconss, maxnconss);

   SCIPinfoMessage(scip, file, "OBJECTIVE\n");
   SCIPinfoMessage(scip, file, "  Sense            : %s\n", objsense == SCIP_OBJSENSE_MINIMIZE ? "minimize" : "maximize");
   if( !SCIPisZero(scip, objoffset) )
      SCIPinfoMessage(scip, file, "  Offset           : %+.15g\n", objoffset);
   if( !SCIPisEQ(scip, objscale, 1.0) )
      SCIPinfoMessage(scip, file, "  Scale            : %.15g\n", objscale);

   if ( nfixedvars > 0 )
   {
      /* set up hash table for variables that have been written property (used for writing out fixed vars in the right order) */
      SCIP_CALL( SCIPhashtableCreate(&varhash, SCIPblkmem(scip), SCIPcalcHashtableSize(10 * (nvars + nfixedvars)), hashGetKeyVar, hashKeyEqVar, hashKeyValVar, NULL) );
   }

   if ( nvars + nfixedvars > 0 )
   {
      SCIPinfoMessage(scip, file, "VARIABLES\n");
   }

   if( nvars > 0 )
   {
      for( i = 0; i < nvars; ++i )
      {
         SCIP_VAR* var;

         var = vars[i];
         assert( var != NULL );
         SCIP_CALL( SCIPprintVar(scip, var, file) );
         if ( varhash != NULL )
         {
            /* add free variable to hashtable */
            if ( ! SCIPhashtableExists(varhash, (void*) var) )
            {
               SCIP_CALL( SCIPhashtableInsert(varhash, (void*) var) );
            }
         }
      }
   }

   readerdata = SCIPreaderGetData(reader);
   assert(readerdata != NULL);

   if( readerdata->writefixedvars && nfixedvars > 0 )
   {
      int nwritten = 0;

      SCIPinfoMessage(scip, file, "FIXED\n");

      /* loop through variables until each has been written after the variables that it depends on have been written; this
       * requires several runs over the variables, but the depth (= number of loops) is usually small. */
      while ( nwritten < nfixedvars )
      {
         SCIPdebugMessage("written %d of %d fixed variables.\n", nwritten, nfixedvars);
         for (i = 0; i < nfixedvars; ++i)
         {
            SCIP_VAR* var;
            SCIP_VAR* tmpvar;

            var = fixedvars[i];
            assert( var != NULL );

            /* skip variables already written */
            if ( SCIPhashtableExists(varhash, (void*) var) )
               continue;

            switch ( SCIPvarGetStatus(var) )
            {
            case SCIP_VARSTATUS_FIXED:

               /* fixed variables can simply be output and added to the hashtable */
               SCIP_CALL( SCIPprintVar(scip, var, file) );
               assert( ! SCIPhashtableExists(varhash, (void*) var) );
               SCIP_CALL( SCIPhashtableInsert(varhash, (void*) var) );
               ++nwritten;

               break;

            case SCIP_VARSTATUS_NEGATED:

               tmpvar = SCIPvarGetNegationVar(var);
               assert( tmpvar != NULL );
               assert( var == SCIPvarGetNegatedVar(tmpvar) );

               /* if the negated variable has been written, we can write the current variable */
               if ( SCIPhashtableExists(varhash, (void*) tmpvar) )
               {
                  SCIP_CALL( SCIPprintVar(scip, var, file) );
                  assert( ! SCIPhashtableExists(varhash, (void*) var) );
                  SCIP_CALL( SCIPhashtableInsert(varhash, (void*) var) );
                  ++nwritten;
               }
               break;

            case SCIP_VARSTATUS_AGGREGATED:

               tmpvar = SCIPvarGetAggrVar(var);
               assert( tmpvar != NULL );

               /* if the aggregating variable has been written, we can write the current variable */
               if ( SCIPhashtableExists(varhash, (void*) tmpvar) )
               {
                  SCIP_CALL( SCIPprintVar(scip, var, file) );
                  assert( ! SCIPhashtableExists(varhash, (void*) var) );
                  SCIP_CALL( SCIPhashtableInsert(varhash, (void*) var) );
                  ++nwritten;
               }
               break;

            case SCIP_VARSTATUS_MULTAGGR:
            {
               SCIP_VAR** aggrvars;
               int naggrvars;
               int j;

               /* get the active representation */
               SCIP_CALL( SCIPflattenVarAggregationGraph(scip, var) );

               naggrvars = SCIPvarGetMultaggrNVars(var);
               aggrvars = SCIPvarGetMultaggrVars(var);
               assert(aggrvars != NULL || naggrvars == 0);

               for (j = 0; j < naggrvars; ++j)
               {
                  if( !SCIPhashtableExists(varhash, (void*) aggrvars[j]) ) /*lint !e613*/
                     break;
               }

               /* if all multi-aggregating variables have been written, we can write the current variable */
               if ( j >= naggrvars )
               {
                  SCIP_CALL( SCIPprintVar(scip, var, file) );
                  assert( ! SCIPhashtableExists(varhash, (void*) var) );
                  SCIP_CALL( SCIPhashtableInsert(varhash, (void*) var) );
                  ++nwritten;
               }
               break;
            }

            case SCIP_VARSTATUS_ORIGINAL:
            case SCIP_VARSTATUS_LOOSE:
            case SCIP_VARSTATUS_COLUMN:
               SCIPerrorMessage("Only fixed variables are allowed to be present in fixedvars list.\n");
               SCIPABORT();
               return SCIP_ERROR; /*lint !e527*/
            }
         }
      }
   }

   if( nconss > 0 )
   {
      SCIPinfoMessage(scip, file, "CONSTRAINTS\n");

      for( i = 0; i < nconss; ++i )
      {
         SCIP_CALL( SCIPprintCons(scip, conss[i], file) );
         SCIPinfoMessage(scip, file, ";\n");
      }
   }
   SCIPinfoMessage(scip, file, "END\n");

   *result = SCIP_SUCCESS;

   if( nfixedvars > 0 )
      SCIPhashtableFree(&varhash);
   else
      assert(varhash == NULL);

   return SCIP_OKAY;
}
/** creates a subproblem for subscip by fixing a number of variables */
static
SCIP_RETCODE setupSubproblem(
   SCIP*                 scip,               /**< original SCIP data structure */
   SCIP*                 subscip,            /**< SCIP data structure for the subproblem */
   SCIP_VAR**            subvars,            /**< the variables of the subproblem */
   int*                  selection,          /**< pool of solutions crossover will use */
   SCIP_HEURDATA*        heurdata,           /**< primal heuristic data */
   SCIP_Bool*            success             /**< pointer to store whether the problem was created successfully */
   )
{
   SCIP_SOL** sols;                         /* array of all solutions found so far         */
   int nsols;                               /* number of all solutions found so far        */
   int nusedsols;                           /* number of solutions to use in crossover     */

   int i;
   char consname[SCIP_MAXSTRLEN];

   /* get solutions' data */
   nsols = SCIPgetNSols(scip);
   sols = SCIPgetSols(scip);
   nusedsols = heurdata->nusedsols;

   assert(nusedsols > 1);
   assert(nsols >= nusedsols);

   /* use nusedsols best solutions if randomization is deactivated or there are only nusedsols solutions at hand
    * or a good new solution was found since last call */
   if( !heurdata->randomization || nsols == nusedsols || heurdata->prevlastsol != sols[nusedsols-1] )
   {
      SOLTUPLE* elem;
      SCIP_HEUR* solheur;
      SCIP_Longint solnodenum;
      SCIP_Bool allsame;

      for( i = 0; i < nusedsols; i++ )
         selection[i] = i;
      SCIP_CALL( createSolTuple(scip, &elem, selection, nusedsols, heurdata) );

      solheur = SCIPsolGetHeur(sols[0]);
      solnodenum = SCIPsolGetNodenum(sols[0]);
      allsame = TRUE;

      /* check, whether all solutions have been found by the same heuristic at the same node; in this case we do not run
       * crossover, since it would probably just optimize over the same space as the other heuristic
       */
      for( i = 1; i < nusedsols; i++ )
      {
         if( SCIPsolGetHeur(sols[i]) != solheur || SCIPsolGetNodenum(sols[i]) != solnodenum )
            allsame = FALSE;
      }
      *success = !allsame && !SCIPhashtableExists(heurdata->hashtable, elem);

      /* check, whether solution tuple has already been tried */
      if( !SCIPhashtableExists(heurdata->hashtable, elem) )
      {
         SCIP_CALL( SCIPhashtableInsert(heurdata->hashtable, elem) );
      }

      /* if solution tuple has already been tried, randomization is allowed and enough solutions are at hand, try
       * to randomize another tuple. E.g., this can happen if the last crossover solution was among the best ones */
      if( !(*success) && heurdata->randomization && nsols > nusedsols )
      {
         SCIP_CALL( selectSolsRandomized(scip, selection, heurdata, success) );
      }

   }
   /* otherwise randomize the set of solutions */
   else
   {
      SCIP_CALL( selectSolsRandomized(scip, selection, heurdata, success) );
   }

   /* no acceptable solution tuple could be created */
   if( !(*success) )
      return SCIP_OKAY;

   /* get name of the original problem and add the string "_crossoversub" */
   (void) SCIPsnprintf(consname, SCIP_MAXSTRLEN, "%s_crossoversub", SCIPgetProbName(scip));

   /* set up the variables of the subproblem */
   SCIP_CALL( fixVariables(scip, subscip, subvars, selection, heurdata, success) );

   /* we copy the rows of the LP, if the enough variables could be fixed and we work on the MIP
      relaxation of the problem */
   if( *success && heurdata->uselprows )
   {
      SCIP_CALL( createRows(scip, subscip, subvars) );
   }

   return SCIP_OKAY;
}
/** execution method of primal heuristic */
static
SCIP_DECL_HEUREXEC(heurExecCrossover)
{  /*lint --e{715}*/
   SCIP_HEURDATA* heurdata;                  /* primal heuristic data                               */
   SCIP* subscip;                            /* the subproblem created by crossover                 */
   SCIP_HASHMAP* varmapfw;                   /* mapping of SCIP variables to sub-SCIP variables */

   SCIP_VAR** vars;                          /* original problem's variables                        */
   SCIP_VAR** subvars;                       /* subproblem's variables                              */
   SCIP_SOL** sols;

   SCIP_Real memorylimit;                    /* memory limit for the subproblem                     */
   SCIP_Real timelimit;                      /* time limit for the subproblem                       */
   SCIP_Real cutoff;                         /* objective cutoff for the subproblem                 */
   SCIP_Real upperbound;
   SCIP_Bool success;

   SCIP_Longint nstallnodes;                 /* node limit for the subproblem                       */

   int* selection;                           /* pool of solutions crossover uses                    */
   int nvars;                                /* number of original problem's variables              */
   int nbinvars;
   int nintvars;
   int nusedsols;
   int i;

   SCIP_RETCODE retcode;

   assert(heur != NULL);
   assert(scip != NULL);
   assert(result != NULL);

   /* get heuristic's data */
   heurdata = SCIPheurGetData(heur);
   assert(heurdata != NULL);
   nusedsols = heurdata->nusedsols;

   *result = SCIP_DELAYED;

   /* only call heuristic, if enough solutions are at hand */
   if( SCIPgetNSols(scip) < nusedsols  )
      return SCIP_OKAY;

   sols = SCIPgetSols(scip);
   assert(sols != NULL);

   /* if one good solution was found, heuristic should not be delayed any longer */
   if( sols[nusedsols-1] != heurdata->prevlastsol )
   {
      heurdata->nextnodenumber = SCIPgetNNodes(scip);
      if( sols[0] != heurdata->prevbestsol )
         heurdata->nfailures = 0;
   }
   /* in nonrandomized mode: only recall heuristic, if at least one new good solution was found in the meantime */
   else if( !heurdata->randomization )
      return SCIP_OKAY;

   /* if heuristic should be delayed, wait until certain number of nodes is reached */
   if( SCIPgetNNodes(scip) < heurdata->nextnodenumber )
      return SCIP_OKAY;

   /* only call heuristic, if enough nodes were processed since last incumbent */
   if( SCIPgetNNodes(scip) - SCIPgetSolNodenum(scip,SCIPgetBestSol(scip))  < heurdata->nwaitingnodes
      && (SCIPgetDepth(scip) > 0 || !heurdata->dontwaitatroot) )
      return SCIP_OKAY;

   *result = SCIP_DIDNOTRUN;

   /* calculate the maximal number of branching nodes until heuristic is aborted */
   nstallnodes = (SCIP_Longint)(heurdata->nodesquot * SCIPgetNNodes(scip));

   /* reward Crossover if it succeeded often */
   nstallnodes = (SCIP_Longint)
      (nstallnodes * (1.0 + 2.0*(SCIPheurGetNBestSolsFound(heur)+1.0)/(SCIPheurGetNCalls(heur)+1.0)));

   /* count the setup costs for the sub-MIP as 100 nodes */
   nstallnodes -= 100 * SCIPheurGetNCalls(heur);
   nstallnodes += heurdata->nodesofs;

   /* determine the node limit for the current process */
   nstallnodes -= heurdata->usednodes;
   nstallnodes = MIN(nstallnodes, heurdata->maxnodes);

   /* check whether we have enough nodes left to call subproblem solving */
   if( nstallnodes < heurdata->minnodes )
      return SCIP_OKAY;

   if( SCIPisStopped(scip) )
     return SCIP_OKAY;

   *result = SCIP_DIDNOTFIND;

   SCIP_CALL( SCIPgetVarsData(scip, &vars, &nvars, &nbinvars, &nintvars, NULL, NULL) );
   assert(nvars > 0);

   /* check whether discrete variables are available */
   if( nbinvars == 0 && nintvars == 0 )
      return SCIP_OKAY;

   /* initializing the subproblem */
   SCIP_CALL( SCIPcreate(&subscip) );

   /* create the variable mapping hash map */
   SCIP_CALL( SCIPhashmapCreate(&varmapfw, SCIPblkmem(subscip), SCIPcalcHashtableSize(5 * nvars)) );
   success = FALSE;

   if( heurdata->uselprows )
   {
      char probname[SCIP_MAXSTRLEN];

      /* copy all plugins */
      SCIP_CALL( SCIPincludeDefaultPlugins(subscip) );

      /* get name of the original problem and add the string "_crossoversub" */
      (void) SCIPsnprintf(probname, SCIP_MAXSTRLEN, "%s_crossoversub", SCIPgetProbName(scip));

      /* create the subproblem */
      SCIP_CALL( SCIPcreateProb(subscip, probname, NULL, NULL, NULL, NULL, NULL, NULL, NULL) );

      /* copy all variables */
      SCIP_CALL( SCIPcopyVars(scip, subscip, varmapfw, NULL, TRUE) );
   }
   else
   {
      SCIP_CALL( SCIPcopy(scip, subscip, varmapfw, NULL, "crossover", TRUE, FALSE, TRUE, &success) );

      if( heurdata->copycuts )
      {
         /** copies all active cuts from cutpool of sourcescip to linear constraints in targetscip */
         SCIP_CALL( SCIPcopyCuts(scip, subscip, varmapfw, NULL, TRUE, NULL) );
      }
   }

   SCIP_CALL( SCIPallocBufferArray(scip, &subvars, nvars) );
   SCIP_CALL( SCIPallocBufferArray(scip, &selection, nusedsols) );

   for( i = 0; i < nvars; i++ )
     subvars[i] = (SCIP_VAR*) (size_t) SCIPhashmapGetImage(varmapfw, vars[i]);

   /* free hash map */
   SCIPhashmapFree(&varmapfw);

   success = FALSE;

   /* create a new problem, which fixes variables with same value in a certain set of solutions */
   SCIP_CALL( setupSubproblem(scip, subscip, subvars, selection, heurdata, &success) );

   heurdata->prevbestsol = SCIPgetBestSol(scip);
   heurdata->prevlastsol = sols[heurdata->nusedsols-1];

   /* if creation of sub-SCIP was aborted (e.g. due to number of fixings), free sub-SCIP and abort */
   if( !success )
   {
      *result = SCIP_DIDNOTRUN;

      /* this run will be counted as a failure since no new solution tuple could be generated or the neighborhood of the
       * solution was not fruitful in the sense that it was too big
       */
      updateFailureStatistic(scip, heurdata);

      goto TERMINATE;
   }

   /* do not abort subproblem on CTRL-C */
   SCIP_CALL( SCIPsetBoolParam(subscip, "misc/catchctrlc", FALSE) );

   /* disable output to console */
   SCIP_CALL( SCIPsetIntParam(subscip, "display/verblevel", 0) );

   /* check whether there is enough time and memory left */
   SCIP_CALL( SCIPgetRealParam(scip, "limits/time", &timelimit) );
   if( !SCIPisInfinity(scip, timelimit) )
      timelimit -= SCIPgetSolvingTime(scip);
   SCIP_CALL( SCIPgetRealParam(scip, "limits/memory", &memorylimit) );

   /* substract the memory already used by the main SCIP and the estimated memory usage of external software */
   if( !SCIPisInfinity(scip, memorylimit) )
   {
      memorylimit -= SCIPgetMemUsed(scip)/1048576.0;
      memorylimit -= SCIPgetMemExternEstim(scip)/1048576.0;
   }

   /* abort if no time is left or not enough memory to create a copy of SCIP, including external memory usage */
   if( timelimit <= 0.0 || memorylimit <= 2.0*SCIPgetMemExternEstim(scip)/1048576.0 )
      goto TERMINATE;

   /* set limits for the subproblem */
   SCIP_CALL( SCIPsetLongintParam(subscip, "limits/nodes", nstallnodes) );
   SCIP_CALL( SCIPsetRealParam(subscip, "limits/time", timelimit) );
   SCIP_CALL( SCIPsetRealParam(subscip, "limits/memory", memorylimit) );

   /* forbid recursive call of heuristics and separators solving subMIPs */
   SCIP_CALL( SCIPsetSubscipsOff(subscip, TRUE) );

   /* disable cutting plane separation */
   SCIP_CALL( SCIPsetSeparating(subscip, SCIP_PARAMSETTING_OFF, TRUE) );

   /* disable expensive presolving */
   SCIP_CALL( SCIPsetPresolving(subscip, SCIP_PARAMSETTING_FAST, TRUE) );

   /* use best estimate node selection */
   if( SCIPfindNodesel(subscip, "estimate") != NULL && !SCIPisParamFixed(subscip, "nodeselection/estimate/stdpriority") )
   {
      SCIP_CALL( SCIPsetIntParam(subscip, "nodeselection/estimate/stdpriority", INT_MAX/4) );
   }

   /* use inference branching */
   if( SCIPfindBranchrule(subscip, "inference") != NULL && !SCIPisParamFixed(subscip, "branching/inference/priority") )
   {
      SCIP_CALL( SCIPsetIntParam(subscip, "branching/inference/priority", INT_MAX/4) );
   }

   /* disable conflict analysis */
   if( !SCIPisParamFixed(subscip, "conflict/useprop") )
   {
      SCIP_CALL( SCIPsetBoolParam(subscip, "conflict/useprop", FALSE) );
   }
   if( !SCIPisParamFixed(subscip, "conflict/useinflp") )
   {
      SCIP_CALL( SCIPsetBoolParam(subscip, "conflict/useinflp", FALSE) );
   }
   if( !SCIPisParamFixed(subscip, "conflict/useboundlp") )
   {
      SCIP_CALL( SCIPsetBoolParam(subscip, "conflict/useboundlp", FALSE) );
   }
   if( !SCIPisParamFixed(subscip, "conflict/usesb") )
   {
      SCIP_CALL( SCIPsetBoolParam(subscip, "conflict/usesb", FALSE) );
   }
   if( !SCIPisParamFixed(subscip, "conflict/usepseudo") )
   {
      SCIP_CALL( SCIPsetBoolParam(subscip, "conflict/usepseudo", FALSE) );
   }

   /* add an objective cutoff */
   cutoff = SCIPinfinity(scip);
   assert(!SCIPisInfinity(scip, SCIPgetUpperbound(scip)));

   upperbound = SCIPgetUpperbound(scip) - SCIPsumepsilon(scip);
   if( !SCIPisInfinity(scip,-1.0*SCIPgetLowerbound(scip)) )
   {
      cutoff = (1-heurdata->minimprove)*SCIPgetUpperbound(scip) + heurdata->minimprove*SCIPgetLowerbound(scip);
   }
   else
   {
      if( SCIPgetUpperbound ( scip ) >= 0 )
         cutoff = ( 1 - heurdata->minimprove ) * SCIPgetUpperbound ( scip );
      else
         cutoff = ( 1 + heurdata->minimprove ) * SCIPgetUpperbound ( scip );
   }
   cutoff = MIN(upperbound, cutoff );
   SCIP_CALL( SCIPsetObjlimit(subscip, cutoff) );

   /* permute the subproblem to increase diversification */
   if( heurdata->permute )
   {
      SCIP_CALL( SCIPpermuteProb(subscip, (unsigned int) SCIPheurGetNCalls(heur), TRUE, TRUE, TRUE, TRUE, TRUE) );
   }

   /* solve the subproblem */
   SCIPdebugMessage("Solve Crossover subMIP\n");
   retcode = SCIPsolve(subscip);

   /* Errors in solving the subproblem should not kill the overall solving process.
    * Hence, the return code is caught and a warning is printed, only in debug mode, SCIP will stop. */
   if( retcode != SCIP_OKAY )
   {
#ifndef NDEBUG
      SCIP_CALL( retcode );
#endif
      SCIPwarningMessage(scip, "Error while solving subproblem in Crossover heuristic; sub-SCIP terminated with code <%d>\n", retcode);
   }

   heurdata->usednodes += SCIPgetNNodes(subscip);

   /* check, whether a solution was found */
   if( SCIPgetNSols(subscip) > 0 )
   {
      SCIP_SOL** subsols;
      int nsubsols;
      int solindex;                             /* index of the solution created by crossover          */

      /* check, whether a solution was found;
       * due to numerics, it might happen that not all solutions are feasible -> try all solutions until one was accepted */
      nsubsols = SCIPgetNSols(subscip);
      subsols = SCIPgetSols(subscip);
      success = FALSE;
      solindex = -1;
      for( i = 0; i < nsubsols && !success; ++i )
      {
         SCIP_CALL( createNewSol(scip, subscip, subvars, heur, subsols[i], &solindex, &success) );
      }

      if( success )
      {
         int tmp;

         assert(solindex != -1);

         *result = SCIP_FOUNDSOL;

         /* insert all crossings of the new solution and (nusedsols-1) of its parents into the hashtable
          * in order to avoid incest ;)
          */
         for( i = 0; i < nusedsols; i++ )
         {
            SOLTUPLE* elem;
            tmp = selection[i];
            selection[i] = solindex;

            SCIP_CALL( createSolTuple(scip, &elem, selection, nusedsols, heurdata) );
            SCIP_CALL( SCIPhashtableInsert(heurdata->hashtable, elem) );
            selection[i] = tmp;
         }

         /* if solution was among the best ones, crossover should not be called until another good solution was found */
         if( !heurdata->randomization )
         {
            heurdata->prevbestsol = SCIPgetBestSol(scip);
            heurdata->prevlastsol = SCIPgetSols(scip)[heurdata->nusedsols-1];
         }
      }

      /* if solution is not better then incumbent or could not be added to problem => run is counted as a failure */
      if( !success || solindex != SCIPsolGetIndex(SCIPgetBestSol(scip)) )
         updateFailureStatistic(scip, heurdata);
   }
   else
   {
      /* if no new solution was found, run was a failure */
      updateFailureStatistic(scip, heurdata);
   }

 TERMINATE:
   /* free subproblem */
   SCIPfreeBufferArray(scip, &selection);
   SCIPfreeBufferArray(scip, &subvars);
   SCIP_CALL( SCIPfree(&subscip) );

   return SCIP_OKAY;
}
/** randomly selects the solutions crossover will use from the pool of all solutions found so far */
static
SCIP_RETCODE selectSolsRandomized(
   SCIP*                 scip,               /**< original SCIP data structure */
   int*                  selection,          /**< pool of solutions crossover uses  */
   SCIP_HEURDATA*        heurdata,           /**< primal heuristic data */
   SCIP_Bool*            success             /**< pointer to store whether the process was successful */
   )
{

   int i;
   int j;
   int lastsol;          /* the worst solution possible to choose */
   int nusedsols;        /* number of solutions which will be chosen */

   SOLTUPLE* elem;
   SCIP_SOL** sols;

   /* initialization */
   nusedsols = heurdata->nusedsols;
   lastsol = SCIPgetNSols(scip);
   sols = SCIPgetSols(scip);
   assert(nusedsols < lastsol);

   i = 0;
   *success = FALSE;

   /* perform at maximum 10 restarts and stop as soon as a new set of solutions is found */
   while( !*success && i < 10 )
   {
      SCIP_Bool validtuple;

      validtuple = TRUE;
      for( j = 0; j < nusedsols && validtuple; j++ )
      {
         int k;
         k = SCIPgetRandomInt(nusedsols-j-1, lastsol-1, &heurdata->randseed);

         /* ensure that the solution does not have a similar source as the others */
         while( k >= nusedsols-j-1 && !solHasNewSource(sols, selection, j, k) )
            k--;

         validtuple = (k >= nusedsols-j-1);
         selection[j] = k;
         lastsol = k;
      }

      if( validtuple )
      {
         /* creates an object ready to be inserted into the hashtable */
         SCIP_CALL( createSolTuple(scip, &elem, selection, nusedsols, heurdata) );

         /* check whether the randomized set is already in the hashtable, if not, insert it */
         if( !SCIPhashtableExists(heurdata->hashtable, elem) )
         {
            SCIP_CALL( SCIPhashtableInsert(heurdata->hashtable, elem) );
            *success = TRUE;
         }
      }
      i++;
   }

   return SCIP_OKAY;
}