expose objective offset (#17)

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* expose objective offset

Author: dance858

include/PSLP/PSLP_API.h

@@ -70,6 +70,12 @@ extern "C"
         // variable bounds lbs <= x <= ubs
         double *lbs;
         double *ubs;
+
+        /* offset to be added to the objective value. (When the presolver fixes
+           variables, it adds an offset to the objective. This offset should
+           possibly be taken into account when evaluating the relative optimality gap
+           of the reduced problem.) */
+        double obj_offset;
     } PresolvedProblem;
 
     /* struct corresponding to the presolver:

src/core/Presolver.c

@@ -536,6 +536,7 @@ void populate_presolved_problem(Presolver *presolver)
     ps_prob->rhs = constraints->rhs;
     ps_prob->lhs = constraints->lhs;
     ps_prob->c = presolver->prob->obj->c;
+    ps_prob->obj_offset = presolver->prob->obj->offset;
 
     // create bounds arrays
     ps_prob->lbs = (double *) malloc(A->n * sizeof(double));