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### genconstr_c.c

/* Copyright 2018, Gurobi Optimization, LLC */

/* In this example we show the use of general constraints for modeling
* some common expressions. We use as an example a SAT-problem where we
* want to see if it is possible to satisfy at least four (or all) clauses
* of the logical for
*
* L = (x0 or ~x1 or x2)  and (x1 or ~x2 or x3)  and
*     (x2 or ~x3 or x0)  and (x3 or ~x0 or x1)  and
*     (~x0 or ~x1 or x2) and (~x1 or ~x2 or x3) and
*     (~x2 or ~x3 or x0) and (~x3 or ~x0 or x1)
*
* We do this by introducing two variables for each literal (itself and its
* negated value), a variable for each clause, and then two
* variables for indicating if we can satisfy four, and another to identify
* the minimum of the clauses (so if it one, we can satisfy all clauses)
* and put these two variables in the objective.
* i.e. the Objective function will be
*
* maximize Obj0 + Obj1
*
*  Obj0 = MIN(Clause1, ... , Clause8)
*  Obj1 = 1 -> Clause1 + ... + Clause8 >= 4
*
* thus, the objective value will be two if and only if we can satisfy all
* clauses; one if and only if at least four clauses can be satisfied, and
* zero otherwise.
*/

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <string.h>
#include "gurobi_c.h"

#define MAXSTR    128
#define NLITERALS 4
#define NCLAUSES  8
#define NOBJ      2
#define NVARS     (2 * NLITERALS + NCLAUSES + NOBJ)
#define LIT(n)    (n)
#define NOTLIT(n) (NLITERALS + n)
#define CLA(n)    (2 * NLITERALS + n)
#define OBJ(n)    (2 * NLITERALS + NCLAUSES + n)

int
main(void)
{
GRBenv   *env   = NULL;
GRBmodel *model = NULL;
int       error = 0;
int       cind[NVARS];
double    cval[NVARS];
char      buffer[MAXSTR];
int col, i, status, nSolutions;
double objval;

/* Example data */
const int Clauses[][3] = {{LIT(0), NOTLIT(1), LIT(2)},
{LIT(1), NOTLIT(2), LIT(3)},
{LIT(2), NOTLIT(3), LIT(0)},
{LIT(3), NOTLIT(0), LIT(1)},
{NOTLIT(0), NOTLIT(1), LIT(2)},
{NOTLIT(1), NOTLIT(2), LIT(3)},
{NOTLIT(2), NOTLIT(3), LIT(0)},
{NOTLIT(3), NOTLIT(0), LIT(1)}};

/* Create environment */
if (error) goto QUIT;

/* Create initial model */
error = GRBnewmodel(env, &model, "genconstr_c", NVARS, NULL,
NULL, NULL, NULL, NULL);
if (error) goto QUIT;

/* Initialize decision variables and objective */
for (i = 0; i < NLITERALS; i++) {
col = LIT(i);
sprintf(buffer, "X%d", i);
error = GRBsetcharattrelement(model, "VType", col, GRB_BINARY);
if (error) goto QUIT;

error = GRBsetstrattrelement(model, "VarName", col, buffer);
if (error) goto QUIT;

col = NOTLIT(i);
sprintf(buffer, "notX%d", i);
error = GRBsetcharattrelement(model, "VType", col, GRB_BINARY);
if (error) goto QUIT;

error = GRBsetstrattrelement(model, "VarName", col, buffer);
if (error) goto QUIT;
}

for (i = 0; i < NCLAUSES; i++) {
col = CLA(i);
sprintf(buffer, "Clause%d", i);
error = GRBsetcharattrelement(model, "VType", col, GRB_BINARY);
if (error) goto QUIT;

error = GRBsetstrattrelement(model, "VarName", col, buffer);
if (error) goto QUIT;
}

for (i = 0; i < NOBJ; i++) {
col = OBJ(i);
sprintf(buffer, "Obj%d", i);
error = GRBsetcharattrelement(model, "VType", col, GRB_BINARY);
if (error) goto QUIT;

error = GRBsetstrattrelement(model, "VarName", col, buffer);
if (error) goto QUIT;

error = GRBsetdblattrelement(model, "Obj", col, 1.0);
if (error) goto QUIT;
}

/* Link Xi and notXi */
for (i = 0; i < NLITERALS; i++) {
sprintf(buffer,"CNSTR_X%d",i);
cind[0] = LIT(i);
cind[1] = NOTLIT(i);
cval[0] = cval[1] = 1;
error = GRBaddconstr(model, 2, cind, cval, GRB_EQUAL, 1.0, buffer);
if (error) goto QUIT;
}

/* Link clauses and literals */
for (i = 0; i < NCLAUSES; i++) {
sprintf(buffer,"CNSTR_Clause%d",i);
error = GRBaddgenconstrOr(model, buffer, CLA(i), 3, Clauses[i]);
if (error) goto QUIT;
}

/* Link objs with clauses */
for (i = 0; i < NCLAUSES; i++) {
cind[i] = CLA(i);
cval[i] = 1;
}
error = GRBaddgenconstrMin(model, "CNSTR_Obj0", OBJ(0), NCLAUSES, cind, GRB_INFINITY);
if (error) goto QUIT;

/* note that passing 4 instead of 4.0 will produce undefined behavior */
OBJ(1), 1, NCLAUSES, cind, cval,
GRB_GREATER_EQUAL, 4.0);
if (error) goto QUIT;

/* Set global objective sense */
error = GRBsetintattr(model, GRB_INT_ATTR_MODELSENSE, GRB_MAXIMIZE);
if (error) goto QUIT;

/* Save problem */
error = GRBwrite(model, "genconstr_c.mps");
if (error) goto QUIT;

error = GRBwrite(model, "genconstr_c.lp");
if (error) goto QUIT;

/* Optimize */
error = GRBoptimize(model);
if (error) goto QUIT;

/* Status checking */
error = GRBgetintattr(model, "Status", &status);
if (error) goto QUIT;

if (status == GRB_INF_OR_UNBD ||
status == GRB_INFEASIBLE  ||
status == GRB_UNBOUNDED     ) {
printf("The model cannot be solved "
"because it is infeasible or unbounded\n");
goto QUIT;
}
if (status != GRB_OPTIMAL) {
printf("Optimization was stopped with status %i\n", status);
goto QUIT;
}

/* Print result */
error = GRBgetdblattr(model, GRB_DBL_ATTR_OBJVAL, &objval);
if (error) goto QUIT;

if (objval > 1.9)
printf("Logical expression is satisfiable\n");
else if (objval > 0.9)
printf("At least four clauses can be satisfied\n");
else
printf("At most three clauses may be satisfied\n");

QUIT:

if (model != NULL) GRBfreemodel(model);
if (env != NULL)   GRBfreeenv(env);

return error;
}


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