int lpx_dual_ratio_test(LPX *lp, int len, const int ind[], const double val[], int how, double tol) { /* perform dual ratio test */ int piv; piv = glp_dual_rtest(lp, len, ind, val, how, tol); xassert(0 <= piv && piv <= len); return piv == 0 ? 0 : ind[piv]; }
static int branch_drtom(glp_tree *T, int *_next) { glp_prob *mip = T->mip; int m = mip->m; int n = mip->n; char *non_int = T->non_int; int j, jj, k, t, next, kase, len, stat, *ind; double x, dk, alfa, delta_j, delta_k, delta_z, dz_dn, dz_up, dd_dn, dd_up, degrad, *val; /* basic solution of LP relaxation must be optimal */ xassert(glp_get_status(mip) == GLP_OPT); /* allocate working arrays */ ind = xcalloc(1+n, sizeof(int)); val = xcalloc(1+n, sizeof(double)); /* nothing has been chosen so far */ jj = 0, degrad = -1.0; /* walk through the list of columns (structural variables) */ for (j = 1; j <= n; j++) { /* if j-th column is not marked as fractional, skip it */ if (!non_int[j]) continue; /* obtain (fractional) value of j-th column in basic solution of LP relaxation */ x = glp_get_col_prim(mip, j); /* since the value of j-th column is fractional, the column is basic; compute corresponding row of the simplex table */ len = glp_eval_tab_row(mip, m+j, ind, val); /* the following fragment computes a change in the objective function: delta Z = new Z - old Z, where old Z is the objective value in the current optimal basis, and new Z is the objective value in the adjacent basis, for two cases: 1) if new upper bound ub' = floor(x[j]) is introduced for j-th column (down branch); 2) if new lower bound lb' = ceil(x[j]) is introduced for j-th column (up branch); since in both cases the solution remaining dual feasible becomes primal infeasible, one implicit simplex iteration is performed to determine the change delta Z; it is obvious that new Z, which is never better than old Z, is a lower (minimization) or upper (maximization) bound of the objective function for down- and up-branches. */ for (kase = -1; kase <= +1; kase += 2) { /* if kase < 0, the new upper bound of x[j] is introduced; in this case x[j] should decrease in order to leave the basis and go to its new upper bound */ /* if kase > 0, the new lower bound of x[j] is introduced; in this case x[j] should increase in order to leave the basis and go to its new lower bound */ /* apply the dual ratio test in order to determine which auxiliary or structural variable should enter the basis to keep dual feasibility */ k = glp_dual_rtest(mip, len, ind, val, kase, 1e-9); if (k != 0) k = ind[k]; /* if no non-basic variable has been chosen, LP relaxation of corresponding branch being primal infeasible and dual unbounded has no primal feasible solution; in this case the change delta Z is formally set to infinity */ if (k == 0) { delta_z = (T->mip->dir == GLP_MIN ? +DBL_MAX : -DBL_MAX); goto skip; } /* row of the simplex table that corresponds to non-basic variable x[k] choosen by the dual ratio test is: x[j] = ... + alfa * x[k] + ... where alfa is the influence coefficient (an element of the simplex table row) */ /* determine the coefficient alfa */ for (t = 1; t <= len; t++) if (ind[t] == k) break; xassert(1 <= t && t <= len); alfa = val[t]; /* since in the adjacent basis the variable x[j] becomes non-basic, knowing its value in the current basis we can determine its change delta x[j] = new x[j] - old x[j] */ delta_j = (kase < 0 ? floor(x) : ceil(x)) - x; /* and knowing the coefficient alfa we can determine the corresponding change delta x[k] = new x[k] - old x[k], where old x[k] is a value of x[k] in the current basis, and new x[k] is a value of x[k] in the adjacent basis */ delta_k = delta_j / alfa; /* Tomlin noticed that if the variable x[k] is of integer kind, its change cannot be less (eventually) than one in the magnitude */ if (k > m && glp_get_col_kind(mip, k-m) != GLP_CV) { /* x[k] is structural integer variable */ if (fabs(delta_k - floor(delta_k + 0.5)) > 1e-3) { if (delta_k > 0.0) delta_k = ceil(delta_k); /* +3.14 -> +4 */ else delta_k = floor(delta_k); /* -3.14 -> -4 */ } } /* now determine the status and reduced cost of x[k] in the current basis */ if (k <= m) { stat = glp_get_row_stat(mip, k); dk = glp_get_row_dual(mip, k); } else { stat = glp_get_col_stat(mip, k-m); dk = glp_get_col_dual(mip, k-m); } /* if the current basis is dual degenerate, some reduced costs which are close to zero may have wrong sign due to round-off errors, so correct the sign of d[k] */ switch (T->mip->dir) { case GLP_MIN: if (stat == GLP_NL && dk < 0.0 || stat == GLP_NU && dk > 0.0 || stat == GLP_NF) dk = 0.0; break; case GLP_MAX: if (stat == GLP_NL && dk > 0.0 || stat == GLP_NU && dk < 0.0 || stat == GLP_NF) dk = 0.0; break; default: xassert(T != T); } /* now knowing the change of x[k] and its reduced cost d[k] we can compute the corresponding change in the objective function delta Z = new Z - old Z = d[k] * delta x[k]; note that due to Tomlin's modification new Z can be even worse than in the adjacent basis */ delta_z = dk * delta_k; skip: /* new Z is never better than old Z, therefore the change delta Z is always non-negative (in case of minimization) or non-positive (in case of maximization) */ switch (T->mip->dir) { case GLP_MIN: xassert(delta_z >= 0.0); break; case GLP_MAX: xassert(delta_z <= 0.0); break; default: xassert(T != T); } /* save the change in the objective fnction for down- and up-branches, respectively */ if (kase < 0) dz_dn = delta_z; else dz_up = delta_z; } /* thus, in down-branch no integer feasible solution can be better than Z + dz_dn, and in up-branch no integer feasible solution can be better than Z + dz_up, where Z is value of the objective function in the current basis */ /* following the heuristic by Driebeck and Tomlin we choose a column (i.e. structural variable) which provides largest degradation of the objective function in some of branches; besides, we select the branch with smaller degradation to be solved next and keep other branch with larger degradation in the active list hoping to minimize the number of further backtrackings */ if (degrad < fabs(dz_dn) || degrad < fabs(dz_up)) { jj = j; if (fabs(dz_dn) < fabs(dz_up)) { /* select down branch to be solved next */ next = GLP_DN_BRNCH; degrad = fabs(dz_up); } else { /* select up branch to be solved next */ next = GLP_UP_BRNCH; degrad = fabs(dz_dn); } /* save the objective changes for printing */ dd_dn = dz_dn, dd_up = dz_up; /* if down- or up-branch has no feasible solution, we does not need to consider other candidates (in principle, the corresponding branch could be pruned right now) */ if (degrad == DBL_MAX) break; } } /* free working arrays */ xfree(ind); xfree(val); /* something must be chosen */ xassert(1 <= jj && jj <= n); #if 1 /* 02/XI-2009 */ if (degrad < 1e-6 * (1.0 + 0.001 * fabs(mip->obj_val))) { jj = branch_mostf(T, &next); goto done; } #endif if (T->parm->msg_lev >= GLP_MSG_DBG) { xprintf("branch_drtom: column %d chosen to branch on\n", jj); if (fabs(dd_dn) == DBL_MAX) xprintf("branch_drtom: down-branch is infeasible\n"); else xprintf("branch_drtom: down-branch bound is %.9e\n", lpx_get_obj_val(mip) + dd_dn); if (fabs(dd_up) == DBL_MAX) xprintf("branch_drtom: up-branch is infeasible\n"); else xprintf("branch_drtom: up-branch bound is %.9e\n", lpx_get_obj_val(mip) + dd_up); } done: *_next = next; return jj; }