Update LP offset in HPresolveInitialSweep::emptyCol

check/TestPresolve.cpp

@@ -2333,3 +2333,46 @@ TEST_CASE("presolve-mode", "[highs_test_presolve]") {
   }
   h.resetThreadScheduler(true);
 }
+TEST_CASE("issue-3545", "[highs_test_presolve]") {
+  HighsLp lp;
+  lp.num_col_ = 3;
+  lp.num_row_ = 2;
+  lp.col_cost_ = {1, 1, 3};
+  lp.col_lower_ = {0, 0, 2};
+  lp.col_upper_ = {kHighsInf, kHighsInf, 5};
+  lp.row_lower_ = {1, -kHighsInf};
+  lp.row_upper_ = {kHighsInf, 0.5};
+  lp.a_matrix_.start_ = {0, 2, 4, 4};
+  lp.a_matrix_.index_ = {0, 1, 0, 1};
+  lp.a_matrix_.value_ = {1, 1, 1, -1};
+  HighsInt z = 2;
+  Highs h;
+  h.setOptionValue("output_flag", dev_run);
+  REQUIRE(h.passModel(lp) == HighsStatus::kOk);
+
+  // Get the optimal objective value
+  REQUIRE(h.run() == HighsStatus::kOk);
+  REQUIRE(h.getModelStatus() == HighsModelStatus::kOptimal);
+  double optimal_objective_value = h.getObjectiveValue();
+  REQUIRE(h.clearSolver() == HighsStatus::kOk);
+
+  REQUIRE(h.setOptionValue("presolve_mode", kPresolveModeInitialSweep) ==
+          HighsStatus::kOk);
+
+  REQUIRE(h.presolve() == HighsStatus::kOk);
+  // Presolve with just initial sweep fixes the empty column (z) at
+  // its lower bound, so the presolved LP should have an offset of 6 =
+  // cost(z)*lower(z)
+  HighsLp presolved_lp = h.getPresolvedLp();
+  REQUIRE(presolved_lp.offset_ == lp.col_cost_[z] * lp.col_lower_[z]);
+  REQUIRE(h.passModel(presolved_lp) == HighsStatus::kOk);
+  // The absence of the offset leads to an incorrect optimal objective
+  // if the presolved LP is solved
+  REQUIRE(h.setOptionValue(kPresolveString, kHighsOffString) ==
+          HighsStatus::kOk);
+  REQUIRE(h.run() == HighsStatus::kOk);
+  REQUIRE(h.getModelStatus() == HighsModelStatus::kOptimal);
+  REQUIRE(h.getObjectiveValue() == optimal_objective_value);
+
+  h.resetThreadScheduler(true);
+}

highs/presolve/HPresolveInitialSweep.cpp

@@ -87,6 +87,7 @@ HPresolveInitialSweep::Result HPresolveInitialSweep::emptyCol(
   assert(fixval != kHighsInf);
   postsolve_stack.removedModelFixedCol(col, fixval, cost, col_nnz, nullptr,
                                        nullptr);
+  lp_.offset_ += cost * fixval;
   if (col == lp_.fme_obj_col_) lp_.fme_obj_col_ = -1;
   return Result::kOk;
 }