Merge pull request ERGO-Code#2330 from ERGO-Code/fix-2326

Avoided false claim of optimality when presolve is terminated by `presolve_reduction_limit` when reduced model has no columns but zero row activity is infeasible.

Author: jajhall

check/TestMipSolver.cpp

@@ -27,8 +27,8 @@ TEST_CASE("MIP-distillation", "[highs_test_mip_solver]") {
   highs.resetGlobalScheduler(true);
 }
 
-// Fails but the cases work separately in 
-// MIP-rowless-1 and 
+// Fails but the cases work separately in
+// MIP-rowless-1 and
 // MIP-rowless-2 below
 // TEST_CASE("MIP-rowless", "[highs_test_mip_solver]") {
 //   Highs highs;
@@ -844,7 +844,6 @@ void rowlessMIP1(Highs& highs) {
   // solve(highs, kHighsOffString, require_model_status, optimal_objective);
 }
 
-
 void rowlessMIP2(Highs& highs) {
   HighsLp lp;
   HighsModelStatus require_model_status;

highs/lp_data/HConst.h

@@ -262,7 +262,8 @@ enum PresolveRuleType : int {
   kPresolveRuleDependentFreeCols,
   kPresolveRuleAggregator,
   kPresolveRuleParallelRowsAndCols,
-  kPresolveRuleMax = kPresolveRuleParallelRowsAndCols,
+  kPresolveRuleProbing,
+  kPresolveRuleMax = kPresolveRuleProbing,
   kPresolveRuleLastAllowOff = kPresolveRuleMax,
   kPresolveRuleCount,
 };

highs/lp_data/HighsModelUtils.cpp

@@ -1421,6 +1421,8 @@ std::string utilPresolveRuleTypeToString(const HighsInt rule_type) {
     return "Aggregator";
   } else if (rule_type == kPresolveRuleParallelRowsAndCols) {
     return "Parallel rows and columns";
+  } else if (rule_type == kPresolveRuleProbing) {
+    return "Probing";
   }
   assert(1 == 0);
   return "????";

highs/presolve/HPresolve.cpp

@@ -4365,7 +4365,8 @@ HPresolve::Result HPresolve::presolve(HighsPostsolveStack& postsolve_stack) {
     bool trySparsify =
         mipsolver != nullptr || !options->lp_presolve_requires_basis_postsolve;
 #endif
-    bool tryProbing = mipsolver != nullptr;
+    bool tryProbing =
+        mipsolver != nullptr && analysis_.allow_rule_[kPresolveRuleProbing];
     HighsInt numCliquesBeforeProbing = -1;
     bool domcolAfterProbingCalled = false;
     bool dependentEquationsCalled = mipsolver != nullptr;
@@ -4744,14 +4745,21 @@ HighsModelStatus HPresolve::run(HighsPostsolveStack& postsolve_stack) {
       }
       mipsolver->mipdata_->lower_bound = 0;
     } else {
-      assert(model->num_row_ == 0);
+      // An LP with no columns must have no rows, unless the reduction
+      // limit has been reached
+      assert(model->num_row_ == 0 ||
+             postsolve_stack.numReductions() >= reductionLimit);
       if (model->num_row_ != 0) {
         presolve_status_ = HighsPresolveStatus::kNotPresolved;
         return HighsModelStatus::kNotset;
       }
     }
     presolve_status_ = HighsPresolveStatus::kReducedToEmpty;
-    return HighsModelStatus::kOptimal;
+    // Make sure that zero row activity from the column-less model is
+    // consistent with the bounds
+    return model->num_row_ == 0 || zeroRowActivityFeasible()
+               ? HighsModelStatus::kOptimal
+               : HighsModelStatus::kInfeasible;
   } else if (postsolve_stack.numReductions() > 0) {
     // Reductions performed
     presolve_status_ = HighsPresolveStatus::kReduced;
@@ -6885,6 +6893,17 @@ HPresolve::Result HPresolve::sparsify(HighsPostsolveStack& postsolve_stack) {
   return Result::kOk;
 }
 
+bool HPresolve::zeroRowActivityFeasible() const {
+  // Check that zero row activity is feasible - called when reduced model
+  // has no columns to assess whether the HighsModelStatus returned is
+  // kOptimal or kInfeasible (as was required for 2326)
+  for (HighsInt iRow = 0; iRow < model->num_row_; iRow++)
+    if (model->row_lower_[iRow] > primal_feastol ||
+        model->row_upper_[iRow] < -primal_feastol)
+      return false;
+  return true;
+}
+
 HighsInt HPresolve::debugGetCheckCol() const {
   const std::string check_col_name = "";  // c37";
   HighsInt check_col = -1;

highs/presolve/HPresolve.h

@@ -394,6 +394,8 @@ class HPresolve {
 
   HighsPresolveStatus getPresolveStatus() const { return presolve_status_; }
 
+  bool zeroRowActivityFeasible() const;
+
   HighsInt debugGetCheckCol() const;
   HighsInt debugGetCheckRow() const;