From 0ab3f0fabd4428f26060cbedbbb7db5880bf366b Mon Sep 17 00:00:00 2001
From: Pavlo Muts <pavlo.muts@gmail.com>
Date: Tue, 22 Jul 2025 10:29:52 +0200
Subject: [PATCH] setSolution() from Highs accepts solution hint

---
 ortools/math_opt/solvers/highs_solver.cc      | 16 ++++++++++++++++
 ortools/math_opt/solvers/highs_solver_test.cc |  4 ++--
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/ortools/math_opt/solvers/highs_solver.cc b/ortools/math_opt/solvers/highs_solver.cc
index 25f66f1e7f..5d5bccf7e1 100644
--- a/ortools/math_opt/solvers/highs_solver.cc
+++ b/ortools/math_opt/solvers/highs_solver.cc
@@ -925,6 +925,22 @@ absl::StatusOr<SolveResultProto> HighsSolver::Solve(
     return absl::OkStatus();
   };
 
+  if (model_parameters.solution_hints_size() > 0) {
+    // Take the first solution hint and set the solution.
+    const SolutionHintProto& hint = model_parameters.solution_hints(0);
+    const int num_vars = highs_->getModel().lp_.num_col_;
+    // Highs accepts only full solutions, on partial solutions it will
+    // return an error.
+    if (hint.variable_values().ids_size() == num_vars) {
+      HighsSolution sol = HighsSolution();
+      sol.col_value.resize(num_vars);
+      for (const auto [id, val] : MakeView(hint.variable_values())) {
+        sol.col_value[variable_data_.at(id).index] = val;
+      }
+      RETURN_IF_ERROR(ToStatus(highs_->setSolution(sol)));
+    }
+  }
+
   RETURN_IF_ERROR(ListInvertedBounds().ToStatus());
   // TODO(b/271595607): delete this code once we upgrade HiGHS, if HiGHS does
   // return a proper infeasibility status for models with empty integer bounds.
diff --git a/ortools/math_opt/solvers/highs_solver_test.cc b/ortools/math_opt/solvers/highs_solver_test.cc
index ccb276129b..3afa835800 100644
--- a/ortools/math_opt/solvers/highs_solver_test.cc
+++ b/ortools/math_opt/solvers/highs_solver_test.cc
@@ -164,8 +164,8 @@ INSTANTIATE_TEST_SUITE_P(HighsLpModelSolveParametersTest,
                              /*supports_duals=*/true,
                              /*supports_primal_only_warm_starts=*/false)));
 
-// MIP hint appears to be supported by Highs::setSolution, this is not yet
-// implemented.
+// Highs::setSolution is implemented, but it only accepts complete solutions.
+// The test below generates partial solutuions, so we skip it.
 GTEST_ALLOW_UNINSTANTIATED_PARAMETERIZED_TEST(MipSolutionHintTest);
 
 // HiGHS does not support branching priority.