Add basic support for IISs

changelog.md

@@ -4,6 +4,20 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased] - [Doc:Unreleased]
 
+### Added
+
+- [PR41](https://github.com/scipopt/SCIPpp/pull/41) Basic support of IIS extraction via `Model::generateIIS`.
+  This requires that the plugin `IISfinderGreedy` is loaded (which is done by default) or any other plugin that can
+  find an IIS. Example usage:
+  ```cpp
+  Model model = ...;
+  auto iis { model.generateIIS() };
+  std::cout << "Contradicting constraints are:\n";
+  for (const auto& consId : iis.consIds) {
+    std::cout << "  " << consId << "\n";
+  }
+  ```
+
 ### Changed
 
 - [PR38](https://github.com/scipopt/SCIPpp/pull/38) Update to SCIP 10.0.0.

include/scippp/iis.hpp

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace scippp {
+
+/**
+ * A data structure to hold an %IIS (%Irreducible %Infeasible %Subsystem).
+ * @since 1.4.0
+ */
+struct IIS {
+    //! Ids of the constraints in the %IIS.
+    std::vector<std::string> consIds;
+};
+
+}

include/scippp/model.hpp

@@ -11,6 +11,7 @@
 #include <vector>
 
 #include "scippp/constant_coefficient.hpp"
+#include "scippp/iis.hpp"
 #include "scippp/initial_solution.hpp"
 #include "scippp/lin_expr.hpp"
 #include "scippp/lin_ineq.hpp"
@@ -347,5 +348,13 @@ class Model {
         bool checkBounds = true,
         bool checkIntegrality = true,
         bool checkLpRows = true);
+
+    /**
+     * Creates an %Irreducible %Infeasible %Subsystem (%IIS) of the current model.
+     *
+     * @since 1.4.0
+     * @return The generated %IIS.
+     */
+    [[nodiscard]] IIS generateIIS() const;
 };
 }

source/model.cpp

@@ -175,4 +175,27 @@ bool Model::addSolution(
     return isStored;
 }
 
+IIS Model::generateIIS() const
+{
+    m_scipCallWrapper(SCIPgenerateIIS(m_scip));
+    auto* iis { SCIPgetIIS(m_scip) };
+    assert(iis); // GCOVR_EXCL_LINE
+    auto* subscip { SCIPiisGetSubscip(iis) };
+    assert(subscip); // GCOVR_EXCL_LINE
+    auto nConss { SCIPgetNOrigConss(subscip) };
+
+    IIS result;
+    if (nConss > 0) {
+        auto** conss { SCIPgetOrigConss(subscip) };
+        assert(conss); // GCOVR_EXCL_LINE
+        result.consIds.reserve(nConss);
+        for (size_t i { 0 }; i < nConss; ++i) {
+            auto* cons = conss[i];
+            assert(cons); // GCOVR_EXCL_LINE
+            result.consIds.emplace_back(SCIPconsGetName(cons));
+        }
+    }
+    return result;
+}
+
 }

test/test_iis.cpp

@@ -0,0 +1,52 @@
+#include <boost/test/unit_test.hpp>
+
+#include "scippp/model.hpp"
+#include "scippp/parameters.hpp"
+
+#include <algorithm>
+
+using namespace scippp;
+using namespace std;
+
+BOOST_AUTO_TEST_SUITE(IIS)
+
+BOOST_AUTO_TEST_CASE(CanBeCreated)
+{
+    Model model("Simple");
+    const auto& [x1, x2] = model.addVars<2>("x_");
+    model.addConstr(x1 + x2 >= 2, "lower");
+    model.addConstr(x1 + x2 <= 1, "upper");
+    model.addConstr(x1 + x2 >= 0, "useless");
+    model.setObjsense(Sense::MINIMIZE);
+
+    model.setParam(params::IIS::SILENT, true); // to avoid output during testing
+    model.solve();
+    BOOST_REQUIRE(model.getStatus() == SCIP_STATUS_INFEASIBLE);
+    auto iis { model.generateIIS() };
+    BOOST_TEST(model.getLastReturnCode() == SCIP_OKAY);
+
+    const auto& icons { iis.consIds };
+    BOOST_TEST(icons.size() == 2);
+    BOOST_TEST((std::find(icons.begin(), icons.end(), "lower") != iis.consIds.end()));
+    BOOST_TEST((std::find(icons.begin(), icons.end(), "upper") != iis.consIds.end()));
+    BOOST_TEST((std::find(icons.begin(), icons.end(), "useless") == iis.consIds.end()));
+}
+
+BOOST_AUTO_TEST_CASE(WorksOnEmptyProblem)
+{
+    Model model("Simple");
+    model.solve();
+    auto iis { model.generateIIS() };
+    BOOST_TEST(model.getLastReturnCode() == SCIP_OKAY);
+    BOOST_TEST(iis.consIds.size() == 0);
+}
+
+BOOST_AUTO_TEST_CASE(WorksWithoutSolving)
+{
+    Model model("Simple");
+    auto iis { model.generateIIS() };
+    BOOST_TEST(model.getLastReturnCode() == SCIP_OKAY);
+    BOOST_TEST(iis.consIds.size() == 0);
+}
+
+BOOST_AUTO_TEST_SUITE_END()