more spans

Author: lperron

ortools/algorithms/python/set_cover.cc

@@ -208,6 +208,7 @@ PYBIND11_MODULE(set_cover, m) {
           },
           arg("subset"), arg("cost"))
       .def("create_sparse_row_view", &SetCoverModel::CreateSparseRowView)
+      .def("sort_elements_in_subsets", &SetCoverModel::SortElementsInSubsets)
       .def("compute_feasibility", &SetCoverModel::ComputeFeasibility)
       .def(
           "reserve_num_subsets",

ortools/algorithms/python/set_cover_test.py

@@ -56,6 +56,7 @@ class SetCoverTest(absltest.TestCase):
 
     def test_save_reload(self):
         model = create_knights_cover_model(10, 10)
+        model.sort_elements_in_subsets()
         proto = model.export_model_as_proto()
         reloaded = set_cover.SetCoverModel()
         reloaded.import_model_from_proto(proto)

ortools/algorithms/set_cover_heuristics.cc

@@ -95,7 +95,7 @@ bool GreedySolutionGenerator::NextSolution() {
 }
 
 bool GreedySolutionGenerator::NextSolution(
-    const std::vector<SubsetIndex>& focus) {
+    absl::Span<const SubsetIndex> focus) {
   return NextSolution(focus, inv_->model()->subset_costs());
 }
 

ortools/algorithms/set_cover_heuristics.h

@@ -122,7 +122,7 @@ class GreedySolutionGenerator {
 
   // Computes the next partial solution considering only the subsets whose
   // indices are in focus.
-  bool NextSolution(const std::vector<SubsetIndex>& focus);
+  bool NextSolution(absl::Span<const SubsetIndex> focus);
 
   // Same with a different set of costs.
   bool NextSolution(absl::Span<const SubsetIndex> focus,

ortools/algorithms/set_cover_model.cc

@@ -183,6 +183,7 @@ void SetCoverModel::UpdateAllSubsetsList() {
 }
 
 void SetCoverModel::AddEmptySubset(Cost cost) {
+  elements_in_subsets_are_sorted_ = false;
   subset_costs_.push_back(cost);
   columns_.push_back(SparseColumn());
   all_subsets_.push_back(SubsetIndex(num_subsets_));
@@ -194,6 +195,7 @@ void SetCoverModel::AddEmptySubset(Cost cost) {
 }
 
 void SetCoverModel::AddElementToLastSubset(BaseInt element) {
+  elements_in_subsets_are_sorted_ = false;
   columns_.back().push_back(ElementIndex(element));
   num_elements_ = std::max(num_elements_, element + 1);
   // No need to update the list all_subsets_.
@@ -206,6 +208,7 @@ void SetCoverModel::AddElementToLastSubset(ElementIndex element) {
 }
 
 void SetCoverModel::SetSubsetCost(BaseInt subset, Cost cost) {
+  elements_in_subsets_are_sorted_ = false;
   CHECK(std::isfinite(cost));
   DCHECK_GE(subset, 0);
   if (subset >= num_subsets()) {
@@ -223,6 +226,7 @@ void SetCoverModel::SetSubsetCost(SubsetIndex subset, Cost cost) {
 }
 
 void SetCoverModel::AddElementToSubset(BaseInt element, BaseInt subset) {
+  elements_in_subsets_are_sorted_ = false;
   if (subset >= num_subsets()) {
     num_subsets_ = subset + 1;
     subset_costs_.resize(num_subsets_, 0.0);
@@ -264,6 +268,13 @@ void SetCoverModel::ReserveNumElementsInSubset(ElementIndex num_elements,
   ReserveNumElementsInSubset(num_elements.value(), subset.value());
 }
 
+void SetCoverModel::SortElementsInSubsets() {
+  for (const SubsetIndex subset : SubsetRange()) {
+    std::sort(columns_[subset].begin(), columns_[subset].end());
+  }
+  elements_in_subsets_are_sorted_ = true;
+}
+
 void SetCoverModel::CreateSparseRowView() {
   if (row_view_is_valid_) {
     return;
@@ -287,6 +298,7 @@ void SetCoverModel::CreateSparseRowView() {
     }
   }
   row_view_is_valid_ = true;
+  elements_in_subsets_are_sorted_ = true;
 }
 
 bool SetCoverModel::ComputeFeasibility() const {
@@ -319,13 +331,15 @@ bool SetCoverModel::ComputeFeasibility() const {
   return true;
 }
 
-SetCoverProto SetCoverModel::ExportModelAsProto() {
+SetCoverProto SetCoverModel::ExportModelAsProto() const {
+  CHECK(elements_in_subsets_are_sorted_);
   SetCoverProto message;
   for (const SubsetIndex subset : SubsetRange()) {
     SetCoverProto::Subset* subset_proto = message.add_subset();
     subset_proto->set_cost(subset_costs_[subset]);
-    std::sort(columns_[subset].begin(), columns_[subset].end());
-    for (const ElementIndex element : columns_[subset]) {
+    SparseColumn column = columns_[subset];
+    std::sort(column.begin(), column.end());
+    for (const ElementIndex element : column) {
       subset_proto->add_element(element.value());
     }
   }
@@ -433,8 +447,9 @@ std::vector<T> ComputeDeciles(std::vector<T> values) {
   const int kNumDeciles = 10;
   std::vector<T> deciles;
   deciles.reserve(kNumDeciles);
+  const float step = values.size() / kNumDeciles;
   for (int i = 1; i <= kNumDeciles; ++i) {
-    const size_t point = values.size() * i / kNumDeciles - 1;
+    const size_t point = std::max<float>(0, i * step - 1);
     std::nth_element(values.begin(), values.begin() + point, values.end());
     deciles.push_back(values[point]);
   }

ortools/algorithms/set_cover_model.h

@@ -109,6 +109,7 @@ class SetCoverModel {
         num_subsets_(0),
         num_nonzeros_(0),
         row_view_is_valid_(false),
+        elements_in_subsets_are_sorted_(false),
         subset_costs_(),
         columns_(),
         rows_(),
@@ -204,7 +205,12 @@ class SetCoverModel {
   void AddElementToSubset(BaseInt element, BaseInt subset);
   void AddElementToSubset(ElementIndex element, SubsetIndex subset);
 
-  // Creates the sparse ("dual") representation of the problem.
+  // Sorts the elements in each subset. Should be called before exporting the
+  // model to a proto.
+  void SortElementsInSubsets();
+
+  // Creates the sparse ("dual") representation of the problem. This also sorts
+  // the elements in each subset.
   void CreateSparseRowView();
 
   // Returns true if the problem is feasible, i.e. if the subsets cover all
@@ -220,10 +226,12 @@ class SetCoverModel {
   void ReserveNumElementsInSubset(ElementIndex num_elements,
                                   SubsetIndex subset);
 
-  // Returns the model as a SetCoverProto. The function is not const because
-  // the element indices in the columns need to be sorted for the representation
-  // as a protobuf to be canonical.
-  SetCoverProto ExportModelAsProto();
+  // Returns the model as a SetCoverProto. Note that the elements of each subset
+  // are sorted locally before being exported to the proto. This is done to
+  // ensure that the proto is deterministic. The function is const because it
+  // does not modify the model. Therefore, the model as exported by this
+  // function may be different from the initial model.
+  SetCoverProto ExportModelAsProto() const;
 
   // Imports the model from a SetCoverProto.
   void ImportModelFromProto(const SetCoverProto& message);
@@ -284,6 +292,9 @@ class SetCoverModel {
   // True when the SparseRowView is up-to-date.
   bool row_view_is_valid_;
 
+  // True when the elements in each subset are sorted.
+  bool elements_in_subsets_are_sorted_;
+
   // Costs for each subset.
 
   SubsetCostVector subset_costs_;

ortools/algorithms/set_cover_test.cc

@@ -123,6 +123,7 @@ class KnightsCover {
 
 TEST(SetCoverProtoTest, SaveReload) {
   SetCoverModel model = KnightsCover(10, 10).model();
+  model.SortElementsInSubsets();
   SetCoverProto proto = model.ExportModelAsProto();
   SetCoverModel reloaded;
   reloaded.ImportModelFromProto(proto);