[CP-SAT] more work on routing cuts; graph based LNS start from the objective; better enforcement of time limits

Author: lperron

ortools/sat/BUILD.bazel

@@ -209,6 +209,7 @@ cc_library(
         "//ortools/util:sorted_interval_list",
         "@com_google_absl//absl/container:flat_hash_map",
         "@com_google_absl//absl/container:flat_hash_set",
+        "@com_google_absl//absl/flags:flag",
         "@com_google_absl//absl/log:check",
         "@com_google_absl//absl/status",
         "@com_google_absl//absl/strings",
@@ -2709,6 +2710,33 @@ cc_test(
     ],
 )
 
+proto_library(
+    name = "routes_support_graph_proto",
+    srcs = ["routes_support_graph.proto"],
+)
+
+cc_proto_library(
+    name = "routes_support_graph_cc_proto",
+    deps = [":routes_support_graph_proto"],
+)
+
+cc_binary(
+    name = "render_routes_support_graph",
+    srcs = ["render_routes_support_graph.cc"],
+    deps = [
+        ":routes_support_graph_cc_proto",
+        "//ortools/base",
+        "//ortools/base:file",
+        "//ortools/routing/parsers:solomon_parser",
+        "//ortools/util:file_util",
+        "@com_google_absl//absl/flags:flag",
+        "@com_google_absl//absl/log",
+        "@com_google_absl//absl/log:check",
+        "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/strings:string_view",
+    ],
+)
+
 cc_library(
     name = "routing_cuts",
     srcs = ["routing_cuts.cc"],
@@ -2724,7 +2752,9 @@ cc_library(
         ":linear_constraint_manager",
         ":model",
         ":precedences",
+        ":routes_support_graph_cc_proto",
         ":sat_base",
+        ":sat_parameters_cc_proto",
         ":synchronization",
         ":util",
         "//ortools/base",
@@ -2734,12 +2764,12 @@ cc_library(
         "//ortools/graph",
         "//ortools/graph:connected_components",
         "//ortools/graph:max_flow",
-        "//ortools/util:bitset",
         "//ortools/util:strong_integers",
         "@com_google_absl//absl/algorithm:container",
         "@com_google_absl//absl/cleanup",
         "@com_google_absl//absl/container:flat_hash_map",
         "@com_google_absl//absl/container:flat_hash_set",
+        "@com_google_absl//absl/flags:flag",
         "@com_google_absl//absl/log",
         "@com_google_absl//absl/log:check",
         "@com_google_absl//absl/log:vlog_is_on",

ortools/sat/CMakeLists.txt

@@ -18,6 +18,7 @@ list(REMOVE_ITEM _SRCS
   ${CMAKE_CURRENT_SOURCE_DIR}/opb_reader.h
   ${CMAKE_CURRENT_SOURCE_DIR}/sat_cnf_reader.h
   ${CMAKE_CURRENT_SOURCE_DIR}/sat_runner.cc
+  ${CMAKE_CURRENT_SOURCE_DIR}/render_routes_support_graph.cc
 )
 set(NAME ${PROJECT_NAME}_sat)
 

ortools/sat/cp_model_checker.cc

@@ -505,8 +505,8 @@ std::string ValidateElementConstraint(const CpModelProto& model,
       RETURN_IF_NOT_EMPTY(ValidateAffineExpression(model, expr));
       LinearExpressionProto overflow_detection = ct.element().linear_target();
       AppendToOverflowValidator(expr, &overflow_detection, -1);
-      overflow_detection.set_offset(overflow_detection.offset() -
-                                    expr.offset());
+      const int64_t offset = CapSub(overflow_detection.offset(), expr.offset());
+      overflow_detection.set_offset(offset);
       if (PossibleIntegerOverflow(model, overflow_detection.vars(),
                                   overflow_detection.coeffs(),
                                   overflow_detection.offset())) {

ortools/sat/cp_model_lns.cc

@@ -197,9 +197,15 @@ void NeighborhoodGeneratorHelper::InitializeHelperData() {
 
   const int num_variables = model_proto_.variables().size();
   is_in_objective_.resize(num_variables, false);
+  has_positive_objective_coefficient_.resize(num_variables, false);
   if (model_proto_.has_objective()) {
-    for (const int ref : model_proto_.objective().vars()) {
+    for (int i = 0; i < model_proto_.objective().vars_size(); ++i) {
+      const int ref = model_proto_.objective().vars(i);
+      const int64_t coeff = model_proto_.objective().coeffs(i);
+      DCHECK_NE(coeff, 0);
       is_in_objective_[PositiveRef(ref)] = true;
+      has_positive_objective_coefficient_[PositiveRef(ref)] =
+          ref == PositiveRef(ref) ? coeff > 0 : coeff < 0;
     }
   }
 }
@@ -1318,6 +1324,26 @@ double NeighborhoodGenerator::Synchronize() {
   return total_dtime;
 }
 
+std::vector<int>
+NeighborhoodGeneratorHelper::ImprovableObjectiveVariablesWhileHoldingLock(
+    const CpSolverResponse& initial_solution) const {
+  std::vector<int> result;
+  absl::ReaderMutexLock lock(&domain_mutex_);
+  for (const int var : active_objective_variables_) {
+    const auto& domain =
+        model_proto_with_only_variables_.variables(var).domain();
+    bool at_best_value = false;
+    if (has_positive_objective_coefficient_[var]) {
+      at_best_value = initial_solution.solution(var) == domain[0];
+    } else {
+      at_best_value =
+          initial_solution.solution(var) == domain[domain.size() - 1];
+    }
+    if (!at_best_value) result.push_back(var);
+  }
+  return result;
+}
+
 namespace {
 
 template <class T>
@@ -1407,21 +1433,20 @@ Neighborhood VariableGraphNeighborhoodGenerator::Generate(
   {
     absl::ReaderMutexLock graph_lock(&helper_.graph_mutex_);
 
+    std::vector<int> initial_vars =
+        helper_.ImprovableObjectiveVariablesWhileHoldingLock(initial_solution);
+    if (initial_vars.empty()) {
+      initial_vars = helper_.ActiveVariablesWhileHoldingLock();
+    }
     // The number of active variables can decrease asynchronously.
     // We read the exact number while locked.
     const int num_active_vars =
         helper_.ActiveVariablesWhileHoldingLock().size();
-    const int num_objective_variables =
-        helper_.ActiveObjectiveVariablesWhileHoldingLock().size();
     const int target_size = std::ceil(data.difficulty * num_active_vars);
     if (target_size == num_active_vars) return helper_.FullNeighborhood();
 
     const int first_var =
-        num_objective_variables > 0  // Prefer objective variables.
-            ? helper_.ActiveObjectiveVariablesWhileHoldingLock()
-                  [absl::Uniform<int>(random, 0, num_objective_variables)]
-            : helper_.ActiveVariablesWhileHoldingLock()[absl::Uniform<int>(
-                  random, 0, num_active_vars)];
+        initial_vars[absl::Uniform<int>(random, 0, initial_vars.size())];
     visited_variables_set[first_var] = true;
     visited_variables.push_back(first_var);
     relaxed_variables.push_back(first_var);
@@ -1478,7 +1503,8 @@ Neighborhood ArcGraphNeighborhoodGenerator::Generate(
   {
     absl::ReaderMutexLock graph_lock(&helper_.graph_mutex_);
     num_active_vars = helper_.ActiveVariablesWhileHoldingLock().size();
-    active_objective_vars = helper_.ActiveObjectiveVariablesWhileHoldingLock();
+    active_objective_vars =
+        helper_.ImprovableObjectiveVariablesWhileHoldingLock(initial_solution);
     constraints_to_vars = helper_.ConstraintToVar();
     vars_to_constraints = helper_.VarToConstraint();
   }
@@ -1569,13 +1595,12 @@ Neighborhood ConstraintGraphNeighborhoodGenerator::Generate(
     const int target_size = std::ceil(data.difficulty * num_active_vars);
     if (target_size == num_active_vars) return helper_.FullNeighborhood();
 
-    // Start by a random constraint.
+    // Start from a random active constraint.
     const int num_active_constraints = helper_.ConstraintToVar().size();
-    if (num_active_constraints != 0) {
-      next_constraints.push_back(
-          absl::Uniform<int>(random, 0, num_active_constraints));
-      added_constraints[next_constraints.back()] = true;
-    }
+    if (num_active_constraints == 0) return helper_.NoNeighborhood();
+    next_constraints.push_back(
+        absl::Uniform<int>(random, 0, num_active_constraints));
+    added_constraints[next_constraints.back()] = true;
 
     while (relaxed_variables.size() < target_size) {
       // Stop if we have a full connected component.
@@ -1667,9 +1692,9 @@ Neighborhood DecompositionGraphNeighborhoodGenerator::Generate(
       elements[i].tie_break = absl::Uniform<double>(random, 0.0, 1.0);
     }
 
-    // We start by a random active variable.
+    // We start from a random active variable.
     //
-    // Note that while num_vars contains all variables, all the fixed variable
+    // Note that while num_vars contains all variables, all the fixed variables
     // will have no associated constraint, so we don't want to start from a
     // random variable.
     //

ortools/sat/cp_model_lns.h

@@ -195,6 +195,14 @@ class NeighborhoodGeneratorHelper : public SubSolver {
     return result;
   }
 
+  // Returns the vector of objective variables that are not already at their
+  // best possible value. The graph_mutex_ must be locked before calling this
+  // method.
+  std::vector<int> ImprovableObjectiveVariablesWhileHoldingLock(
+      const CpSolverResponse& initial_solution) const
+      ABSL_SHARED_LOCKS_REQUIRED(graph_mutex_)
+          ABSL_LOCKS_EXCLUDED(domain_mutex_);
+
   // Constraints <-> Variables graph.
   // Important:
   //   - The constraint index is NOT related to the one in the cp_model.
@@ -332,9 +340,12 @@ class NeighborhoodGeneratorHelper : public SubSolver {
   // Constraints by types. This never changes.
   std::vector<std::vector<int>> type_to_constraints_;
 
-  // Whether a model_proto_ variable appear in the objective. This never
+  // Whether a model_proto_ variable appears in the objective. This never
   // changes.
   std::vector<bool> is_in_objective_;
+  // If a model_proto_ variable has a positive coefficient in the objective.
+  // This never changes.
+  std::vector<bool> has_positive_objective_coefficient_;
 
   // A copy of CpModelProto where we did some basic presolving to remove all
   // constraint that are always true. The Variable-Constraint graph is based on
@@ -368,7 +379,7 @@ class NeighborhoodGeneratorHelper : public SubSolver {
 
   std::vector<int> tmp_row_;
 
-  mutable absl::Mutex domain_mutex_;
+  mutable absl::Mutex domain_mutex_ ABSL_ACQUIRED_AFTER(graph_mutex_);
 };
 
 // Base class for a CpModelProto neighborhood generator.

ortools/sat/cp_model_presolve.cc

@@ -7731,6 +7731,8 @@ void CpModelPresolver::Probe() {
     return (void)context_->NotifyThatModelIsUnsat("during probing");
   }
 
+  time_limit_->ResetHistory();
+
   // Update the presolve context with fixed Boolean variables.
   int num_fixed = 0;
   CHECK_EQ(sat_solver->CurrentDecisionLevel(), 0);
@@ -8663,6 +8665,7 @@ void CpModelPresolver::MergeNoOverlapConstraints() {
   // We reuse the max-clique code from sat.
   Model local_model;
   local_model.GetOrCreate<Trail>()->Resize(num_constraints);
+  local_model.GetOrCreate<TimeLimit>()->MergeWithGlobalTimeLimit(time_limit_);
   auto* graph = local_model.GetOrCreate<BinaryImplicationGraph>();
   graph->Resize(num_constraints);
   for (const std::vector<Literal>& clique : cliques) {
@@ -8699,6 +8702,7 @@ void CpModelPresolver::MergeNoOverlapConstraints() {
                             new_num_intervals, " intervals).");
     context_->UpdateRuleStats("no_overlap: merged constraints");
   }
+  time_limit_->ResetHistory();
 }
 
 // TODO(user): Should we take into account the exactly_one constraints? note

ortools/sat/cp_model_solver_helpers.cc

@@ -88,25 +88,6 @@
 #include "ortools/util/strong_integers.h"
 #include "ortools/util/time_limit.h"
 
-ABSL_FLAG(bool, cp_model_dump_models, false,
-          "DEBUG ONLY. When set to true, SolveCpModel() will dump its model "
-          "protos (original model, presolved model, mapping model) in text "
-          "format to 'FLAGS_cp_model_dump_prefix'{model|presolved_model|"
-          "mapping_model}.pb.txt.");
-
-#if defined(_MSC_VER)
-ABSL_FLAG(std::string, cp_model_dump_prefix, ".\\",
-          "Prefix filename for all dumped files");
-#else
-ABSL_FLAG(std::string, cp_model_dump_prefix, "/tmp/",
-          "Prefix filename for all dumped files");
-#endif
-
-ABSL_FLAG(bool, cp_model_dump_submodels, false,
-          "DEBUG ONLY. When set to true, solve will dump all "
-          "lns or objective_shaving submodels proto in text format to "
-          "'FLAGS_cp_model_dump_prefix'xxx.pb.txt.");
-
 ABSL_FLAG(
     std::string, cp_model_load_debug_solution, "",
     "DEBUG ONLY. When this is set to a non-empty file name, "

ortools/sat/cp_model_solver_helpers.h

@@ -16,12 +16,9 @@
 
 #include <cstdint>
 #include <memory>
-#include <string>
-#include <tuple>
 #include <utility>
 #include <vector>
 
-#include "absl/flags/declare.h"
 #include "absl/types/span.h"
 #include "ortools/base/timer.h"
 #include "ortools/sat/cp_model.pb.h"
@@ -34,11 +31,6 @@
 #include "ortools/sat/work_assignment.h"
 #include "ortools/util/logging.h"
 
-ABSL_DECLARE_FLAG(bool, cp_model_dump_models);
-ABSL_DECLARE_FLAG(std::string, cp_model_dump_prefix);
-ABSL_DECLARE_FLAG(bool, cp_model_dump_problematic_lns);
-ABSL_DECLARE_FLAG(bool, cp_model_dump_submodels);
-
 namespace operations_research {
 namespace sat {
 

ortools/sat/cp_model_utils.cc

@@ -23,6 +23,7 @@
 
 #include "absl/container/flat_hash_map.h"
 #include "absl/container/flat_hash_set.h"
+#include "absl/flags/flag.h"
 #include "absl/log/check.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/string_view.h"
@@ -36,6 +37,25 @@
 #include "ortools/util/saturated_arithmetic.h"
 #include "ortools/util/sorted_interval_list.h"
 
+ABSL_FLAG(bool, cp_model_dump_models, false,
+          "DEBUG ONLY. When set to true, SolveCpModel() will dump its model "
+          "protos (original model, presolved model, mapping model) in text "
+          "format to 'FLAGS_cp_model_dump_prefix'{model|presolved_model|"
+          "mapping_model}.pb.txt.");
+
+#if defined(_MSC_VER)
+ABSL_FLAG(std::string, cp_model_dump_prefix, ".\\",
+          "Prefix filename for all dumped files");
+#else
+ABSL_FLAG(std::string, cp_model_dump_prefix, "/tmp/",
+          "Prefix filename for all dumped files");
+#endif
+
+ABSL_FLAG(bool, cp_model_dump_submodels, false,
+          "DEBUG ONLY. When set to true, solve will dump all "
+          "lns or objective_shaving submodels proto in text format to "
+          "'FLAGS_cp_model_dump_prefix'xxx.pb.txt.");
+
 namespace operations_research {
 namespace sat {
 

ortools/sat/cp_model_utils.h

@@ -24,6 +24,7 @@
 #if !defined(__PORTABLE_PLATFORM__)
 #include "ortools/base/helpers.h"
 #endif  // !defined(__PORTABLE_PLATFORM__)
+#include "absl/flags/declare.h"
 #include "absl/log/check.h"
 #include "absl/status/status.h"
 #include "absl/strings/match.h"
@@ -37,6 +38,11 @@
 #include "ortools/util/bitset.h"
 #include "ortools/util/sorted_interval_list.h"
 
+ABSL_DECLARE_FLAG(bool, cp_model_dump_models);
+ABSL_DECLARE_FLAG(std::string, cp_model_dump_prefix);
+ABSL_DECLARE_FLAG(bool, cp_model_dump_problematic_lns);
+ABSL_DECLARE_FLAG(bool, cp_model_dump_submodels);
+
 namespace operations_research {
 namespace sat {
 

ortools/sat/precedences_test.cc

@@ -501,8 +501,11 @@ std::vector<Relation> GetRelations(Model& model) {
   for (int i = 0; i < repository.size(); ++i) {
     Relation r = repository.relation(i);
     if (r.a.coeff < 0) {
-      r = Relation({r.enforcement, {r.a.var, -r.a.coeff}, {r.b.var, -r.b.coeff},
-                   -r.rhs, -r.lhs});
+      r = Relation({r.enforcement,
+                    {r.a.var, -r.a.coeff},
+                    {r.b.var, -r.b.coeff},
+                    -r.rhs,
+                    -r.lhs});
     }
     relations.push_back(r);
   }