shaving_solver.h

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// Copyright 2010-2024 Google LLC
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
 
#ifndef OR_TOOLS_SAT_SHAVING_SOLVER_H_
#define OR_TOOLS_SAT_SHAVING_SOLVER_H_
 
#include <atomic>
#include <cstdint>
#include <functional>
#include <memory>
#include <string>
#include <vector>
 
#include "absl/base/thread_annotations.h"
#include "absl/synchronization/mutex.h"
#include "ortools/sat/cp_model.pb.h"
#include "ortools/sat/cp_model_lns.h"
#include "ortools/sat/cp_model_solver_helpers.h"
#include "ortools/sat/integer.h"
#include "ortools/sat/model.h"
#include "ortools/sat/sat_parameters.pb.h"
#include "ortools/sat/subsolver.h"
#include "ortools/util/sorted_interval_list.h"
 
namespace operations_research {
namespace sat {
 
class ObjectiveShavingSolver : public SubSolver {
 public:
  ObjectiveShavingSolver(const SatParameters& local_parameters,
                         NeighborhoodGeneratorHelper* helper,
                         SharedClasses* shared);
 
  ~ObjectiveShavingSolver() override;
 
  bool TaskIsAvailable() override;
 
  std::function<void()> GenerateTask(int64_t task_id) override;
  void Synchronize() override;
 
 private:
  std::string Info();
 
  bool ResetModel(int64_t task_id);
 
  // This is fixed at construction.
  SatParameters local_params_;
  NeighborhoodGeneratorHelper* helper_;
  SharedClasses* shared_;
 
  // Allow to control the local time limit in addition to a potential user
  // defined external Boolean.
  std::atomic<bool> stop_current_chunk_;
 
  // Local singleton repository and presolved local model.
  std::unique_ptr<Model> local_sat_model_;
  CpModelProto local_proto_;
 
  // For postsolving a feasible solution or improving objective lb.
  std::vector<int> postsolve_mapping_;
  CpModelProto mapping_proto_;
 
  absl::Mutex mutex_;
  IntegerValue objective_lb_ ABSL_GUARDED_BY(mutex_);
  IntegerValue objective_ub_ ABSL_GUARDED_BY(mutex_);
  IntegerValue current_objective_target_ub_ ABSL_GUARDED_BY(mutex_);
  bool task_in_flight_ ABSL_GUARDED_BY(mutex_) = false;
};
 
class VariablesShavingSolver : public SubSolver {
 public:
  struct State {
    int var_index;
    bool minimize;
    Domain reduced_domain;
  };
 
  VariablesShavingSolver(const SatParameters& local_parameters,
                         SharedClasses* shared);
 
  ~VariablesShavingSolver() override;
 
  bool TaskIsAvailable() override;
 
  void ProcessLocalResponse(const CpSolverResponse& local_response,
                            const State& state);
 
  std::function<void()> GenerateTask(int64_t task_id) override;
 
  void Synchronize() override;
 
 private:
  std::string Info();
 
  int64_t DomainSize(int var) const ABSL_SHARED_LOCKS_REQUIRED(mutex_);
 
  bool VarIsFixed(int int_var) const ABSL_SHARED_LOCKS_REQUIRED(mutex_);
 
  bool ConstraintIsInactive(int c) const ABSL_SHARED_LOCKS_REQUIRED(mutex_);
 
  bool FindNextVar(State* state) ABSL_SHARED_LOCKS_REQUIRED(mutex_);
 
  void CopyModelConnectedToVar(State* state, Model* local_sat_model,
                               CpModelProto* shaving_proto)
      ABSL_SHARED_LOCKS_REQUIRED(mutex_);
 
  bool ResetModel(int64_t task_id, State* state, Model* local_sat_model,
                  CpModelProto* shaving_proto);
 
  // This is fixed at construction.
  SatParameters local_params_;
  SharedClasses* shared_;
  int shared_bounds_id_ = -1;
 
  // Allow to control the local time limit in addition to a potential user
  // defined external Boolean.
  std::atomic<bool> stop_current_chunk_;
 
  const CpModelProto& model_proto_;
 
  absl::Mutex mutex_;
  int current_index_ = -1;
  std::vector<Domain> var_domains_ ABSL_GUARDED_BY(mutex_);
 
  // Stats.
  int num_vars_tried_ ABSL_GUARDED_BY(mutex_) = 0;
  int num_vars_shaved_ ABSL_GUARDED_BY(mutex_) = 0;
  int num_infeasible_found_ ABSL_GUARDED_BY(mutex_) = 0;
};
 
}  // namespace sat
}  // namespace operations_research
 
#endif  // OR_TOOLS_SAT_SHAVING_SOLVER_H_