linear_expr.h

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// Copyright 2010-2018 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_LINEAR_SOLVER_LINEAR_EXPR_H_
#define OR_TOOLS_LINEAR_SOLVER_LINEAR_EXPR_H_

#include "absl/container/flat_hash_map.h"

namespace operations_research {

// NOTE(user): forward declaration is necessary due to cyclic dependencies,
// MPVariable is defined in linear_solver.h, which depends on LinearExpr.
class MPVariable;

class LinearExpr {
 public:
  LinearExpr();
  LinearExpr(double constant);  // NOLINT

  /***
   * Possible implicit conversions are intentional.
   * 
   * Warning: var is not owned.
   */
  LinearExpr(const MPVariable* var);  // NOLINT

  static LinearExpr NotVar(LinearExpr var);

  LinearExpr& operator+=(const LinearExpr& rhs);
  LinearExpr& operator-=(const LinearExpr& rhs);
  LinearExpr& operator*=(double rhs);
  LinearExpr& operator/=(double rhs);
  LinearExpr operator-() const;

  double offset() const { return offset_; }
  const absl::flat_hash_map<const MPVariable*, double>& terms() const {
    return terms_;
  }

  double SolutionValue() const;

 private:
  double offset_;
  absl::flat_hash_map<const MPVariable*, double> terms_;
};

// NOTE(user): in the ops below, the non-"const LinearExpr&" are intentional.
// We need to create a new LinearExpr for the result, so we lose nothing by
// passing one argument by value, mutating it, and then returning it. In
// particular, this allows (with move semantics and RVO) an optimized
// evaluation of expressions such as
// a + b + c + d
// (see http://en.cppreference.com/w/cpp/language/operators).
LinearExpr operator+(LinearExpr lhs, const LinearExpr& rhs);
LinearExpr operator-(LinearExpr lhs, const LinearExpr& rhs);
LinearExpr operator*(LinearExpr lhs, double rhs);
LinearExpr operator/(LinearExpr lhs, double rhs);
LinearExpr operator*(double lhs, LinearExpr rhs);

class LinearRange {
 public:
  LinearRange() : lower_bound_(0), upper_bound_(0) {}
  // The bounds of the linear range are updated so that they include the offset
  // from "linear_expr", i.e., we form the range:
  // lower_bound - offset <= linear_expr - offset <= upper_bound - offset.
  LinearRange(double lower_bound, const LinearExpr& linear_expr,
              double upper_bound);

  double lower_bound() const { return lower_bound_; }
  const LinearExpr& linear_expr() const { return linear_expr_; }
  double upper_bound() const { return upper_bound_; }

 private:
  double lower_bound_;
  // invariant: linear_expr_.offset() == 0.
  LinearExpr linear_expr_;
  double upper_bound_;
};

LinearRange operator<=(const LinearExpr& lhs, const LinearExpr& rhs);
LinearRange operator==(const LinearExpr& lhs, const LinearExpr& rhs);
LinearRange operator>=(const LinearExpr& lhs, const LinearExpr& rhs);

// TODO(user,user): explore defining more overloads to support:
// solver.AddRowConstraint(0.0 <= x + y + z <= 1.0);

}  // namespace operations_research

#endif  // OR_TOOLS_LINEAR_SOLVER_LINEAR_EXPR_H_