text: // Ceres Solver - A fast non-linear least squares minimizer
  // Copyright 2019 Google Inc. All rights reserved.
  // http://code.google.com/p/ceres-solver/
  //
  // Redistribution and use in source and binary forms, with or without
  // modification, are permitted provided that the following conditions are met:
  //
  // * Redistributions of source code must retain the above copyright notice,
  //   this list of conditions and the following disclaimer.
  // * Redistributions in binary form must reproduce the above copyright notice,
  //   this list of conditions and the following disclaimer in the documentation
  //   and/or other materials provided with the distribution.
  // * Neither the name of Google Inc. nor the names of its contributors may be
  //   used to endorse or promote products derived from this software without
  //   specific prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  // AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  // IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  // ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
  // LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  // CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  // SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  // INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  // CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  // ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  // POSSIBILITY OF SUCH DAMAGE.
  //
  // Author: darius.rueckert@fau.de (Darius Rueckert)
  
  #ifndef CERES_PUBLIC_EXPRESSION_TREE_H_
  #define CERES_PUBLIC_EXPRESSION_TREE_H_
  
  #include <vector>
  
  #include "expression.h"
  
  namespace ceres {
  namespace internal {
  
  // A directed, acyclic, unconnected graph containing all expressions of a
  // program.
  //
  // The expression graph is stored linear in the expressions_ array. The order is
  // identical to the execution order. Each expression can have multiple children
  // and multiple parents.
  // A is child of B     <=>  B has A as a parameter    <=> B.DirectlyDependsOn(A)
  // A is parent of B    <=>  A has B as a parameter    <=> A.DirectlyDependsOn(B)
  class ExpressionGraph {
   public:
    // Creates an expression and adds it to expressions_.
    // The returned reference will be invalid after this function is called again.
    Expression& CreateExpression(ExpressionType type);
  
    // Checks if A depends on B.
    // -> B is a descendant of A
    bool DependsOn(ExpressionId A, ExpressionId B) const;
  
    Expression& ExpressionForId(ExpressionId id) { return expressions_[id]; }
    const Expression& ExpressionForId(ExpressionId id) const {
      return expressions_[id];
    }
  
    int Size() const { return expressions_.size(); }
  
   private:
    // All Expressions are referenced by an ExpressionId. The ExpressionId is the
    // index into this array. Each expression has a list of ExpressionId as
    // arguments. These references form the graph.
    std::vector<Expression> expressions_;
  };
  
  // After calling this method, all operations on 'ExpressionRef' objects will be
  // recorded into an ExpressionGraph. You can obtain this graph by calling
  // StopRecordingExpressions.
  //
  // Performing expression operations before calling StartRecordingExpressions or
  // calling StartRecodring. twice is an error.
  void StartRecordingExpressions();
  
  // Stops recording and returns all expressions that have been executed since the
  // call to StartRecordingExpressions. The internal ExpressionGraph will be
  // invalidated and a second consecutive call to this method results in an error.
  ExpressionGraph StopRecordingExpressions();
  
  // Returns a pointer to the active expression tree.
  // Normal users should not use this functions.
  ExpressionGraph* GetCurrentExpressionGraph();
  
  }  // namespace internal
  }  // namespace ceres
  #endif
  