include/ceres/internal/expression_graph.h

// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2019 Google Inc. All rights reserved.
// http://code.google.com/p/ceres-solver/
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// 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 arithmetic expression of the following form:
  // <lhs> = <rhs>;
  //
  // For example:
  //   CreateArithmeticExpression(PLUS, 5)
  // will generate:
  //   v_5 = __ + __;
  // The place holders are then set by the CreateXX functions of Expression.
  //
  // If lhs_id == kInvalidExpressionId, then a new lhs_id will be generated and
  // assigned to the created expression.
  // Calling this function with a lhs_id that doesn't exist results in an
  // error.
  Expression& CreateArithmeticExpression(ExpressionType type,
                                         ExpressionId lhs_id);

  // Control expression don't have a left hand side.
  // Supported types: IF/ELSE/ENDIF/NOP
  Expression& CreateControlExpression(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