include/ceres/codegen/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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//   used to endorse or promote products derived from this software without
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// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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// Author: darius.rueckert@fau.de (Darius Rueckert)

#ifndef CERES_PUBLIC_CODEGEN_INTERNAL_EXPRESSION_GRAPH_H_
#define CERES_PUBLIC_CODEGEN_INTERNAL_EXPRESSION_GRAPH_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:
  // Checks if A depends on B.
  // -> B is a descendant of A
  bool DependsOn(ExpressionId A, ExpressionId B) const;

  bool operator==(const ExpressionGraph& other) const;
  bool operator!=(const ExpressionGraph& other) const {
    return !(*this == other);
  }

  Expression& ExpressionForId(ExpressionId id) { return expressions_[id]; }
  const Expression& ExpressionForId(ExpressionId id) const {
    return expressions_[id];
  }

  int Size() const { return expressions_.size(); }

  // Erases the expression at "location". All expression after "location" are
  // moved by one element to the front. References to moved expressions are
  // updated. Removing an expression that is still referenced somewhere is
  // undefined behaviour.
  void Erase(ExpressionId location);

  // Insert a new expression at "location" into the graph. All expression
  // after "location" are moved by one element to the back. References to
  // moved expressions are updated.
  void Insert(ExpressionId location, const Expression& expression);

  // Adds an Expression to the end of the expression list and creates a new
  // variable for the result. The id of the result variable is returned so it
  // can be used for further operations.
  ExpressionId InsertBack(const Expression& expression);

  // Finds the closing ENDIF expression for a given IF expression. Calling this
  // method is only valid on IF expressions. If no suitable ENDIF is found,
  // kInvalidExpressionId is returned. Example:
  // <id> <expr>    FindMatchingEndif(id)
  //  0  IF         7
  //  1    IF       3
  //  2    ELSE     -
  //  3    ENDIF    -
  //  4  ELSE       -
  //  5    IF       6
  //  6    ENDIF    -
  //  7  ENDIF      -
  ExpressionId FindMatchingEndif(ExpressionId id) const;

  // Similar to FindMatchingEndif, but returns the matching ELSE expression. If
  // no suitable ELSE is found, kInvalidExpressionId is returned.
  // FindMatchingElse does not throw an error is this case, because IF without
  // ELSE is allowed.
  // <id> <expr>    FindMatchingEndif(id)
  //  0  IF         4
  //  1    IF       2
  //  2    ELSE     -
  //  3    ENDIF    -
  //  4  ELSE       -
  //  5    IF       kInvalidEpressionId
  //  6    ENDIF    -
  //  7  ENDIF      -
  ExpressionId FindMatchingElse(ExpressionId id) const;

 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  // CERES_PUBLIC_CODEGEN_INTERNAL_EXPRESSION_GRAPH_H_