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//
// G4DataInterpolation
//
// Class description:
//
// The class consists of some methods for data interpolations and
// extrapolations. The methods based mainly on recommendations given in the
// book: An introduction to NUMERICAL METHODS IN C++, B.H. Flowers,
//       Claredon Press, Oxford, 1995.

// Author: V.Grichine, 03.04.1997
// --------------------------------------------------------------------
#ifndef G4DATAINTERPOLATION_HH
#define G4DATAINTERPOLATION_HH 1

#include "globals.hh"

class G4DataInterpolation
{
 public:
  G4DataInterpolation(G4double pX[], G4double pY[], G4int number);
  // Constructor for initializing data members.

  G4DataInterpolation(G4double pX[], G4double pY[], G4int number,
                      G4double pFirstDerStart, G4double pFirstDerFinish);
  // Constructor for cubic spline interpolation. It creates fSecond Deivative
  // array as well as fArgument and fFunction.

  ~G4DataInterpolation();
  // Destructor deletes dynamically created arrays for data members: fArgument,
  // fFunction and fSecondDerivative, all have dimension of fNumber.

  G4DataInterpolation(const G4DataInterpolation&) = delete;
  G4DataInterpolation& operator=(const G4DataInterpolation&) = delete;
  // Copy constructor and assignement operator not allowed.

  G4double PolynomInterpolation(G4double pX, G4double& deltaY) const;
  // This function returns the value P(pX), where P(x) is polynom of fNumber-1
  // degree such that P(fArgument[i]) = fFunction[i], for i = 0, ..., fNumber-1.

  void PolIntCoefficient(G4double cof[]) const;
  // Given arrays fArgument[0,..,fNumber-1] and fFunction[0,..,fNumber-1], this
  // function calculates an array of coefficients.
  // The coefficients don't provide usually (fNumber>10) better accuracy for
  // polynom interpolation, as compared with PolynomInterpolation() function.
  // They could be used instead for derivate calculations and some other
  // applications.

  G4double RationalPolInterpolation(G4double pX, G4double& deltaY) const;
  // The function returns diagonal rational function (Bulirsch and Stoer
  // algorithm of Neville type) Pn(x)/Qm(x) where P and Q are polynoms.
  // Tests showed the method is not stable and hasn't advantage if compared
  // with polynomial interpolation.

  G4double CubicSplineInterpolation(G4double pX) const;
  // Cubic spline interpolation in point pX for function given by the table:
  // fArgument, fFunction. The constructor, which creates fSecondDerivative,
  // must be called before. The function works optimal, if sequential calls
  // are in random values of pX.

  G4double FastCubicSpline(G4double pX, G4int index) const;
  // Return cubic spline interpolation in the point pX which is located between
  // fArgument[index] and fArgument[index+1]. It is usually called in sequence
  // of known from external analysis values of index.

  G4int LocateArgument(G4double pX) const;
  // Given argument pX, returns index k, so that pX bracketed by fArgument[k]
  // and fArgument[k+1].

  void CorrelatedSearch(G4double pX, G4int& index) const;
  // Given a value pX, returns a value 'index' such that pX is between
  // fArgument[index] and fArgument[index+1]. fArgument MUST BE MONOTONIC,
  // either increasing or decreasing. If index = -1 or fNumber, this indicates
  // that pX is out of range. The value index on input is taken as the initial
  // approximation for index on output.

 private:
  // pointers to data table to be interpolated for y[i] and x[i] respectively
  G4double* fArgument = nullptr;
  G4double* fFunction = nullptr;

  G4double* fSecondDerivative = nullptr;

  G4int fNumber = 0;  // the corresponding table size
};

#endif