ChebyshevSeries.cpp

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// Mantid Repository : https://github.com/mantidproject/mantid
//
// Copyright © 2018 ISIS Rutherford Appleton Laboratory UKRI,
//   NScD Oak Ridge National Laboratory, European Spallation Source,
//   Institut Laue - Langevin & CSNS, Institute of High Energy Physics, CAS
// SPDX - License - Identifier: GPL - 3.0 +
//-----------------------------------------------------------------------------
// Includes
//-----------------------------------------------------------------------------
#include "MantidKernel/Math/Distributions/ChebyshevSeries.h"
#include <algorithm>
#include <cassert>
 
//-----------------------------------------------------------------------------
// Anonmouys static helpers
//-----------------------------------------------------------------------------
 
namespace Mantid::Kernel {
 
//-----------------------------------------------------------------------------
// Public members
//-----------------------------------------------------------------------------
ChebyshevSeries::ChebyshevSeries(const size_t degree) : m_bk(degree + 3, 0.0) {
  // The algorithm requires computing upto n+2 terms so space is
  // reserved for (n+1)+2 values.
}
 
double ChebyshevSeries::operator()(const std::vector<double> &c, const double x) {
  const size_t degree(m_bk.size() - 3);
  assert(c.size() >= degree + 1);
 
  m_bk.resize(degree + 3);
  std::fill(m_bk.begin(), m_bk.end(), 0.0);
  for (size_t i = 0; i <= degree; ++i) {
    const size_t k = degree - i;
    m_bk[k] = c[k] + 2. * x * m_bk[k + 1] - m_bk[k + 2];
  }
  return m_bk[0] - x * m_bk[1];
}
 
} // namespace Mantid::Kernel