welch_satterthwaite.h

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/* Copyright (c) 2008-2022 the MRtrix3 contributors.
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Covered Software is provided under this License on an "as is"
 * basis, without warranty of any kind, either expressed, implied, or
 * statutory, including, without limitation, warranties that the
 * Covered Software is free of defects, merchantable, fit for a
 * particular purpose or non-infringing.
 * See the Mozilla Public License v. 2.0 for more details.
 *
 * For more details, see http://www.mrtrix.org/.
 */
 
#ifndef __math_welch_satterthwaite_h__
#define __math_welch_satterthwaite_h__
 
#include "math/math.h"
 
namespace MR {
  namespace Math {
 
 
 
    template <class VarArrayType, class CountArrayType>
    default_type welch_satterthwaite (const VarArrayType& variances, const CountArrayType& counts)
    {
      assert (size_t(variances.size()) == size_t(counts.size()));
      default_type numerator = 0.0, denominator = 0.0;
      for (size_t i = 0; i != size_t(variances.size()); ++i) {
        const default_type ks2 = (1.0 / (counts[i] - 1)) * variances[i];
        numerator += ks2;
        denominator += Math::pow2 (ks2) / (counts[i] - 1);
      }
      return Math::pow2 (numerator) / denominator;
    }
 
 
 
  }
}
 
#endif