ChiSquareAccumulator.h

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#ifndef LARDATA_UTILITIES_CHISQUAREACCUMULATOR_H
#define LARDATA_UTILITIES_CHISQUAREACCUMULATOR_H

// C/C++ standard libraries
#include <utility> // std::move(), std::forward()

namespace lar {
  namespace util {

    template <typename F, typename T = double>
    class ChiSquareAccumulator {
    public:
      using Function_t = F; 
      using Data_t = T;     


      ChiSquareAccumulator(Function_t const& expected) : fExpected(expected) {}
      // @}

      // --- BEGIN -- Access to results ----------------------------------------

      // @{
      Data_t chiSquare() const { return fChiSq; }
      Data_t operator()() const { return chiSquare(); }
      operator Data_t() const { return chiSquare(); }

      unsigned int N() const { return fN; }

      Data_t expected(Data_t x) const { return fExpected(x); }

      // --- END -- Access to results ------------------------------------------

      // --- BEGIN -- Data manipulation ----------------------------------------

      void add(Data_t x, Data_t y)
      {
        fChiSq += sqr(y - expected(x));
        ++fN;
      }

      void add(Data_t x, Data_t y, Data_t s)
      {
        fChiSq += sqr(z(y, expected(x), s));
        ++fN;
      }

      void clear()
      {
        fChiSq = Data_t{0};
        fN = 0U;
      }

      // --- END -- Data manipulation ------------------------------------------

    private:
      unsigned int fN = 0U; 
      Data_t fChiSq = 0.0;  

      Function_t fExpected; 

      static Data_t z(Data_t x, Data_t mu, Data_t sigma) { return (x - mu) / sigma; }

      static Data_t sqr(Data_t v) { return v * v; }

    }; // ChiSquareAccumulator<>

    //--------------------------------------------------------------------------
    template <typename F>
    auto makeChiSquareAccumulator(F&& e)
    {
      return lar::util::ChiSquareAccumulator<F>(std::forward<F>(e));
    }

    template <typename T, typename F>
    auto makeChiSquareAccumulator(F&& e)
    {
      return lar::util::ChiSquareAccumulator<F, T>(std::forward<F>(e));
    }

    //--------------------------------------------------------------------------

  } // namespace util
} // namespace lar

#endif // LARDATA_UTILITIES_CHISQUAREACCUMULATOR_H