Sfx_UpwindScheme1st.hpp

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#pragma once
 
// C++ includes
#include <cassert>
#include <vector>
#include <ranges>  // C++20 features
// STL include
#include <numeric> // adjacent_difference
// SfxBase19 include
#include "include/Sfx_UniversalConstants.h"
 
namespace Sfx 
{
    template<std::ranges::input_range Rng> // Range of values 
  requires std::same_as<Rng,std::vector<double>> // only support std::vector for now
    auto UpwindDerivatr1st( Rng aRng) //bias forward stencil (left to right propagation)
    {
    static_assert(Sfx::EMCNEILNbSections::value==std::ranges::size(aRng));
 
    using value_type = std::ranges::range_value_t<Rng>;
    using diff_t = std::ranges::range_difference_t<Rng>;
 
    if constexpr (std::ranges::range<decltype(aRng)>)
    {
      Rng w_deriValues;
      w_rngOfValues.reserve(std::ranges::distance(aRng));
      
      // compute derivative at first order (bias upwind forward stencil)
      std::adjacent_difference(std::ranges::begin(aRng), std::ranges::end(aRng), 
        std::back_inserter(w_deriValues));
     
      // adjacent_difference leave the first element as is, it shall not be considered 
      // in the computation since it doesn't correspond to any gradient (container value)
      return std::move(w_deriValues) | std::views::drop(1);
    }
    else
    {
      Rng{}; // is that make sense???
    }
#if 0
        //diff_t w_Length = std::distance( std::begin(aRng), std::end(aRng));
    //const auto w_Length = std::distance(std::begin(aRng), std::end(aRng));
    const auto w_Length = std::ranges::distance(aRng);
 
    // create a vector of w_length initialized to zero, is it?
    std::vector<value_type> w_Cont;
    w_Cont.reserve(w_Length);
        
    // NOTE:
    // adjacent_difference leave the first element as is, it shall not be considered 
    // in the computation since it doesn't correspond to any gradient (container value)
    // In our case, i do think we use it exclusively for flux derivative F = F_i - F_i-1    
    // cell face ... to be completed
    // compute the gradient (U_i - U_i-1) at first order
        std::adjacent_difference( std::ranges::begin(aRng), std::ranges::end(aRng), std::back_inserter(w_Cont));
        assert( w_Cont.size() == w_Length); // really need it?
 
    //      w_Cont.pop_front();  // remove first element, not part of the result
//      assert( w_Cont.size()==w_Length-1);
 
        // return value optimization (remove first element, not part of the result)
        return std::vector<value_type>( std::next(w_Cont.cbegin()), w_Cont.cend()); 
#endif // 0
    }
} // End of namespace