superbee_limiter.hh

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#pragma once
 
#include <DiFfRG/discretization/FV/limiter/abstract_limiter.hh>
 
#include <cmath>
 
namespace DiFfRG
{
  namespace def
  {
    class SuperbeeLimiter
    {
    public:
      template <typename NumberType> static NumberType slope_limit(const NumberType &du_1, const NumberType &du_2)
      {
        using std::abs;
        using std::max;
        using std::min;
 
        // Branch-free same-sign indicator: 1 when du_1 and du_2 share strict sign, else 0.
        // Identity: |sgn(a)+sgn(b)| / 2.
        const int s1 = limiter_utils::sgn(du_1);
        const int s2 = limiter_utils::sgn(du_2);
        if (s1 == 0 || s2 == 0 || s1 != s2) return NumberType(0);
 
        // Both slopes strictly positive or strictly negative. Work in the
        // common-sign convention then re-apply the sign at the end.
        const auto a = abs(du_1);
        const auto b = abs(du_2);
 
        // minmod(2a, b) and minmod(a, 2b) in this regime collapse to
        // min(2a, b) and min(a, 2b); maxmod is then max() of those.
        const auto m1 = min(NumberType(2) * a, b);
        const auto m2 = min(a, NumberType(2) * b);
        const auto limited = max(m1, m2);
 
        return NumberType(s1) * limited;
      }
    };
    static_assert(HasSlopeLimiter<SuperbeeLimiter>);
 
  } // namespace def
} // namespace DiFfRG