/*
This file implements the Central upwind flux

Copyright (C) 2016, 2017, 2018 SINTEF ICT

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/



/**
  * Central upwind flux function
  */
__device__ float3 CentralUpwindFlux(const float3 Qm, float3 Qp, const float g) {
    const float3 Fp = F_func(Qp, g);
    const float up = Qp.y / Qp.x;   // hu / h
    const float cp = sqrt(g*Qp.x); // sqrt(g*h)

    const float3 Fm = F_func(Qm, g);
    const float um = Qm.y / Qm.x;   // hu / h
    const float cm = sqrt(g*Qm.x); // sqrt(g*h)
    
    const float am = min(min(um-cm, up-cp), 0.0f); // largest negative wave speed
    const float ap = max(max(um+cm, up+cp), 0.0f); // largest positive wave speed
    
    return ((ap*Fm - am*Fp) + ap*am*(Qp-Qm))/(ap-am);
}