using System;
using FluidSim.Components;

namespace FluidSim.Core
{
    public static class NozzleFlow
    {
        public static void Compute(Volume0D vol, double area, double downstreamPressure,
                                   out double massFlow, out double rhoFace, out double uFace, out double pFace,
                                   double gamma = 1.4)
        {
            massFlow = 0.0;
            rhoFace = 0.0;
            uFace = 0.0;
            pFace = 0.0;

            if (vol == null || vol.Mass <= 0 || vol.Volume <= 0)
                return;

            double p0 = vol.Pressure;
            double T0 = vol.Temperature;
            double R = vol.GasConstant;
            double rho0 = vol.Density;

            if (double.IsNaN(p0) || double.IsNaN(T0) || double.IsNaN(rho0) ||
                p0 <= 0 || T0 <= 0 || rho0 <= 0)
                return;

            double pr = downstreamPressure / p0;
            double choked = Math.Pow(2.0 / (gamma + 1.0), gamma / (gamma - 1.0));

            // If pr > 1, flow is INTO the cylinder (reverse), so we swap the roles.
            bool reverse = (pr > 1.0);
            if (reverse)
            {
                // Treat the cylinder as the downstream, the pipe as the upstream.
                double p_up = downstreamPressure;
                double T_up = 300.0;        // pipe temperature (ambient)
                double rho_up = downstreamPressure / (R * T_up);

                double pr_rev = p0 / p_up;  // now cylinder / pipe
                if (pr_rev < choked) pr_rev = choked;

                double M = Math.Sqrt((2.0 / (gamma - 1.0)) * (Math.Pow(pr_rev, -(gamma - 1.0) / gamma) - 1.0));
                if (double.IsNaN(M)) return;

                // Flow from pipe INTO cylinder (positive mass flow into volume)
                uFace = M * Math.Sqrt(gamma * R * T_up);
                rhoFace = rho_up * Math.Pow(pr_rev, 1.0 / gamma);
                pFace = p_up * pr_rev;
                massFlow = rhoFace * uFace * area;
                // massFlow is positive = into cylinder
            }
            else
            {
                // Normal flow out of cylinder
                if (pr < choked) pr = choked;

                double M = Math.Sqrt((2.0 / (gamma - 1.0)) * (Math.Pow(pr, -(gamma - 1.0) / gamma) - 1.0));
                if (double.IsNaN(M)) return;

                uFace = M * Math.Sqrt(gamma * R * T0);
                rhoFace = rho0 * Math.Pow(pr, 1.0 / gamma);
                pFace = p0 * pr;
                massFlow = -rhoFace * uFace * area;   // negative = out of cylinder
            }

            if (double.IsNaN(massFlow) || double.IsInfinity(massFlow))
                massFlow = 0.0;
        }
    }
}