#pragma once
#include <cmath>

namespace fluidsim::port {
// Causal unflanged circular-pipe reflection fit, Silva et al. (2009), Eq. 16.
// R(s)=-(1+n*a/c*s)/(1+d*a/c*s+e*(a/c*s)^2).
// Trapezoidal state integration supports the solver's variable CFL timestep.
class OpenEndRadiation {
public:
    double process(double incident, double scaled_dt) {
        if (!initialized_) { position_ = incident; previous_ = incident; initialized_ = true; }
        constexpr double n = .167, d = n + 2*.6133;
        constexpr double e = (d*d-n*n-1)/2;
        const double h = scaled_dt*.5;
        const double rhs = velocity_ + h/e*(previous_ + incident - position_ - d*velocity_);
        const double next_velocity = (rhs - h/e*(position_ + h*velocity_)) /
                                     (1 + h*d/e + h*h/e);
        position_ += h*(velocity_ + next_velocity);
        velocity_ = next_velocity;
        previous_ = incident;
        return -(position_ + n*velocity_);
    }
private:
    double position_{}, velocity_{}, previous_{};
    bool initialized_{};
};
} // namespace fluidsim::port