package dev.lumareth.improvedAerodynamics.flight;

/**
 * Rise of zero-lift drag around the speed of sound: shock waves build up from Mach 0.8, drag peaks at Mach 1.05 and
 * settles to a lower, still high, level once the flow is fully supersonic from Mach 1.3.
 */
final class TransonicDrag {
    private static final double DRAG_RISE_START = 0.80;
    private static final double PEAK = 1.05;
    private static final double SUPERSONIC_START = 1.30;

    private TransonicDrag() {
    }

    /**
     * @param mach           airspeed divided by the speed of sound
     * @param peakDrag       multiplier at the peak
     * @param supersonicDrag multiplier once fully supersonic
     * @return factor for parasitic drag, 1 in subsonic flight
     */
    static double multiplier(double mach, double peakDrag, double supersonicDrag) {
        if (mach <= DRAG_RISE_START) {
            return 1.0;
        }
        if (mach < PEAK) {
            return 1.0 + (peakDrag - 1.0) * ImprovedAero.smoothstep(DRAG_RISE_START, PEAK, mach);
        }
        if (mach < SUPERSONIC_START) {
            return peakDrag + (supersonicDrag - peakDrag) * ImprovedAero.smoothstep(PEAK, SUPERSONIC_START, mach);
        }
        return supersonicDrag;
    }
}