namespace GameCli;

/// <summary>
/// Pure physics stepper. All constants are public so training/tuning can inspect them.
/// Explicit Euler integration at fixed dt.
/// </summary>
public static class Physics
{
    public const float Dt = 0.02f;                 // 50 Hz
    public const float Gravity = 0.5f;             // world units / s^2, +Y (downward)
    public const float MainThrust = 1.2f;          // world units / s^2 along body-up
    public const float SideTorque = 4.0f;          // rad / s^2
    public const float SideLateralImpulse = 0.15f; // world units / s^2 along body-x
    public const float LinearDrag = 0.10f;         // per-second
    public const float AngularDrag = 0.50f;        // per-second

    public const int ActionNoop = 0;
    public const int ActionLeft = 1;
    public const int ActionMain = 2;
    public const int ActionRight = 3;

    /// <summary>
    /// Advance the ship state by one dt given a discrete action.
    /// Gravity is always applied. Thrusters add to acceleration/torque.
    /// </summary>
    public static ShipState Step(ShipState s, int action)
    {
        // Start with gravity.
        float ax = 0f;
        float ay = Gravity;
        float torque = 0f;

        // Body-up direction in world coords, given angle:
        //   angle=0    → (0, -1)    "up" on screen (Y is down)
        //   angle=+π/2 → (+1,  0)   right
        //   angle=+π   → (0, +1)    down
        float sinA = MathF.Sin(s.Angle);
        float cosA = MathF.Cos(s.Angle);
        float bodyUpX = sinA;
        float bodyUpY = -cosA;
        // Body-right direction (perpendicular, rotated +90°):
        float bodyRightX = cosA;
        float bodyRightY = sinA;

        switch (action)
        {
            case ActionMain:
                ax += bodyUpX * MainThrust;
                ay += bodyUpY * MainThrust;
                break;
            case ActionLeft:
                torque -= SideTorque;
                ax += bodyRightX * SideLateralImpulse;
                ay += bodyRightY * SideLateralImpulse;
                break;
            case ActionRight:
                torque += SideTorque;
                ax -= bodyRightX * SideLateralImpulse;
                ay -= bodyRightY * SideLateralImpulse;
                break;
            case ActionNoop:
            default:
                break;
        }

        // Integrate velocity, apply linear drag, integrate position.
        float vx = (s.VX + ax * Dt) * (1f - LinearDrag * Dt);
        float vy = (s.VY + ay * Dt) * (1f - LinearDrag * Dt);
        float x = s.X + vx * Dt;
        float y = s.Y + vy * Dt;

        // Angular: same pattern.
        float w = (s.AngularVelocity + torque * Dt) * (1f - AngularDrag * Dt);
        float angle = s.Angle + w * Dt;

        return new ShipState(x, y, vx, vy, angle, w);
    }
}