SLerp

public const float PiOverTwo = (float)Math.PI * 2f;

public static float SLerp(float x, float y, float amount)
{
    float diff = (float)Math.Abs(y - x);
    if (diff > Math.PI)
    {
        if (y > x)
            x += PiOverTwo;
        else
            y += PiOverTwo;
    }

    float value = (x + ((y - x)*amount));

    if (value >= 0f && value <= PiOverTwo)
        return value;

    return (value % PiOverTwo);
}