#if !defined(TWODIMS) && !defined(ONEDIM)
#define  NUMDIMS 3		/*!< For 3D-normalized kernel */
#define  NORM_COEFF      4.188790204786	/*!< Coefficient for kernel normalization. Note:  4.0/3 * PI = 4.188790204786 */
#else
#ifndef  ONEDIM
#define  NUMDIMS 2		/*!< For 2D-normalized kernel */
#define  NORM_COEFF      M_PI	/*!< Coefficient for kernel normalization. */
#else
#define  NUMDIMS 1             /*!< For 1D-normalized kernel */
#define  NORM_COEFF      2.0
#endif
#endif

typedef struct {
    double H;
    double HH;
    double Hinv; /* convert from r to u*/
    int type;
    double support;
    char * name;
    /* private: */
    double Wknorm;
    double dWknorm;
} DensityKernel;

double
density_kernel_desnumngb(DensityKernel * kernel, double eta);
void
density_kernel_init(DensityKernel * kernel, double H);
void
density_kernel_init_with_type(DensityKernel * kernel, int type, double H);
double
density_kernel_wk(DensityKernel * kernel, double u);
double
density_kernel_dwk(DensityKernel * kernel, double u);
double
density_kernel_volume(DensityKernel * kernel);

static inline double
density_kernel_dW(DensityKernel * kernel, double u, double wk, double dwk)
{
    return - (NUMDIMS * kernel->Hinv * wk + u * dwk);
}