/*
    Gstat, a program for geostatistical modelling, prediction and simulation
    Copyright 1992, 2003 (C) Edzer J. Pebesma

    Edzer J. Pebesma, e.pebesma@geo.uu.nl
    Department of physical geography, Utrecht University
    P.O. Box 80.115, 3508 TC Utrecht, The Netherlands

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version. As a special exception, linking 
    this program with the Qt library is permitted.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

    (read also the files COPYING and Copyright)
*/

/*
 * vario_fn.c: contains elementary variogram model functions
 */
#include <stdio.h> /* req'd by userio.h */
#include <math.h>

#include "defs.h" /* config.h may define USING_R */
#ifdef USING_R
/* #define MATHLIB_STANDALONE */
#include <Rmath.h>
#endif

#include "userio.h"
#include "vario_fn.h"
static double bessi1(double x);
static double bessk1(double x);

/*
 * Copyright (C) 1994, Edzer J. Pebesma
 * 
 * basic variogram functions
 */

#define MIN_BESS 1.0e-3
#ifndef PI
#   define PI 3.14159265359
#endif


double fn_nugget(double h, double *r) {
	return (h == 0.0 ? 0.0 : 1.0);
}

double fn_linear(double h, double *r) {
	if (h == 0)
		return 0.0;
	if (*r == 0)
		return h; /* 1lin() or 1 lin(0): slope 1 (and no range) */
	return (h >= *r ? 1.0 : h/(*r));
}

double da_fn_linear(double h, double *r) {
	if (*r == 0)
		return 0.0; /* 1lin() or 1 lin(0): slope 1 (and no range) */
	if (h > *r)
		return 0.0;
	return -h/((*r) * (*r));
}

double fn_circular(double h, double *r) {
	double hr;
	if (h == 0.0)
		return 0.0;
	if (h >= *r)
		return 1.0;
	hr = h/(*r);
	/*
	 * return 1.0 + (2.0/PI) * (hr * sqrt(1.0 - hr * hr) - acos(hr));
	 * probably equivalent to:
	 */
	return (2.0/PI) * (hr * sqrt(1.0 - hr * hr) + asin(hr));
}

double fn_spherical(double h, double *r) {
	double hr;
	if (h == 0)
		return 0.0;
	if (h >= *r)
		return 1.0;
	hr = h/(*r);
	return hr * (1.5 - 0.5 * hr * hr);
}

double da_fn_spherical(double h, double *r) {
	double hr2;
	if (h > *r)
		return 0.0;
	hr2 = h / ((*r) * (*r));
	return 1.5 * hr2 * (-1.0 + h * hr2);
}

double fn_bessel(double h, double *r) {
	double hr;
	hr = h/(*r);
	if (hr < MIN_BESS) 
		return 0.0;
	return 1.0 - hr * bessk1(hr);
}

double fn_gaussian(double h, double *r) {
	double hr;
	if (h == 0.0)
		return 0.0;
	hr = h/(*r);
	return 1.0 - exp(-(hr*hr)); 
}

double da_fn_gaussian(double h, double *r) {
	double hr;
	hr = h / (*r);
	return (-hr /(*r)) * exp(-(hr * hr));
}

double fn_exponential(double h, double *r) {
	if (h == 0.0)
		return 0.0;
	return 1.0 - exp(-h/(*r)); 
}

double fn_exclass(double h, double *r) {
	if (h == 0.0)
		return 0.0;
	return 1.0 - exp(-pow(h/r[0],r[1])); 
}

double da_fn_exponential(double h, double *r) {
	double hr;
	hr = -h/(*r);
	return (hr / (*r))  * exp(hr); 
}

double fn_pentaspherical(double h, double *r) {
	double hr, h2r2;
	if (h == 0.0)
		return 0.0;
	if (h >= *r)
		return 1.0;
	hr = h/(*r);
	h2r2 = hr * hr;
	return hr * ((15.0/8.0) + h2r2 * ((-5.0/4.0) + h2r2 * (3.0/8.0)));
}

double da_fn_pentaspherical(double h, double *r) {
	double hr2;
	if (h >= *r)
		return 0.0;
	hr2 = h / ((*r) * (*r));
	return hr2*((-15.0/8.0) + hr2*((15.0/4.0)*h - (15.0/8.0)*h*h*hr2));
}

double fn_periodic(double h, double *r) {
	if (h == 0.0)
		return 0.0;
	return 1.0 - cos(2.0 * PI * h/(*r));
}

double da_fn_periodic(double h, double *r) {
	return (2.0 * PI * h/((*r) * (*r))) * sin(2.0 * PI * h/(*r));
}

double fn_hole(double h, double *r) {
	if (h == 0.0)
		return 0.0;
	return 1.0 - sin(h/(*r))/(h/(*r));
}

double da_fn_hole(double h, double *r) {
	double hr, hr2;
	hr = h/(*r);
	hr2 = h/((*r)*(*r));
	return hr2 * sin(hr) + hr * hr2 * cos(hr);
}

double fn_logarithmic(double h, double *r) {
	if (h == 0.0)
		return 0.0;
	return log(h + *r); 
}

double da_fn_logarithmic(double h, double *r) {
	return 1/(*r);
}

double fn_power(double h, double *r) {
	if (h == 0.0)
		return 0.0;
	return pow(h, *r); 
}

double da_fn_power(double h, double *r) {
	return log(h) * pow(h, *r);
}

double fn_spline(double h, double *r) {
	if (h == 0.0)
		return 0.0;
	return h * h * log(h); 
}

double fn_intercept(double h, double *r) {
	return 1.0;
}

double da_is_zero(double h, double *r) {
	return 0.0;
}

#ifdef USING_R
double fn_matern(double h, double *p) {
	double hr, ans, phi, kappa;

	phi = p[0];
	kappa = p[1];
    if (h == 0.0)
		return(0.0);
	if (h > 600 * phi)
		return(1.0);
	hr = h/phi;
	ans = (pow(2.0, -(kappa - 1.0))/gammafn(kappa)) *
			pow(hr, kappa) * bessel_k(hr, kappa, 1.0);
	/* ans was for correlation; */
    return 1.0 - ans;
}
#endif

static double bessi1(double x)
/*
 * bessi1 from numerical recipes 
 */
{
	double ax,ans;
	double y;

	if ((ax=fabs(x)) < 3.75) {
		y=x/3.75;
		y*=y;
		ans=ax*(0.5+y*(0.87890594+y*(0.51498869+y*(0.15084934
			+y*(0.2658733e-1+y*(0.301532e-2+y*0.32411e-3))))));
	} else {
		y=3.75/ax;
		ans=0.2282967e-1+y*(-0.2895312e-1+y*(0.1787654e-1
			-y*0.420059e-2));
		ans=0.39894228+y*(-0.3988024e-1+y*(-0.362018e-2
			+y*(0.163801e-2+y*(-0.1031555e-1+y*ans))));
		ans *= (exp(ax)/sqrt(ax));
	}
	return (double) x < 0.0 ? -ans : ans;
}

static double bessk1(double x)
/*
 * bessk1 from numerical recipes 
 */
{
	double y,ans;

	if (x <= 2.0) {
		y=x*x/4.0;
		ans=(log(x/2.0)*bessi1(x))+(1.0/x)*(1.0+y*(0.15443144
			+y*(-0.67278579+y*(-0.18156897+y*(-0.1919402e-1
			+y*(-0.110404e-2+y*(-0.4686e-4)))))));
	} else {
		y=2.0/x;
		ans=(exp(-x)/sqrt(x))*(1.25331414+y*(0.23498619
			+y*(-0.3655620e-1+y*(0.1504268e-1+y*(-0.780353e-2
			+y*(0.325614e-2+y*(-0.68245e-3)))))));
	}
	return (double) ans;
}