/*******************************************************************************
 * Hermite orbit interpolator based on fortran code of Werner Gunter           *
 * 13th International Workshop on Laser Ranging, 2002, Washington, DC          *
 *******************************************************************************/
/********************************************************************************
 * Creator:  David T. Sandwell and Rob Mellors * (San Diego State University,
 *Scripps Institution of Oceanography)  * Date   :  06/07/2007 *
 ********************************************************************************/
/********************************************************************************
 * Modification history: * Date: * 10/03/2007   -   converted from FORTRAN to C
 * *
 * *****************************************************************************/

#include "gmtsar.h"
#include "lib_functions.h"
#include <math.h>
#include <stdio.h>
#include <stdlib.h>

void hermite_c(double *x, double *y, double *z, int nmax, int nval, double xp, double *yp, int *ir) {
	/*

	  interpolation by a polynomial using nval out of nmax given data points

	  input:  x(i)  - arguments of given values (i=1,...,nmax)
	          y(i)  - functional values y=f(x)
	          z(i)  - derivatives       z=f'(x)
	          nmax  - number of given points in list
	          nval  - number of points to use for interpolation
	          xp    - interpolation argument

	  output: yp    - interpolated value at xp
	          ir    - return code
	                  0 = ok
	                  1 = interpolation not in center interval
	                  2 = argument out of range

	***** calls no other routines
	*/
	int n, i, j, i0;
	double sj, hj, f0, f1;

	/*  check to see if interpolation point is inside data range */

	*yp = 0.0;
	n = nval - 1;
	*ir = 0;

	/* reduced index by 1 */
	if (xp < x[0] || xp > x[nmax - 1]) {
		fprintf(stderr, "interpolation point outside of data constraints, %f %f %f\n", xp, x[0], x[nmax - 1]);
		*ir = 2;
		exit(1);
	}

	/*  look for given value immediately preceeding interpolation argument */

	for (i = 0; i < nmax; i++) {
		if (x[i] >= xp)
			break;
	}
	/*  check to see if interpolation point is centered in  data range */
	i0 = i - (n + 1) / 2;

	if (i0 <= 0) {
		// fprintf(stderr,"hermite: interpolation not in center interval\n");
		i0 = 0;
		*ir = 0;
	}

	/* reduced index by 1 */
	if (i0 + n > nmax) {
		fprintf(stderr, "hermite: interpolation not in center interval\n");
		i0 = nmax - n - 1;
		*ir = 0;
	}

	/*  do Hermite interpolation */
	for (i = 0; i <= n; i++) {
		sj = 0.0;
		hj = 1.0;
		for (j = 0; j <= n; j++) {
			if (j != i) {
				hj = hj * (xp - x[j + i0]) / (x[i + i0] - x[j + i0]);
				sj = sj + 1.0 / (x[i + i0] - x[j + i0]);
			}
		}

		f0 = 1.0 - 2.0 * (xp - x[i + i0]) * sj;
		f1 = xp - x[i + i0];

		*yp = *yp + (y[i + i0] * f0 + z[i + i0] * f1) * hj * hj;
		if (isnan(*yp) != 0) {
			fprintf(stderr, "nan!\n");
			exit(1);
		}
	}

	/*	done 	*/
}
