spline.c

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#define NRANSI
#include "nrutil.h"

void spline(float x[], float y[], int n, float yp1, float ypn, float y2[])
{
	int i,k;
	float p,qn,sig,un,*u;

	u=vector(1,n-1);
	if (yp1 > 0.99e30)
		y2[1]=u[1]=0.0;
	else {
		y2[1] = -0.5;
		u[1]=(3.0/(x[2]-x[1]))*((y[2]-y[1])/(x[2]-x[1])-yp1);
	}
	for (i=2;i<=n-1;i++) {
		sig=(x[i]-x[i-1])/(x[i+1]-x[i-1]);
		p=sig*y2[i-1]+2.0;
		y2[i]=(sig-1.0)/p;
		u[i]=(y[i+1]-y[i])/(x[i+1]-x[i]) - (y[i]-y[i-1])/(x[i]-x[i-1]);
		u[i]=(6.0*u[i]/(x[i+1]-x[i-1])-sig*u[i-1])/p;
	}
	if (ypn > 0.99e30)
		qn=un=0.0;
	else {
		qn=0.5;
		un=(3.0/(x[n]-x[n-1]))*(ypn-(y[n]-y[n-1])/(x[n]-x[n-1]));
	}
	y2[n]=(un-qn*u[n-1])/(qn*y2[n-1]+1.0);
	for (k=n-1;k>=1;k--)
		y2[k]=y2[k]*y2[k+1]+u[k];
	free_vector(u,1,n-1);
}
#undef NRANSI