add numerical recipes library

lib/nr/k_and_r/recipes/polint.c

@@ -0,0 +1,39 @@
+
+#include <math.h>
+#include "nrutil.h"
+
+void polint(xa,ya,n,x,y,dy)
+float *dy,*y,x,xa[],ya[];
+int n;
+{
+	int i,m,ns=1;
+	float den,dif,dift,ho,hp,w;
+	float *c,*d;
+
+	dif=fabs(x-xa[1]);
+	c=vector(1,n);
+	d=vector(1,n);
+	for (i=1;i<=n;i++) {
+		if ( (dift=fabs(x-xa[i])) < dif) {
+			ns=i;
+			dif=dift;
+		}
+		c[i]=ya[i];
+		d[i]=ya[i];
+	}
+	*y=ya[ns--];
+	for (m=1;m<n;m++) {
+		for (i=1;i<=n-m;i++) {
+			ho=xa[i]-x;
+			hp=xa[i+m]-x;
+			w=c[i+1]-d[i];
+			if ( (den=ho-hp) == 0.0) nrerror("Error in routine polint");
+			den=w/den;
+			d[i]=hp*den;
+			c[i]=ho*den;
+		}
+		*y += (*dy=(2*ns < (n-m) ? c[ns+1] : d[ns--]));
+	}
+	free_vector(d,1,n);
+	free_vector(c,1,n);
+}