add numerical recipes library

lib/nr/k_and_r/recipes/ratint.c

@@ -0,0 +1,47 @@
+
+#include <math.h>
+#include "nrutil.h"
+#define TINY 1.0e-25
+#define FREERETURN {free_vector(d,1,n);free_vector(c,1,n);return;}
+
+void ratint(xa,ya,n,x,y,dy)
+float *dy,*y,x,xa[],ya[];
+int n;
+{
+	int m,i,ns=1;
+	float w,t,hh,h,dd,*c,*d;
+
+	c=vector(1,n);
+	d=vector(1,n);
+	hh=fabs(x-xa[1]);
+	for (i=1;i<=n;i++) {
+		h=fabs(x-xa[i]);
+		if (h == 0.0) {
+			*y=ya[i];
+			*dy=0.0;
+			FREERETURN
+		} else if (h < hh) {
+			ns=i;
+			hh=h;
+		}
+		c[i]=ya[i];
+		d[i]=ya[i]+TINY;
+	}
+	*y=ya[ns--];
+	for (m=1;m<n;m++) {
+		for (i=1;i<=n-m;i++) {
+			w=c[i+1]-d[i];
+			h=xa[i+m]-x;
+			t=(xa[i]-x)*d[i]/h;
+			dd=t-c[i+1];
+			if (dd == 0.0) nrerror("Error in routine ratint");
+			dd=w/dd;
+			d[i]=c[i+1]*dd;
+			c[i]=t*dd;
+		}
+		*y += (*dy=(2*ns < (n-m) ? c[ns+1] : d[ns--]));
+	}
+	FREERETURN
+}
+#undef TINY
+#undef FREERETURN