add numerical recipes library

lib/nr/cpp/recipes/spread.cpp

@@ -0,0 +1,25 @@
+#include "nr.h"
+
+void NR::spread(const DP y, Vec_IO_DP &yy, const DP x, const int m)
+{
+	static int nfac[11]={0,1,1,2,6,24,120,720,5040,40320,362880};
+	int ihi,ilo,ix,j,nden;
+	DP fac;
+
+	int n=yy.size();
+	if (m > 10) nrerror("factorial table too small in spread");
+	ix=int(x);
+	if (x == DP(ix)) yy[ix-1] += y;
+	else {
+		ilo=MIN(MAX(int(x-0.5*m),0),int(n-m));
+		ihi=ilo+m;
+		nden=nfac[m];
+		fac=x-ilo-1;
+		for (j=ilo+1;j<ihi;j++) fac *= (x-j-1);
+		yy[ihi-1] += y*fac/(nden*(x-ihi));
+		for (j=ihi-1;j>ilo;j--) {
+			nden=(nden/(j-ilo))*(j-ihi);
+			yy[j-1] += y*fac/(nden*(x-j));
+		}
+	}
+}