add numerical recipes library

lib/nr/cpp/recipes/rtflsp.cpp

@@ -0,0 +1,40 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::rtflsp(DP func(const DP), const DP x1, const DP x2, const DP xacc)
+{
+	const int MAXIT=30;
+	int j;
+	DP fl,fh,xl,xh,dx,del,f,rtf;
+
+	fl=func(x1);
+	fh=func(x2);
+	if (fl*fh > 0.0) nrerror("Root must be bracketed in rtflsp");
+	if (fl < 0.0) {
+		xl=x1;
+		xh=x2;
+	} else {
+		xl=x2;
+		xh=x1;
+		SWAP(fl,fh);
+	}
+	dx=xh-xl;
+	for (j=0;j<MAXIT;j++) {
+		rtf=xl+dx*fl/(fl-fh);
+		f=func(rtf);
+		if (f < 0.0) {
+			del=xl-rtf;
+			xl=rtf;
+			fl=f;
+		} else {
+			del=xh-rtf;
+			xh=rtf;
+			fh=f;
+		}
+		dx=xh-xl;
+		if (fabs(del) < xacc || f == 0.0) return rtf;
+	}
+	nrerror("Maximum number of iterations exceeded in rtflsp");
+	return 0.0;
+}