add numerical recipes library

lib/nr/ansi/recipes/rtbis.c

@@ -0,0 +1,23 @@
+
+#include <math.h>
+#define JMAX 40
+
+float rtbis(float (*func)(float), float x1, float x2, float xacc)
+{
+	void nrerror(char error_text[]);
+	int j;
+	float dx,f,fmid,xmid,rtb;
+
+	f=(*func)(x1);
+	fmid=(*func)(x2);
+	if (f*fmid >= 0.0) nrerror("Root must be bracketed for bisection in rtbis");
+	rtb = f < 0.0 ? (dx=x2-x1,x1) : (dx=x1-x2,x2);
+	for (j=1;j<=JMAX;j++) {
+		fmid=(*func)(xmid=rtb+(dx *= 0.5));
+		if (fmid <= 0.0) rtb=xmid;
+		if (fabs(dx) < xacc || fmid == 0.0) return rtb;
+	}
+	nrerror("Too many bisections in rtbis");
+	return 0.0;
+}
+#undef JMAX