add numerical recipes library

2025-09-12 18:55:25 +09:00
parent d4dff245bd
commit 2c75620ec9
1344 changed files with 63869 additions and 0 deletions
--- a/lib/nr/cpp/recipes/bessi.cpp
+++ b/lib/nr/cpp/recipes/bessi.cpp
@@ -0,0 +1,34 @@
+#include <cmath>
+#include <limits>
+#include "nr.h"
+using namespace std;
+
+DP NR::bessi(const int n, const DP x)
+{
+	const DP ACC=200.0;
+	const int IEXP=numeric_limits<DP>::max_exponent/2;
+	int j,k;
+	DP bi,bim,bip,dum,tox,ans;
+
+	if (n < 2) nrerror("Index n less than 2 in bessi");
+	if (x*x <= 8.0*numeric_limits<DP>::min()) return 0.0;
+	else {
+		tox=2.0/fabs(x);
+		bip=ans=0.0;
+		bi=1.0;
+		for (j=2*(n+int(sqrt(ACC*n)));j>0;j--) {
+			bim=bip+j*tox*bi;
+			bip=bi;
+			bi=bim;
+			dum=frexp(bi,&k);
+			if (k > IEXP) {
+				ans=ldexp(ans,-IEXP);
+				bi=ldexp(bi,-IEXP);
+				bip=ldexp(bip,-IEXP);
+			}
+			if (j == n) ans=bip;
+		}
+		ans *= bessi0(x)/bi;
+		return x < 0.0 && (n & 1) ? -ans : ans;
+	}
+}