#include <cmath>
#include "nr.h"
using namespace std;

DP NR::bessj0(const DP x)
{
	DP ax,z,xx,y,ans,ans1,ans2;

	if ((ax=fabs(x)) < 8.0) {
		y=x*x;
		ans1=57568490574.0+y*(-13362590354.0+y*(651619640.7
			+y*(-11214424.18+y*(77392.33017+y*(-184.9052456)))));
		ans2=57568490411.0+y*(1029532985.0+y*(9494680.718
			+y*(59272.64853+y*(267.8532712+y*1.0))));
		ans=ans1/ans2;
	} else {
		z=8.0/ax;
		y=z*z;
		xx=ax-0.785398164;
		ans1=1.0+y*(-0.1098628627e-2+y*(0.2734510407e-4
			+y*(-0.2073370639e-5+y*0.2093887211e-6)));
		ans2 = -0.1562499995e-1+y*(0.1430488765e-3
			+y*(-0.6911147651e-5+y*(0.7621095161e-6
			-y*0.934945152e-7)));
		ans=sqrt(0.636619772/ax)*(cos(xx)*ans1-z*sin(xx)*ans2);
	}
	return ans;
}