rd.cpp

C++数值算法原书代码

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

DP NR::rd(const DP x, const DP y, const DP z)
{
	const DP ERRTOL=0.0015, TINY=1.0e-25, BIG=4.5e21;
	const DP C1=3.0/14.0, C2=1.0/6.0, C3=9.0/22.0;
	const DP C4=3.0/26.0, C5=0.25*C3, C6=1.5*C4;
	DP alamb,ave,delx,dely,delz,ea,eb,ec,ed,ee,fac,sqrtx,sqrty,
		sqrtz,sum,xt,yt,zt;

	if (MIN(x,y) < 0.0 || MIN(x+y,z) < TINY || MAX(MAX(x,y),z) > BIG)
		nrerror("invalid arguments in rd");
	xt=x;
	yt=y;
	zt=z;
	sum=0.0;
	fac=1.0;
	do {
		sqrtx=sqrt(xt);
		sqrty=sqrt(yt);
		sqrtz=sqrt(zt);
		alamb=sqrtx*(sqrty+sqrtz)+sqrty*sqrtz;
		sum += fac/(sqrtz*(zt+alamb));
		fac=0.25*fac;
		xt=0.25*(xt+alamb);
		yt=0.25*(yt+alamb);
		zt=0.25*(zt+alamb);
		ave=0.2*(xt+yt+3.0*zt);
		delx=(ave-xt)/ave;
		dely=(ave-yt)/ave;
		delz=(ave-zt)/ave;
	} while (MAX(MAX(fabs(delx),fabs(dely)),fabs(delz)) > ERRTOL);
	ea=delx*dely;
	eb=delz*delz;
	ec=ea-eb;
	ed=ea-6.0*eb;
	ee=ed+ec+ec;
	return 3.0*sum+fac*(1.0+ed*(-C1+C5*ed-C6*delz*ee)
		+delz*(C2*ee+delz*(-C3*ec+delz*C4*ea)))/(ave*sqrt(ave));
}