rc.c

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#include <math.h>
#define NRANSI
#include "nrutil.h"
#define ERRTOL 0.04
#define TINY 1.69e-38
#define SQRTNY 1.3e-19
#define BIG 3.e37
#define TNBG (TINY*BIG)
#define COMP1 (2.236/SQRTNY)
#define COMP2 (TNBG*TNBG/25.0)
#define THIRD (1.0/3.0)
#define C1 0.3
#define C2 (1.0/7.0)
#define C3 0.375
#define C4 (9.0/22.0)

float rc(float x, float y)
{
	float alamb,ave,s,w,xt,yt;
	if (x < 0.0 || y == 0.0 || (x+fabs(y)) < TINY || (x+fabs(y)) > BIG ||
		(y<-COMP1 && x > 0.0 && x < COMP2))
			nrerror("invalid arguments in rc");
	if (y > 0.0) {
		xt=x;
		yt=y;
		w=1.0;
	} else {
		xt=x-y;
		yt = -y;
		w=sqrt(x)/sqrt(xt);
	}
	do {
		alamb=2.0*sqrt(xt)*sqrt(yt)+yt;
		xt=0.25*(xt+alamb);
		yt=0.25*(yt+alamb);
		ave=THIRD*(xt+yt+yt);
		s=(yt-ave)/ave;
	} while (fabs(s) > ERRTOL);
	return w*(1.0+s*s*(C1+s*(C2+s*(C3+s*C4))))/sqrt(ave);
}
#undef ERRTOL
#undef TINY
#undef SQRTNY
#undef BIG
#undef TNBG
#undef COMP1
#undef COMP2
#undef THIRD
#undef C1
#undef C2
#undef C3
#undef C4
#undef NRANSI