44 lines
866 B
C++
44 lines
866 B
C++
#include <cmath>
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#include <limits>
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#include "nr.h"
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using namespace std;
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DP NR::betacf(const DP a, const DP b, const DP x)
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{
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const int MAXIT=100;
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const DP EPS=numeric_limits<DP>::epsilon();
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const DP FPMIN=numeric_limits<DP>::min()/EPS;
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int m,m2;
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DP aa,c,d,del,h,qab,qam,qap;
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qab=a+b;
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qap=a+1.0;
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qam=a-1.0;
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c=1.0;
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d=1.0-qab*x/qap;
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if (fabs(d) < FPMIN) d=FPMIN;
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d=1.0/d;
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h=d;
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for (m=1;m<=MAXIT;m++) {
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m2=2*m;
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aa=m*(b-m)*x/((qam+m2)*(a+m2));
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d=1.0+aa*d;
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if (fabs(d) < FPMIN) d=FPMIN;
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c=1.0+aa/c;
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if (fabs(c) < FPMIN) c=FPMIN;
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d=1.0/d;
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h *= d*c;
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aa = -(a+m)*(qab+m)*x/((a+m2)*(qap+m2));
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d=1.0+aa*d;
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if (fabs(d) < FPMIN) d=FPMIN;
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c=1.0+aa/c;
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if (fabs(c) < FPMIN) c=FPMIN;
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d=1.0/d;
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del=d*c;
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h *= del;
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if (fabs(del-1.0) <= EPS) break;
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}
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if (m > MAXIT) nrerror("a or b too big, or MAXIT too small in betacf");
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return h;
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}
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