35 lines
690 B
C++
35 lines
690 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::bessi(const int n, const DP x)
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{
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const DP ACC=200.0;
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const int IEXP=numeric_limits<DP>::max_exponent/2;
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int j,k;
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DP bi,bim,bip,dum,tox,ans;
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if (n < 2) nrerror("Index n less than 2 in bessi");
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if (x*x <= 8.0*numeric_limits<DP>::min()) return 0.0;
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else {
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tox=2.0/fabs(x);
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bip=ans=0.0;
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bi=1.0;
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for (j=2*(n+int(sqrt(ACC*n)));j>0;j--) {
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bim=bip+j*tox*bi;
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bip=bi;
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bi=bim;
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dum=frexp(bi,&k);
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if (k > IEXP) {
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ans=ldexp(ans,-IEXP);
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bi=ldexp(bi,-IEXP);
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bip=ldexp(bip,-IEXP);
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}
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if (j == n) ans=bip;
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}
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ans *= bessi0(x)/bi;
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return x < 0.0 && (n & 1) ? -ans : ans;
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}
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}
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