Subversion Repositories planix.SVN

#include <math.h>
#include <errno.h>
/*
        C program for floating point error function

        erf(x) returns the error function of its argument
        erfc(x) returns 1 - erf(x)

        erf(x) is defined by
        ${2 over sqrt(pi)} int from 0 to x e sup {-t sup 2} dt$

        the entry for erfc is provided because of the
        extreme loss of relative accuracy if erf(x) is
        called for large x and the result subtracted
        from 1. (e.g. for x= 10, 12 places are lost).

        There are no error returns.

        Calls exp.

        Coefficients for large x are #5667 from Hart & Cheney (18.72D).
*/

#define M 7
#define N 9
static double torp = 1.1283791670955125738961589031;
static double p1[] = {
        0.804373630960840172832162e5,
        0.740407142710151470082064e4,
        0.301782788536507577809226e4,
        0.380140318123903008244444e2,
        0.143383842191748205576712e2,
        -.288805137207594084924010e0,
        0.007547728033418631287834e0,
};
static double q1[]  = {
        0.804373630960840172826266e5,
        0.342165257924628539769006e5,
        0.637960017324428279487120e4,
        0.658070155459240506326937e3,
        0.380190713951939403753468e2,
        0.100000000000000000000000e1,
        0.0,
};
static double p2[]  = {
        0.18263348842295112592168999e4,
        0.28980293292167655611275846e4,
        0.2320439590251635247384768711e4,
        0.1143262070703886173606073338e4,
        0.3685196154710010637133875746e3,
        0.7708161730368428609781633646e2,
        0.9675807882987265400604202961e1,
        0.5641877825507397413087057563e0,
        0.0,
};
static double q2[]  = {
        0.18263348842295112595576438e4,
        0.495882756472114071495438422e4,
        0.60895424232724435504633068e4,
        0.4429612803883682726711528526e4,
        0.2094384367789539593790281779e4,
        0.6617361207107653469211984771e3,
        0.1371255960500622202878443578e3,
        0.1714980943627607849376131193e2,
        1.0,
};

double erfc(double);

double
erf(double arg)
{
        int sign;
        double argsq;
        double d, n;
        int i;

        errno = 0;
        sign = 1;
        if(arg < 0) {
                arg = -arg;
                sign = -1;
        }
        if(arg < 0.5) {
                argsq = arg*arg;
                for(n=0,d=0,i=M-1; i>=0; i--) {
                        n = n*argsq + p1[i];
                        d = d*argsq + q1[i];
                }
                return sign*torp*arg*n/d;
        }
        if(arg >= 10)
                return sign;
        return sign*(1 - erfc(arg));
}

double
erfc(double arg)
{
        double n, d;
        int i;

        errno = 0;
        if(arg < 0)
                return 2 - erfc(-arg);
        if(arg < 0.5)
                return 1 - erf(arg);
        if(arg >= 10)
                return 0;

        for(n=0,d=0,i=N-1; i>=0; i--) {
                n = n*arg + p2[i];
                d = d*arg + q2[i];
        }
        return exp(-arg*arg)*n/d;
}