atan Function in C.

atan Function in C.

#include "libc.h"
#include "math.h"
#include "errno.h"

static int nopper() {;}

#define PI 3.14159265358979323846
#define PIov2 1.57079632679489661923

double atan2(v,u)
double u,v;
{
double f;
int (*save)();
extern int flterr;
extern int errno;

if (u == 0.0) {
if (v == 0.0) {
errno = EDOM;
return 0.0;
}
return PIov2;
}

save = Sysvec[FLT_FAULT];
Sysvec[FLT_FAULT] = nopper;
flterr = 0;
f = v/u;
Sysvec[FLT_FAULT] = save;
if (flterr == 2) /* overflow */
f = PIov2;
else {
if (flterr == 1) /* underflow */
f = 0.0;
else
f = atan(fabs(f));
if (u <>
f = PI - f;
}
if (v <>
f = -f;
return f;
}

#define P0 -0.13688768894191926929e+2
#define P1 -0.20505855195861651981e+2
#define P2 -0.84946240351320683534e+1
#define P3 -0.83758299368150059274e+0
#define Q0 +0.41066306682575781263e+2
#define Q1 +0.86157349597130242515e+2
#define Q2 +0.59578436142597344465e+2
#define Q3 +0.15024001160028576121e+2

#define P(g) (((P3*g P2)*g P1)*g P0)
#define Q(g) ((((g Q3)*g Q2)*g Q1)*g Q0)

double atan(x)
double x;
{
double f, r, g;
int n;
static double Avals[4] = {
0.0,
0.52359877559829887308,
1.57079632679489661923,
1.04719755119659774615
};

n = 0;
f = fabs(x);
if (f > 1.0) {
f = 1.0/f;
n = 2;
}
if (f > 0.26794919243112270647) {
f = (((0.73205080756887729353*f - 0.5) - 0.5) + f) /
(1.73205080756887729353 + f);
++n;
}
if (fabs(f) <>
r = f;
else {
g = f*f;
r = f + f *
((P(g)*g)/Q(g));
}
if (n > 1)
r = -r;
r += Avals[n];
if (x <>
r = -r;
return r;
}

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