plan9port

fork of plan9port with libvec, libstr and libsdb
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cubrt.c (328B)

      1 #include <u.h>
      2 #include <libc.h>
      3 #include "map.h"
      4 
      5 double
      6 cubrt(double a)
      7 {
      8 	double x,y,x1;
      9 	if(a==0)
     10 		return(0.);
     11 	y = 1;
     12 	if(a<0) {
     13 		y = -y;
     14 		a = -a;
     15 	}
     16 	while(a<1) {
     17 		a *= 8;
     18 		y /= 2;
     19 	}
     20 	while(a>1) {
     21 		a /= 8;
     22 		y *= 2;
     23 	}
     24 	x = 1;
     25 	do {
     26 		x1 = x;
     27 		x = (2*x1+a/(x1*x1))/3;
     28 	} while(fabs(x-x1)>10.e-15);
     29 	return(x*y);
     30 }