tcubrt.c - plan9port - [fork] Plan 9 from user space
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       ---
       tcubrt.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 }