ref: bda28c7ea313ca00bb21854c1b83c3383a7ed47c
dir: /sys/src/cmd/map/libmap/ccubrt.c/
#include <u.h>
#include <libc.h>
#include "map.h"
void
ccubrt(double zr, double zi, double *wr, double *wi)
{
double r, theta;
theta = atan2(zi,zr);
r = cubrt(hypot(zr,zi));
*wr = r*cos(theta/3);
*wi = r*sin(theta/3);
}