C++ implementation of the Mandelbrot Set function

mandel_func.cpp

static int Mandelbrot(double cr, double ci, int limit)
{
    int count = 0;
    double zr = 0.0;
    double zi = 0.0;
    double zr2 = 0.0;
    double zi2 = 0.0;
    while ((count < limit) && (zr2 + zi2 < 4.001))
    {
        double tzi = 2.0*zr*zi + ci;
        zr = zr2 - zi2 + cr;
        zi = tzi;
        zr2 = zr*zr;
        zi2 = zi*zi;
        ++count;
    }
    return count;
}