/* spiral.c  spiral trough */
#include "spiral.h"
#include <math.h>

double spiral(double x, double y)
{
  double halfpi = 1.57079;
  double r, theta, tz;

  r = sqrt(x*x+y*y);
  if( r < 0.001) theta = 0.0;
  else          theta = atan2(y,x);
  tz = theta/halfpi;
  if(tz < 0.0) tz = tz + 4.0;
  return 0.75*r + sin((r+tz)*halfpi);
} /* end of spiral */