# spiral_trough.py   z=spiral(x,y) function included below
#            usage        from spiral_trough import spiral
import math
def spiral(x, y):  # spiral trough
  halfpi = 1.57079

  r = math.sqrt(x*x+y*y)
  if r < 0.001:
    theta = 0.0
  else:
    theta = math.atan2(y,x)
  tz = theta/halfpi
  if tz < 0.0:
    tz = tz + 4.0
  return 0.75*r + math.sin((r+tz)*halfpi)

  # end spiral