// cycleLength e.g. 360
static double cyclicDistance(double a, double b, double cycleLength) {
  a = mod(a, cycleLength); // a is now between 0 and 360
  b = mod(b, cycleLength); // b is now between 0 and 360
  
  // if e.g. a is 40 and b is 50, then distance is 10
  // if e.g. a is 10 and b is 350, then distance is 20
  // if e.g. b is 10 and a is 350, then distance is 20
  double diff = absDiff(a, b);
  if (a < b)
    diff = min(diff, a+cycleLength-b);
  else
    diff = min(diff, b+cycleLength-a);
  ret diff;
}