// This object locates a floating-point key in a sorted map.
// Then it calls (probabilistically schedules) an action function
// on keys from the map, starting with the key closest to the
// requested one, then moving further to the left AND to the right
// until (at possibly very low probability) even the keys farthest
// away have been called.

// Probability 1 is used only iff there is an exact match.
// Probabilities fall with increasing distance between
// searched and found key.

srecord noeq ProbabilisticDistanceBasedLookup(
  IProbabilisticScheduler scheduler,
  NavigableMap<Double, ?> map,
  IVF1<Double> action,
  double key) {
  
  swappable double distanceToProbability(double distance) {
    ret genericDistanceToProbability(distance);
  }
  
  run  {
    Double closest = closestDoubleKey(map, key);
    if (closest == null) ret;
    
    double p = distanceToProbability(abs(closest-key));
    scheduler.at(p, () -> action.get(closest));
    scheduler.at(p, () -> walkLeft(closest));
    scheduler.at(p, () -> walkRight(closest));
  }

  void walkLeft(double x) {
    Double y = map.lowerKey(x);
    if (y == null) ret;
    double p = distanceToProbability(abs(y-key));
    scheduler.at(p, () -> action.get(y));
    scheduler.at(p, () -> walkLeft(y));
  }

  void walkRight(double x) {
    Double y = map.higherKey(x);
    if (y == null) ret;
    double p = distanceToProbability(abs(y-key));
    scheduler.at(p, () -> action.get(y));
    scheduler.at(p, () -> walkRight(y));
  }
}