Example #1
0
  /**
   * Processes point set using Gift Wrapping algorithm and returns the points of the convex hull of
   * the input set.
   *
   * @param points set of points to run the algorithm on
   * @return convex hull points
   */
  @Override
  public LinkedList useAlgorithm(LinkedList points) {
    if (points.getLength() < 4) {
      return points;
    }

    // Find the point with minimum x coordinate: O(n).
    Point2D.Double minXPoint = findMinX(points);

    LinkedList hullPoints = new LinkedList();
    hullPoints.insert(minXPoint);

    Point2D.Double endPoint = minXPoint;
    Point2D.Double newEndPoint = null;

    // Do until hull closes, that is, for all hull points: O(h).
    // Inner loop checks every point of the set: O(n).
    // Run time is O(n*h).
    while (true) {
      // initial candidate for new end point
      LinkedListNode current = points.getHead();
      newEndPoint = current.getPoint();
      current = current.getNext();
      double totalArea = 0.0;
      while (current != null) {
        Point2D.Double temp = current.getPoint();
        // ignore if temp is the current end point
        if (!temp.equals(endPoint)) {
          double triangleArea = triangleArea(endPoint, newEndPoint, temp);
          // If temp is left of line from endpoint to candidate
          // (area positive), update candidate.
          totalArea += triangleArea;
          if (triangleArea >= 0) {
            newEndPoint = temp;
          }
          if (totalArea == 0.0 && current.getNext() == null) {
            // all collinear so return all
            return points;
          }
        }
        current = current.getNext();
      }
      // if the new end point candidate is the same as the starting point
      // hull is closed and we can exit the loop
      if (newEndPoint.equals(minXPoint)) {
        break;
      }
      // Add to hull points and update end point.
      hullPoints.insert(newEndPoint);
      endPoint = newEndPoint;
    }

    return hullPoints;
  }
Example #2
0
 // Finds the point with minimum x coordinate.
 private Point2D.Double findMinX(LinkedList points) {
   Point2D.Double minXPoint = null;
   LinkedListNode node = points.getHead();
   while (node != null) {
     Point2D.Double point = node.getPoint();
     if (minXPoint == null || point.getX() < minXPoint.getX()) {
       minXPoint = point;
     }
     node = node.getNext();
   }
   return minXPoint;
 }