// 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;
}
/**
* 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;
}