Addition of the Skyline Algorithm

Implementation of the skyline algorithm
TheAlgorithms · yanglbme · Jan 15, 2019 · Jan 9, 2019 · Jan 9, 2019 · Jan 15, 2019
commit 96ad920ce5975e17ac4c644819dc3e204b8e772b
@@ -0,0 +1,190 @@
+package skylinealgorithm;
+
+import java.util.ArrayList;
+import java.util.Comparator;
+
+/**
+ *
+ * @author dimgrichr
+ * 
+ * Space complexity: O(n)
+ * Time complexity: O(nlogn), because it is a divide and conquer algorithm
+ */
+public class SkylineAlgorithm {
+    private ArrayList<Point> points;
+
+
+    /**
+     * Main constructor of the application.
+     * ArrayList points gets created, which represents the sum of all edges.
+     */
+    public SkylineAlgorithm(){
+        points = new ArrayList<>();
+    }
+
+
+    /**
+     * @return points, the ArrayList that includes all points.
+     */
+    public ArrayList<Point> getPoints(){
+        return points;
+    }
+
+
+    /**
+     * The main divide and conquer, and also recursive algorithm.
+     * It gets an ArrayList full of points as an argument.
+     * If the size of that ArrayList is 1 or 2,
+     * the ArrayList is returned as it is, or with one less point
+     * (if the initial size is 2 and one of it's points, is dominated by the other one).
+     * On the other hand, if the ArrayList's size is bigger than 2,
+     * the function is called again, twice, 
+     * with arguments the corresponding half of the initial ArrayList each time.
+     * Once the flashback has ended, the function produceFinalSkyLine gets called,
+     * in order to produce the final skyline, and return it.
+     * @param list, the initial list of points
+     * @return leftSkyLine, the combination of first half's and second half's skyline
+     * @see Point
+     * @see produceFinalSkyLine
+     */
+    public ArrayList<Point> produceSubSkyLines(ArrayList<Point> list){
+
+        //part where function exits flashback
+        int size = list.size();
+        if(size == 1){
+            return list;
+        }
+        else if(size==2){
+            if(list.get(0).dominates(list.get(1))){
+                list.remove(1);
+            }
+            else{
+                if(list.get(1).dominates(list.get(0))){
+                    list.remove(0);
+                }
+            }
+            return list;
+        }
+
+        //recursive part of the function
+        ArrayList<Point> leftHalf=new ArrayList<>();
+        ArrayList<Point> rightHalf=new ArrayList<>();
+        for (int i=0;i<list.size();i++){
+            if(i<list.size()/2){
+                leftHalf.add(list.get(i));
+            }
+            else{
+                rightHalf.add(list.get(i));
+            }
+        }
+        ArrayList<Point> leftSubSkyLine=new ArrayList<>();
+        ArrayList<Point> rightSubSkyLine=new ArrayList<>();
+        leftSubSkyLine=produceSubSkyLines(leftHalf);
+        rightSubSkyLine=produceSubSkyLines(rightHalf);
+
+        //skyline is produced
+        return produceFinalSkyLine(leftSubSkyLine,rightSubSkyLine);
+    }
+
+
+    /**
+     * The first half's skyline gets cleared
+     * from some points that are not part of the final skyline
+     * (Points with same x-value and different y=values. The point with the smallest y-value is kept).
+     * Then, the minimum y-value of the points of first half's skyline is found.
+     * That helps us to clear the second half's skyline, because, the points
+     * of second half's skyline that have greater y-value of the minimum y-value that we found before,
+     * are dominated, so they are not part of the final skyline.
+     * Finally, the "cleaned" first half's and second half's skylines, are combined,
+     * producing the final skyline, which is returned.
+     * @param left the skyline of the left part of points
+     * @param right the skyline of the right part of points
+     * @return left the final skyline
+     */
+    public ArrayList<Point> produceFinalSkyLine(ArrayList<Point> left, ArrayList<Point> right){
+
+        //dominated points of ArrayList left are removed
+        for(int i=0;i<left.size()-1;i++){
+            if(left.get(i).x==left.get(i+1).x && left.get(i).y>left.get(i+1).y){
+                left.remove(i);
+                i--;
+            }
+        }
+
+        //minimum y-value is found 
+        int min=left.get(0).y;
+        for(int i=1;i<left.size();i++){
+            if(min>left.get(i).y){
+                min = left.get(i).y;
+                if(min==1){
+                    i=left.size();
+                }
+            }
+        }
+
+        //dominated points of ArrayList right are removed
+        for(int i=0;i<right.size();i++){
+            if(right.get(i).y>=min){
+                right.remove(i);
+                i--;
+            }
+        }
+
+        //final skyline found and returned
+        left.addAll(right);
+        return left;
+    }
+
+
+
+    public static class Point{
+        private int x;
+        private int y;
+        /**
+         * The main constructor of Point Class, used to represent the 2 Dimension points.
+         * @param x the point's x-value.
+         * @param y the point's y-value.
+         */
+        public Point(int x, int y){
+            this.x=x;
+            this.y=y;
+        }
+        /**
+         * @return x, the x-value
+         */
+        public int getX(){
+            return x;
+        }
+        /**
+         * @return y, the y-value
+         */
+        public int getY(){
+            return y;
+        }
+        /**
+         * Based on the skyline theory,
+         * it checks if the point that calls the function dominates the argument point.
+         * @param p1 the point that is compared
+         * @return true if the point wich calls the function dominates p1
+         *         false otherwise.
+         */
+        public boolean dominates(Point p1){
+
+            //checks if p1 is dominated
+            if((this.x<p1.x && this.y<=p1.y) || (this.x<=p1.x && this.y<p1.y)){
+                return true;
+            }
+            return false;
+        }
+    }
+    /**
+     * It is used to compare the 2 Dimension points,
+     * based on their x-values, in order get sorted later.
+     */
+    class XComparator implements Comparator<Point> {
+    @Override
+    public int compare(Point a, Point b) {
+        return a.x < b.x ? -1 : a.x == b.x ? 0 : 1;
+    }
+    }
+}