Add Graham Scan in Java (#459)

* Add Graham Scan in Java

* refactoring

* reworked code and added point.equals

Author: PaddyKe

contents/graham_scan/code/java/GrahamScan.java

@@ -0,0 +1,95 @@
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.List;
+import java.util.stream.Collectors;
+
+public class GrahamScan {
+
+    static class Point {
+        public double x;
+        public double y;
+
+        public Point(double x, double y) {
+            this.x = x;
+            this.y = y;
+        }
+
+        @Override
+        public boolean equals(Object o) {
+            if (o == null) return false;
+            if (o == this) return true;
+            if (!(o instanceof Point)) return false;
+            Point p = (Point)o;
+            return p.x == this.x && p.y == this.y;
+        }
+    }
+
+    static double ccw(Point a, Point b, Point c) {
+        return (b.x - a.x) * (c.y - a.y) - (b.y - a.y) * (c.x - a.x);
+    }
+
+    static double polarAngle(Point origin, Point p) {
+        return Math.atan2(p.y - origin.y, p.x - origin.x);
+    }
+
+    static List<Point> grahamScan(List<Point> gift) {
+        gift = gift.stream()
+                   .distinct()
+                   .sorted(Comparator.comparingDouble(point -> -point.y))
+                   .collect(Collectors.toList());
+
+        Point pivot = gift.get(0);
+
+        // Sort the remaining Points based on the angle between the pivot and itself
+        List<Point> hull = gift.subList(1, gift.size());
+        hull.sort(Comparator.comparingDouble(point -> polarAngle(point, pivot)));
+
+        // The pivot is always on the hull
+        hull.add(0, pivot);
+
+        int n = hull.size();
+        int m = 1;
+
+        for (int i = 2; i < n; i++) {
+            while (ccw(hull.get(m - 1), hull.get(m), hull.get(i)) <= 0) {
+                if (m > 1) {
+                    m--;
+                } else if (m == 1) {
+                    break;
+                } else {
+                    i++;
+                }
+            }
+            m++;
+
+            Point temp = hull.get(i);
+            hull.set(i, hull.get(m));
+            hull.set(m, temp);
+        }
+        return hull.subList(0, m + 1);
+    }
+
+    public static void main(String[] args) {
+        ArrayList<Point> points = new ArrayList<>();
+
+        points.add(new Point(-5, 2));
+        points.add(new Point(5, 7));
+        points.add(new Point(-6, -12));
+        points.add(new Point(-14, -14));
+        points.add(new Point(9, 9));
+        points.add(new Point(-1, -1));
+        points.add(new Point(-10, 11));
+        points.add(new Point(-6, 15));
+        points.add(new Point(-6, -8));
+        points.add(new Point(15, -9));
+        points.add(new Point(7, -7));
+        points.add(new Point(-2, -9));
+        points.add(new Point(6, -5));
+        points.add(new Point(0, 14));
+        points.add(new Point(2, 8));
+
+        List<Point> convexHull = grahamScan(points);
+
+        convexHull.forEach(p -> System.out.printf("% 1.0f, % 1.0f\n", p.x, p.y));
+    }
+}

contents/graham_scan/graham_scan.md

@@ -22,6 +22,8 @@ We can find whether a rotation is counter-clockwise with trigonometric functions
 [import:36-38, lang:"javascript"](code/javascript/graham-scan.js)
 {% sample lang="py" %}
 [import:4-6, lang:"python"](code/python/grahamScan.py)
+{% sample lang="java" %}
+[import:27-29, lang:"java"](code/java/GrahamScan.java)
 {% endmethod %}
 
 If the output of this function is 0, the points are collinear.
@@ -46,6 +48,8 @@ In the end, the code should look something like this:
 [import:1-30, lang:"javascript"](code/javascript/graham-scan.js)
 {% sample lang="py" %}
 [import:14-27, lang:"python"](code/python/grahamScan.py)
+{% sample lang="java" %}
+[import:35-70, lang:"java"](code/java/GrahamScan.java)
 {% endmethod %}
 
 ### Bibliography
@@ -65,6 +69,8 @@ In the end, the code should look something like this:
 [import, lang:"javascript"](code/javascript/graham-scan.js)
 {% sample lang="py" %}
 [import, lang:"python"](code/python/grahamScan.py)
+{% sample lang="java" %}
+[import, lang:"java"](code/java/GrahamScan.java)
 {% endmethod %}
 
 <script>