89 lines
2.9 KiB
Java
89 lines
2.9 KiB
Java
import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.List;
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import java.util.stream.Collectors;
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import static java.util.Collections.emptyList;
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public class ConvexHull {
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private static class Point implements Comparable<Point> {
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private int x, y;
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public Point(int x, int y) {
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this.x = x;
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this.y = y;
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}
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@Override
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public int compareTo(Point o) {
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return Integer.compare(x, o.x);
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}
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@Override
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public String toString() {
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return String.format("(%d, %d)", x, y);
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}
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}
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private static List<Point> convexHull(List<Point> p) {
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if (p.isEmpty()) return emptyList();
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p.sort(Point::compareTo);
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List<Point> h = new ArrayList<>();
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// lower hull
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for (Point pt : p) {
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while (h.size() >= 2 && !ccw(h.get(h.size() - 2), h.get(h.size() - 1), pt)) {
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h.remove(h.size() - 1);
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}
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h.add(pt);
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}
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// upper hull
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int t = h.size() + 1;
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for (int i = p.size() - 1; i >= 0; i--) {
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Point pt = p.get(i);
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while (h.size() >= t && !ccw(h.get(h.size() - 2), h.get(h.size() - 1), pt)) {
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h.remove(h.size() - 1);
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}
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h.add(pt);
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}
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h.remove(h.size() - 1);
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return h;
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}
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// ccw returns true if the three points make a counter-clockwise turn
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private static boolean ccw(Point a, Point b, Point c) {
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return ((b.x - a.x) * (c.y - a.y)) > ((b.y - a.y) * (c.x - a.x));
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}
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public static void main(String[] args) {
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List<Point> points = Arrays.asList(new Point(16, 3),
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new Point(12, 17),
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new Point(0, 6),
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new Point(-4, -6),
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new Point(16, 6),
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new Point(16, -7),
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new Point(16, -3),
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new Point(17, -4),
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new Point(5, 19),
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new Point(19, -8),
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new Point(3, 16),
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new Point(12, 13),
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new Point(3, -4),
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new Point(17, 5),
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new Point(-3, 15),
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new Point(-3, -9),
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new Point(0, 11),
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new Point(-9, -3),
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new Point(-4, -2),
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new Point(12, 10));
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List<Point> hull = convexHull(points);
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System.out.printf("Convex Hull: %s\n", hull);
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}
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}
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