programming-examples/java/Data_Structures/LineGeometry.java

import java.util.*;

public class LineGeometry {

	static final double EPS = 1e-10;

	public static int sign(double a) {
		return a < -EPS ? -1 : a > EPS ? 1 : 0;
	}

	public static class Point implements Comparable<Point> {
		public double x, y;

		public Point(double x, double y) {
			this.x = x;
			this.y = y;
		}

		public Point minus(Point b) {
			return new Point(x - b.x, y - b.y);
		}

		public double cross(Point b) {
			return x * b.y - y * b.x;
		}

		public double dot(Point b) {
			return x * b.x + y * b.y;
		}

		public Point rotateCCW(double angle) {
			return new Point(x * Math.cos(angle) - y * Math.sin(angle), x * Math.sin(angle) + y * Math.cos(angle));
		}

		@Override
		public int compareTo(Point o) {
			// return Double.compare(Math.atan2(y, x), Math.atan2(o.y, o.x));
			return Double.compare(x, o.x) != 0 ? Double.compare(x, o.x) : Double.compare(y, o.y);
		}
	}

	public static class Line {
		public double a, b, c;

		public Line(double a, double b, double c) {
			this.a = a;
			this.b = b;
			this.c = c;
		}

		public Line(Point p1, Point p2) {
			a = +(p1.y - p2.y);
			b = -(p1.x - p2.x);
			c = p1.x * p2.y - p2.x * p1.y;
		}

		public Point intersect(Line line) {
			double d = a * line.b - line.a * b;
			if (sign(d) == 0) {
				return null;
			}
			double x = -(c * line.b - line.c * b) / d;
			double y = -(a * line.c - line.a * c) / d;
			return new Point(x, y);
		}
	}

	// Returns -1 for clockwise, 0 for straight line, 1 for counterclockwise order
	public static int orientation(Point a, Point b, Point c) {
		Point AB = b.minus(a);
		Point AC = c.minus(a);
		return sign(AB.cross(AC));
	}

	public static boolean cw(Point a, Point b, Point c) {
		return orientation(a, b, c) < 0;
	}

	public static boolean ccw(Point a, Point b, Point c) {
		return orientation(a, b, c) > 0;
	}

	public static boolean isCrossIntersect(Point a, Point b, Point c, Point d) {
		return orientation(a, b, c) * orientation(a, b, d) < 0 && orientation(c, d, a) * orientation(c, d, b) < 0;
	}

	public static boolean isCrossOrTouchIntersect(Point a, Point b, Point c, Point d) {
		if (Math.max(a.x, b.x) < Math.min(c.x, d.x) - EPS || Math.max(c.x, d.x) < Math.min(a.x, b.x) - EPS
				|| Math.max(a.y, b.y) < Math.min(c.y, d.y) - EPS || Math.max(c.y, d.y) < Math.min(a.y, b.y) - EPS) {
			return false;
		}
		return orientation(a, b, c) * orientation(a, b, d) <= 0 && orientation(c, d, a) * orientation(c, d, b) <= 0;
	}

	public static double pointToLineDistance(Point p, Line line) {
		return Math.abs(line.a * p.x + line.b * p.y + line.c) / fastHypot(line.a, line.b);
	}

	public static double fastHypot(double x, double y) {
		return Math.sqrt(x * x + y * y);
	}

	public static double sqr(double x) {
		return x * x;
	}

	public static double angleBetween(Point a, Point b) {
		return Math.atan2(a.cross(b), a.dot(b));
	}

	public static double angle(Line line) {
		return Math.atan2(-line.a, line.b);
	}

	public static double signedArea(Point[] points) {
		int n = points.length;
		double area = 0;
		for (int i = 0, j = n - 1; i < n; j = i++) {
			area += (points[i].x - points[j].x) * (points[i].y + points[j].y);
			// area += points[i].x * points[j].y - points[j].x * points[i].y;
		}
		return area / 2;
	}

	public static enum Position {
		LEFT, RIGHT, BEHIND, BEYOND, ORIGIN, DESTINATION, BETWEEN
	}

	// Classifies position of point p against vector a
	public static Position classify(Point p, Point a) {
		int s = sign(a.cross(p));
		if (s > 0) {
			return Position.LEFT;
		}
		if (s < 0) {
			return Position.RIGHT;
		}
		if (sign(p.x) == 0 && sign(p.y) == 0) {
			return Position.ORIGIN;
		}
		if (sign(p.x - a.x) == 0 && sign(p.y - a.y) == 0) {
			return Position.DESTINATION;
		}
		if (a.x * p.x < 0 || a.y * p.y < 0) {
			return Position.BEYOND;
		}
		if (a.x * a.x + a.y * a.y < p.x * p.x + p.y * p.y) {
			return Position.BEHIND;
		}
		return Position.BETWEEN;
	}

	// cuts right part of poly (returns left part)
	public static Point[] convexCut(Point[] poly, Point p1, Point p2) {
		int n = poly.length;
		List<Point> res = new ArrayList<>();
		for (int i = 0, j = n - 1; i < n; j = i++) {
			int d1 = orientation(p1, p2, poly[j]);
			int d2 = orientation(p1, p2, poly[i]);
			if (d1 >= 0)
				res.add(poly[j]);
			if (d1 * d2 < 0)
				res.add(new Line(p1, p2).intersect(new Line(poly[j], poly[i])));
		}
		return res.toArray(new Point[res.size()]);
	}

	// Usage example
	public static void main(String[] args) {
	}
}