programming-examples/java/Data_Structures/PolygonCircleIntersection.java

import java.awt.geom.Point2D;
import java.util.*;

public class PolygonCircleIntersection {

	static final double eps = 1e-9;

	public static double polygonCircleIntersection(int[] x, int[] y, int r) {
		double area = 0;
		for (int i = 0, j = x.length - 1; i < x.length; j = i++) {
			double cur = circleTriangleIntersection(r, x[j], y[j], x[i], y[i]);
			area += Math.signum(x[j] * y[i] - x[i] * y[j]) * cur;
		}
		return Math.abs(area);
	}

	static double circleTriangleIntersection(double r, int x1, int y1, int x2, int y2) {
		List<Point2D.Double> points = new ArrayList<>();
		points.add(new Point2D.Double(x1, y1));
		for (Point2D.Double p : circleLineIntersection(r, y2 - y1, x1 - x2, -(x1 * y2 - x2 * y1)))
			if (isMiddle(p.x, p.y, x1, y1, x2, y2))
				points.add(p);
		points.add(new Point2D.Double(x2, y2));
		double area = 0;
		for (int i = 0; i + 1 < points.size(); i++) {
			Point2D.Double p1 = points.get(i);
			Point2D.Double p2 = points.get(i + 1);
			if (p1.x * p1.x + p1.y * p1.y > r * r + eps || p2.x * p2.x + p2.y * p2.y > r * r + eps)
				area += angleBetween(p1.x, p1.y, p2.x, p2.y) / 2 * r * r;
			else
				area += Math.abs(p1.x * p2.y - p2.x * p1.y) / 2;
		}
		return area;
	}

	static double angleBetween(double ax, double ay, double bx, double by) {
		double a = Math.abs(Math.atan2(by, bx) - Math.atan2(ay, ax));
		return Math.min(a, 2 * Math.PI - a);
	}

	static Point2D.Double[] circleLineIntersection(double r, double a, double b, double c) {
		double aabb = a * a + b * b;
		double d = c * c / aabb - r * r;
		if (d > eps)
			return new Point2D.Double[0];
		double x0 = -a * c / aabb;
		double y0 = -b * c / aabb;
		if (d > -eps)
			return new Point2D.Double[]{new Point2D.Double(x0, y0)};
		d /= -aabb;
		double k = Math.sqrt(d < 0 ? 0 : d);
		return new Point2D.Double[]{
				new Point2D.Double(x0 + k * b, y0 - k * a),
				new Point2D.Double(x0 - k * b, y0 + k * a)};
	}

	static boolean isMiddle(double x, double y, double x1, double y1, double x2, double y2) {
		return (x < x1 + eps || x < x2 + eps) && (x > x1 - eps || x > x2 - eps)
				&& (y < y1 + eps || y < y2 + eps) && (y > y1 - eps || y > y2 - eps);
	}

	// random tests
	public static void main(String[] args) {
		for (int step = 0; step < 1000; step++) {
			System.out.println(step);
			int n = rnd.nextInt(10) + 3;
			int range = 10;
			int[][] xy = getRandomPolygon(n, range, range);
			int[] x = xy[0];
			int[] y = xy[1];
			int r = rnd.nextInt(10) + 1;
			double res1 = polygonCircleIntersection(x, y, r);
			double res2 = polygonCircleIntersectionMonteCarlo(x, y, r);
			if (!(Math.abs(res1 - res2) < 0.1))
				throw new RuntimeException();
		}
	}

	static double polygonCircleIntersectionMonteCarlo(int[] x, int[] y, int r) {
		int count = 0;
		int hits = 0;
		int minx = Integer.MAX_VALUE;
		int maxx = Integer.MIN_VALUE;
		int miny = Integer.MAX_VALUE;
		int maxy = Integer.MIN_VALUE;
		for (int i = 0; i < x.length; i++) {
			minx = Math.min(minx, x[i]);
			maxx = Math.max(maxx, x[i]);
			miny = Math.min(miny, y[i]);
			maxy = Math.max(maxy, y[i]);
		}
		for (double px = minx; px <= maxx; px += 0.005)
			for (double py = miny; py <= maxy; py += 0.005) {
				++count;
				if (pointInPolygon(px, py, x, y) >= 0 && px * px + py * py <= r * r)
					++hits;
			}
		return (double) hits / count * ((maxx - minx) * (maxy - miny));
	}

	static int pointInPolygon(double qx, double qy, int[] x, int[] y) {
		int n = x.length;
		int cnt = 0;
		for (int i = 0, j = n - 1; i < n; j = i++) {
			if (y[i] == qy && (x[i] == qx || y[j] == qy && (x[i] <= qx || x[j] <= qx) && (x[i] >= qx || x[j] >= qx)))
				return 0; // boundary
			if ((y[i] > qy) != (y[j] > qy)) {
				double det = (x[i] - qx) * (y[j] - qy) - (x[j] - qx) * (y[i] - qy);
				if (det == 0)
					return 0; // boundary
				if ((det > 0) != (y[j] - y[i] > 0))
					++cnt;
			}
		}
		return cnt % 2 == 0 ? -1 /* exterior */ : 1 /* interior */;
	}

	static Random rnd = new Random(1);

	static int[][] getRandomPolygon(int n, int maxWidth, int maxHeight) {
		int[] x = new int[n];
		int[] y = new int[n];
		int[] p = new int[n];
		while (true) {
			for (int i = 0; i < n; i++) {
				x[i] = rnd.nextInt(maxWidth);
				y[i] = rnd.nextInt(maxHeight);
				p[i] = i;
			}
			for (boolean improved = true; improved; ) {
				improved = false;
				for (int i = 0; i < n; i++) {
					for (int j = 0; j < n; j++) {
						int[] p1 = p.clone();
						reverse(p1, i, j);
						if (len(x, y, p) > len(x, y, p1)) {
							p = p1;
							improved = true;
						}
					}
				}
			}
			int[] tx = x.clone();
			int[] ty = y.clone();
			for (int i = 0; i < n; i++) {
				x[i] = tx[p[i]];
				y[i] = ty[p[i]];
			}
			boolean ok = true;
			for (int i = 0; i < n; i++) {
				long x1 = x[(i - 1 + n) % n] - x[i];
				long y1 = y[(i - 1 + n) % n] - y[i];
				long x2 = x[(i + 1) % n] - x[i];
				long y2 = y[(i + 1) % n] - y[i];
				ok &= x1 * y2 - x2 * y1 != 0 || x1 * x2 + y1 * y2 <= 0;
			}
			for (int i2 = 0, i1 = p.length - 1; i2 < p.length; i1 = i2++)
				for (int j2 = 0, j1 = p.length - 1; j2 < p.length; j1 = j2++)
					ok &= i1 == j1 || i1 == j2 || i2 == j1
							|| !isCrossOrTouchIntersect(x[i1], y[i1], x[i2], y[i2], x[j1], y[j1], x[j2], y[j2]);
			if (ok)
				return new int[][]{x, y};
		}
	}

	// http://en.wikipedia.org/wiki/2-opt
	static void reverse(int[] p, int i, int j) {
		int n = p.length;
		// reverse order from i to j
		while (i != j) {
			int t = p[j];
			p[j] = p[i];
			p[i] = t;
			i = (i + 1) % n;
			if (i == j) break;
			j = (j - 1 + n) % n;
		}
	}

	static double len(int[] x, int[] y, int[] p) {
		double res = 0;
		for (int i = 0, j = p.length - 1; i < p.length; j = i++) {
			double dx = x[p[i]] - x[p[j]];
			double dy = y[p[i]] - y[p[j]];
			res += Math.sqrt(dx * dx + dy * dy);
		}
		return res;
	}

	static boolean isCrossOrTouchIntersect(long x1, long y1, long x2, long y2, long x3, long y3, long x4, long y4) {
		if (Math.max(x1, x2) < Math.min(x3, x4) || Math.max(x3, x4) < Math.min(x1, x2)
				|| Math.max(y1, y2) < Math.min(y3, y4) || Math.max(y3, y4) < Math.min(y1, y2))
			return false;
		long z1 = (x2 - x1) * (y3 - y1) - (y2 - y1) * (x3 - x1);
		long z2 = (x2 - x1) * (y4 - y1) - (y2 - y1) * (x4 - x1);
		long z3 = (x4 - x3) * (y1 - y3) - (y4 - y3) * (x1 - x3);
		long z4 = (x4 - x3) * (y2 - y3) - (y4 - y3) * (x2 - x3);
		return (z1 <= 0 || z2 <= 0) && (z1 >= 0 || z2 >= 0) && (z3 <= 0 || z4 <= 0) && (z3 >= 0 || z4 >= 0);
	}
}