diff --git a/src/main/java/com/thealgorithms/geometry/LineIntersection.java b/src/main/java/com/thealgorithms/geometry/LineIntersection.java new file mode 100644 index 000000000000..8d65833816b3 --- /dev/null +++ b/src/main/java/com/thealgorithms/geometry/LineIntersection.java @@ -0,0 +1,105 @@ +package com.thealgorithms.geometry; + +import java.awt.geom.Point2D; +import java.util.Optional; + +/** + * Utility methods for checking and computing 2D line segment intersections. + */ +public final class LineIntersection { + private LineIntersection() { + } + + /** + * Checks whether two line segments intersect. + * + * @param p1 first endpoint of segment 1 + * @param p2 second endpoint of segment 1 + * @param q1 first endpoint of segment 2 + * @param q2 second endpoint of segment 2 + * @return true when the segments intersect (including touching endpoints) + */ + public static boolean intersects(Point p1, Point p2, Point q1, Point q2) { + int o1 = orientation(p1, p2, q1); + int o2 = orientation(p1, p2, q2); + int o3 = orientation(q1, q2, p1); + int o4 = orientation(q1, q2, p2); + + if (o1 != o2 && o3 != o4) { + return true; + } + + if (o1 == 0 && onSegment(p1, q1, p2)) { + return true; + } + if (o2 == 0 && onSegment(p1, q2, p2)) { + return true; + } + if (o3 == 0 && onSegment(q1, p1, q2)) { + return true; + } + if (o4 == 0 && onSegment(q1, p2, q2)) { + return true; + } + + return false; + } + + /** + * Computes the single geometric intersection point between two non-parallel + * segments when it exists. + * + *

For parallel/collinear overlap, this method returns {@code Optional.empty()}. + * + * @param p1 first endpoint of segment 1 + * @param p2 second endpoint of segment 1 + * @param q1 first endpoint of segment 2 + * @param q2 second endpoint of segment 2 + * @return the intersection point when uniquely defined and on both segments + */ + public static Optional intersectionPoint(Point p1, Point p2, Point q1, Point q2) { + if (!intersects(p1, p2, q1, q2)) { + return Optional.empty(); + } + + long x1 = p1.x(); + long y1 = p1.y(); + long x2 = p2.x(); + long y2 = p2.y(); + long x3 = q1.x(); + long y3 = q1.y(); + long x4 = q2.x(); + long y4 = q2.y(); + + long denominator = (x1 - x2) * (y3 - y4) - (y1 - y2) * (x3 - x4); + if (denominator == 0L) { + return sharedEndpoint(p1, p2, q1, q2); + } + + long determinant1 = x1 * y2 - y1 * x2; + long determinant2 = x3 * y4 - y3 * x4; + long numeratorX = determinant1 * (x3 - x4) - (x1 - x2) * determinant2; + long numeratorY = determinant1 * (y3 - y4) - (y1 - y2) * determinant2; + + return Optional.of(new Point2D.Double(numeratorX / (double) denominator, numeratorY / (double) denominator)); + } + + private static int orientation(Point a, Point b, Point c) { + long cross = ((long) b.x() - a.x()) * ((long) c.y() - a.y()) - ((long) b.y() - a.y()) * ((long) c.x() - a.x()); + return Long.compare(cross, 0L); + } + + private static Optional sharedEndpoint(Point p1, Point p2, Point q1, Point q2) { + if (p1.equals(q1) || p1.equals(q2)) { + return Optional.of(new Point2D.Double(p1.x(), p1.y())); + } + if (p2.equals(q1) || p2.equals(q2)) { + return Optional.of(new Point2D.Double(p2.x(), p2.y())); + } + return Optional.empty(); + } + + private static boolean onSegment(Point a, Point b, Point c) { + return b.x() >= Math.min(a.x(), c.x()) && b.x() <= Math.max(a.x(), c.x()) && b.y() >= Math.min(a.y(), c.y()) && b.y() <= Math.max(a.y(), c.y()); + } +} diff --git a/src/test/java/com/thealgorithms/geometry/LineIntersectionTest.java b/src/test/java/com/thealgorithms/geometry/LineIntersectionTest.java new file mode 100644 index 000000000000..9f60df51b65f --- /dev/null +++ b/src/test/java/com/thealgorithms/geometry/LineIntersectionTest.java @@ -0,0 +1,101 @@ +package com.thealgorithms.geometry; + +import static org.junit.jupiter.api.Assertions.assertEquals; +import static org.junit.jupiter.api.Assertions.assertFalse; +import static org.junit.jupiter.api.Assertions.assertTrue; + +import java.awt.geom.Point2D; +import java.util.Optional; +import org.junit.jupiter.api.Test; + +class LineIntersectionTest { + + @Test + void testCrossingSegments() { + Point p1 = new Point(0, 0); + Point p2 = new Point(4, 4); + Point q1 = new Point(0, 4); + Point q2 = new Point(4, 0); + + assertTrue(LineIntersection.intersects(p1, p2, q1, q2)); + Optional intersection = LineIntersection.intersectionPoint(p1, p2, q1, q2); + assertTrue(intersection.isPresent()); + assertEquals(2.0, intersection.orElseThrow().getX(), 1e-9); + assertEquals(2.0, intersection.orElseThrow().getY(), 1e-9); + } + + @Test + void testParallelSegments() { + Point p1 = new Point(0, 0); + Point p2 = new Point(3, 3); + Point q1 = new Point(0, 1); + Point q2 = new Point(3, 4); + + assertFalse(LineIntersection.intersects(p1, p2, q1, q2)); + assertTrue(LineIntersection.intersectionPoint(p1, p2, q1, q2).isEmpty()); + } + + @Test + void testTouchingAtEndpoint() { + Point p1 = new Point(0, 0); + Point p2 = new Point(2, 2); + Point q1 = new Point(2, 2); + Point q2 = new Point(4, 0); + + assertTrue(LineIntersection.intersects(p1, p2, q1, q2)); + Optional intersection = LineIntersection.intersectionPoint(p1, p2, q1, q2); + assertTrue(intersection.isPresent()); + assertEquals(2.0, intersection.orElseThrow().getX(), 1e-9); + assertEquals(2.0, intersection.orElseThrow().getY(), 1e-9); + } + + @Test + void testCollinearOverlapHasNoUniquePoint() { + Point p1 = new Point(0, 0); + Point p2 = new Point(4, 4); + Point q1 = new Point(2, 2); + Point q2 = new Point(6, 6); + + assertTrue(LineIntersection.intersects(p1, p2, q1, q2)); + assertTrue(LineIntersection.intersectionPoint(p1, p2, q1, q2).isEmpty()); + } + + @Test + void testCollinearDisjointSegments() { + Point p1 = new Point(0, 0); + Point p2 = new Point(2, 2); + Point q1 = new Point(3, 3); + Point q2 = new Point(5, 5); + + assertFalse(LineIntersection.intersects(p1, p2, q1, q2)); + assertTrue(LineIntersection.intersectionPoint(p1, p2, q1, q2).isEmpty()); + } + + @Test + void testCollinearSegmentsTouchingAtEndpointHaveUniquePoint() { + Point p1 = new Point(0, 0); + Point p2 = new Point(2, 2); + Point q1 = new Point(2, 2); + Point q2 = new Point(4, 4); + + assertTrue(LineIntersection.intersects(p1, p2, q1, q2)); + Optional intersection = LineIntersection.intersectionPoint(p1, p2, q1, q2); + assertTrue(intersection.isPresent()); + assertEquals(2.0, intersection.orElseThrow().getX(), 1e-9); + assertEquals(2.0, intersection.orElseThrow().getY(), 1e-9); + } + + @Test + void testVerticalAndHorizontalCrossingSegments() { + Point p1 = new Point(2, 0); + Point p2 = new Point(2, 5); + Point q1 = new Point(0, 3); + Point q2 = new Point(4, 3); + + assertTrue(LineIntersection.intersects(p1, p2, q1, q2)); + Optional intersection = LineIntersection.intersectionPoint(p1, p2, q1, q2); + assertTrue(intersection.isPresent()); + assertEquals(2.0, intersection.orElseThrow().getX(), 1e-9); + assertEquals(3.0, intersection.orElseThrow().getY(), 1e-9); + } +}