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package ru.dubov.segmentsintersect;
import ru.dubov.primitives.Point;
import ru.dubov.primitives.Segment;
/**
* Determines whether the two given segments intersect in O(1),
* as depicted in "Introduction to Algorithms" by T.Cormen et al.
*
* @author Mikhail Dubov
*/
public class SegmentsIntersect {
private static double direction(Point p0, Point p1, Point p2) {
return ((p2.getX() - p0.getX()) * (p1.getY() - p0.getY()) -
(p2.getY() - p0.getY()) * (p1.getX() - p0.getX()));
}
private static boolean onSegment(Point pi, Point pj, Point pk) {
return (Math.min(pi.getX(), pj.getX()) <= pk.getX() &&
pk.getX() <= Math.max(pi.getX(), pj.getX()) &&
Math.min(pi.getY(), pj.getY()) <= pk.getY() &&
pk.getY() <= Math.max(pi.getY(), pj.getY()));
}
public static boolean two(Segment s1, Segment s2) {
Point p1 = s1.getLeft();
Point p2 = s1.getRight();
Point p3 = s2.getLeft();
Point p4 = s2.getRight();
double d1 = direction(p3, p4, p1);
double d2 = direction(p3, p4, p2);
double d3 = direction(p1, p2, p3);
double d4 = direction(p1, p2, p4);
if (((d1 > 0 && d2 < 0) || (d1 < 0 && d2 > 0)) &&
((d3 > 0 && d4 < 0) || (d3 < 0 && d4 > 0))) {
return true;
} else if (d1 == 0 && onSegment(p3, p4, p1)) {
return true;
} else if (d2 == 0 && onSegment(p3, p4, p2)) {
return true;
} else if (d3 == 0 && onSegment(p1, p2, p3)) {
return true;
} else if (d4 == 0 && onSegment(p1, p2, p4)) {
return true;
} else {
return false;
}
}
}