1 files changed, 63 insertions, 0 deletions
diff --git a/geometry/segmentIntersection.cpp b/geometry/segmentIntersection.cpp
new file mode 100644
index 0000000..c2961da
--- /dev/null
+++ b/geometry/segmentIntersection.cpp
@@ -0,0 +1,63 @@
+struct seg {
+	pt a, b;
+	int id;
+	bool operator<(const seg& o) const {
+		if (real(a) < real(o.a)) {
+			int s = orientation(a, b, o.a);
+			return (s > 0 || (s == 0 && imag(a) < imag(o.a)));//
+		} else if (real(a) > real(o.a)) {
+			int s = orientation(o.a, o.b, a);
+			return (s < 0 || (s == 0 && imag(a) < imag(o.a)));
+		}
+		return imag(a) < imag(o.a);
+	}
+};
+ 
+struct event {
+	pt p;
+	int id, type;
+	bool operator<(const event& o) const {
+		if (real(p) != real(o.p)) return real(p) < real(o.p);
+		if (type != o.type) return type > o.type;
+		return imag(p) < imag(o.p);
+	}
+};
+ 
+bool lessPT(const pt& a, const pt& b) {
+	return real(a) != real(b) ? real(a) < real(b)
+														: imag(a) < imag(b);
+}
+ 
+bool intersect(const seg& a, const seg& b) {
+	return lineSegmentIntersection(a.a, a.b, b.a, b.b);
+}
+ 
+pair<int, int> intersect(vector<seg>& segs) {
+	vector<event> events;
+	for (seg& s : segs) {
+		if (lessPT(s.b, s.a)) swap(s.b, s.a);
+		events.push_back({s.a, s.id, 1});
+		events.push_back({s.b, s.id, -1});
+	}
+	sort(all(events));
+ 
+	set<seg> q;
+	vector<set<seg>::iterator> where(sz(segs));
+	for (auto e : events) {
+		int id = e.id;
+		if (e.type > 0) {
+			auto it = q.lower_bound(segs[id]);
+			if (it != q.end() && intersect(*it, segs[id]))
+				return {it->id, segs[id].id};
+			if (it != q.begin() && intersect(*prev(it), segs[id]))
+				return {prev(it)->id, segs[id].id};
+			where[id] = q.insert(it, segs[id]);
+		} else {
+			auto it = where[id];
+			if (it != q.begin() && next(it) != q.end() && intersect(*next(it), *prev(it)))
+				return {next(it)->id, prev(it)->id};
+			q.erase(it);
+		}
+	}
+	return {-1, -1};
+}
+\ No newline at end of file