Responded to comments and tried a macro

Author: Chillee

content/geometry/HalfPlane.h

@@ -31,16 +31,15 @@ vector<P> halfPlaneIntersection(vector<Line> vs) {
 	vector<P> ans(sz(vs) + 5);
 	deq[0] = vs[0];
 	int dh = 0, dt = 1, ah = 0, at = 0;
+    #define upd(l, pt, op) while (ah < at && sideOf(sp(l), ans[pt]) < 0) op;
 	for (int i = 1; i < sz(vs); ++i) {
 		if (angDiff(vs[i], vs[i - 1]) == 0) continue;
-		while (ah<at && sideOf(sp(vs[i]),ans[at-1]) < 0) at--,dt--;
-		while (ah<at && sideOf(sp(vs[i]),ans[ah]) < 0) ah++,dh++;
+        upd(vs[i], at-1, (at--,dt--)) upd(vs[i], ah, (ah++,dh++));
 		ans[at++] = lineInter(sp(deq[dt - 1]), sp(vs[i])).second;
 		deq[dt++] = vs[i];
 	}
-	while (ah<at && sideOf(sp(deq[dh]),ans[at-1]) < 0) at--,dt--;
-	while (ah<at && sideOf(sp(deq[dt]),ans[ah]) < 0) ah++, dh++;
+    upd(deq[dh], at-1, (at--,dt--)); upd(deq[dt], ah, (ah++,dh++));
 	if (dt - dh <= 2) return {};
-	ans[at++] = lineInter(sp(deq[dh]), sp(deq[dt - 1])).second;
+	ans[at++] = lineInter(sp(deq[dt-1]), sp(deq[dh])).second;
 	return {ans.begin() + ah, ans.begin() + at};
 }

content/geometry/PolygonCut.h

@@ -28,10 +28,8 @@ vector<P> polygonCut(const vector<P>& poly, P s, P e) {
 	rep(i,0,sz(poly)) {
 		P cur = poly[i], prev = i ? poly[i-1] : poly.back();
 		bool side = s.cross(e, cur) < 0;
-		if (side != (s.cross(e, prev) < 0)) {
-			res.emplace_back();
-			lineIntersection(s, e, cur, prev, res.back());
-		}
+		if (side != (s.cross(e, prev) < 0))
+			res.push_back(lineInter(s, e, cur, prev).second);
 		if (side)
 			res.push_back(cur);
 	}

content/geometry/lineIntersection.h

@@ -1,20 +1,20 @@
 /**
- * Author: Ulf Lundstrom
- * Date: 2009-03-21
+ * Author: Victor Lecomte, chilli
+ * Date: 2019-05-05
  * License: CC0
- * Source:
+ * Source: https://vlecomte.github.io/cp-geo.pdf
  * Description:\\
 \begin{minipage}{75mm}
-If a unique intersection point of the lines going through s1,e1 and s2,e2 exists r is set to this point and 1 is returned. If no intersection point exists 0 is returned and if infinitely many exists -1 is returned. If s1==e1 or s2==e2 -1 is returned. The wrong position will be returned if P is Point<ll> and the intersection point does not have integer coordinates. Products of three coordinates are used in intermediate steps so watch out for overflow if using int or long long.
+If a unique intersection point of the lines going through s1,e1 and s2,e2 exists {1, point} is returned. If no intersection point exists {0, (0,0)} is returned and if infinitely many exists {-1, (0,0)} is returned. The wrong position will be returned if P is Point<ll> and the intersection point does not have integer coordinates. Products of three coordinates are used in intermediate steps so watch out for overflow if using int or long long.
 \end{minipage}
 \begin{minipage}{15mm}
 \includegraphics[width=\textwidth]{content/geometry/lineIntersection}
 \end{minipage}
- * Status: tested
+ * Status: tested through half-plane tests
  * Usage:
- * 	point<double> intersection;
- * 	if (1 == LineIntersection(s1,e1,s2,e2,intersection))
- * 		cout << "intersection point at " << intersection << endl;
+ * 	auto res = lineInter(s1,e1,s2,e2);
+ * 	if (res.first == 1)
+ * 		cout << "intersection point at " << res.second << endl;
  */
 #pragma once
 
@@ -24,7 +24,7 @@ template<class P>
 pair<int, P> lineInter(P s1, P e1, P s2, P e2) {
 	auto d = (e1-s1).cross(e2-s2);
 	if (d == 0)  //if parallel
-		return {-((e1-s1).cross(s2-s1)==0 || s2==e2), P(0,0)};
+		return {-(s1.cross(e1, s2)==0 || s2==e2), P(0,0)};
 	else
-		return {1, s2-(e2-s2)*(e1-s1).cross(s2-s1)/d};
+		return {1, s2-(e2-s2)*s1.cross(e1, s2)/d};
 }

fuzz-tests/geometry/HalfPlane

@@ -227,9 +227,6 @@ void testRandom() {
         assert(diff < EPS);
     }
 }
-void testSelf() {
-    int lim = 1e2;
-}
 
 int main() {
     test1();
@@ -238,13 +235,6 @@ int main() {
     testPoint();
     testEmpty();
     testRandom();
-    vector<Line> t({{P(0,0), P(5,0)}, {P(0,1), P(5,1)}});
-    reverse(all(t));
-    addInf(t);
-
-    auto res = halfPlaneIntersection(t);
-    for (auto i: res) cout<<i<<' ';
-    cout<<endl;
     // Failure case for MIT's half plane code
     // vector<Line> t({{P(9, 8), P(9, 1)}, {P(3, 3), P(3, 5)}, {P(7, 6), P(0, 8)}});
     // addInf(t);