From 0211492d96838c111bfbbf284b0f7957036e1e24 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Gustaf=20Alh=C3=A4ll?= <gustaf@hanicef.me>
Date: Thu, 4 Jan 2024 20:12:20 +0100
Subject: [PATCH] Change point-in-polygon algorithm

---
 src/r_main.c | 75 +++++++++++++---------------------------------------
 1 file changed, 19 insertions(+), 56 deletions(-)

diff --git a/src/r_main.c b/src/r_main.c
index 5b01ddfc5..940104d7d 100644
--- a/src/r_main.c
+++ b/src/r_main.c
@@ -1020,76 +1020,39 @@ void R_Init(void)
 boolean R_IsPointInSector(sector_t *sector, fixed_t x, fixed_t y)
 {
 	size_t i;
-	line_t *closest = NULL;
-	fixed_t closestdist = INT32_MAX;
+	size_t passes = 0;
 
 	for (i = 0; i < sector->linecount; i++)
 	{
-		vertex_t v, *v1, *v2;
-		fixed_t dist;
-
-		// find the line closest to the point we're looking for.
-		P_ClosestPointOnLine(x, y, sector->lines[i], &v);
+		vertex_t *v1, *v2;
 
 		v1 = sector->lines[i]->v1;
 		v2 = sector->lines[i]->v2;
 
-		// do some correction in order to prevent points outside the line from being measured.
-		if (abs(v1->x - v2->x) > abs(v1->y - v2->y))
+		// make sure v1 is below v2
+		if (v1->y > v2->y)
 		{
-			if (v.x < v1->x && v.x < v2->x)
-				v = v1->x < v2->x ? *v1 : *v2;
-			else if (v.x > v1->x && v.x > v2->x)
-				v = v1->x > v2->x ? *v1 : *v2;
+			vertex_t *tmp = v1;
+			v1 = v2;
+			v2 = tmp;
 		}
-		else
+		else if (v1->y == v2->y)
+			// horizontal line, we can't match this
+			continue;
+
+		if (v1->y <= y && y < v2->y)
 		{
-			if (v.y < v1->y && v.y < v2->y)
-				v = v1->y < v2->y ? *v1 : *v2;
-			else if (v.y > v1->y && v.y > v2->y)
-				v = v1->y > v2->y ? *v1 : *v2;
-		}
+			// if the y axis in inside the line, find the point where we intersect on the x axis...
+			fixed_t vx = v1->x + (INT64)(v2->x - v1->x) * (y - v1->y) / (v2->y - v1->y);
 
-		dist = R_PointToDist2(v.x, v.y, x, y);
-		if (dist < closestdist)
-		{
-			closest = sector->lines[i];
-			closestdist = dist;
-		}
-		else if (dist == closestdist)
-		{
-			// coordinates are the same; we're most likely matching against a corner here.
-			// check closest angle instead.
-			vertex_t *cv, *dv;
-			angle_t angle = R_PointToAngle2(v.x, v.y, x, y);
-			angle_t ca, da;
-			if (closest->v1->x == v.x && closest->v1->y == v.y)
-				cv = closest->v2;
-			else
-				cv = closest->v1;
-
-			if (v1->x == v.x && v1->y == v.y)
-				dv = v2;
-			else
-				dv = v1;
-
-			ca = R_PointToAngle2(v.x, v.y, cv->x, cv->y) - angle;
-			da = R_PointToAngle2(v.x, v.y, dv->x, dv->y) - angle;
-			if (ca > ANGLE_180)
-				ca = -ca;
-			if (da > ANGLE_180)
-				da = -da;
-
-			if (ca > da)
-			{
-				closest = sector->lines[i];
-				closestdist = dist;
-			}
+			// ...and if that point is to the left of the point, count it as inside.
+			if (vx < x)
+				passes++;
 		}
 	}
 
-	// if the side of the closest line is in this sector, we're inside of it.
-	return P_PointOnLineSide(x, y, closest) == 0 ? closest->frontsector == sector : closest->backsector == sector;
+	// and odd number of passes means we're inside the polygon.
+	return passes % 2;
 }
 
 //