diff --git a/source/build/src/engine.cpp b/source/build/src/engine.cpp
index 57fea10ab..9dcda73d1 100644
--- a/source/build/src/engine.cpp
+++ b/source/build/src/engine.cpp
@@ -697,6 +697,46 @@ void initspritelists(void)
     Numsprites = 0;
 }
 
+//
+// inside
+//
+// See http://fabiensanglard.net/duke3d/build_engine_internals.php,
+// "Inside details" for the idea behind the algorithm.
+
+int32_t inside(int32_t x, int32_t y, int sectnum)
+{
+	if (validSectorIndex(sectnum))
+    {
+        uint32_t cnt = 0;
+        auto wal       = (uwallptr_t)&wall[sector[sectnum].wallptr];
+        int  wallsleft = sector[sectnum].wallnum;
+
+        do
+        {
+            // Get the x and y components of the [tested point]-->[wall
+            // point{1,2}] vectors.
+            vec2_t v1 = { wal->x - x, wal->y - y };
+            auto const &wal2 = *(uwallptr_t)&wall[wal->point2];
+            vec2_t v2 = { wal2.x - x, wal2.y - y };
+
+            // If their signs differ[*], ...
+            //
+            // [*] where '-' corresponds to <0 and '+' corresponds to >=0.
+            // Equivalently, the branch is taken iff
+            //   y1 != y2 AND y_m <= y < y_M,
+            // where y_m := min(y1, y2) and y_M := max(y1, y2).
+            if ((v1.y^v2.y) < 0)
+                cnt ^= (((v1.x^v2.x) >= 0) ? v1.x : (v1.x*v2.y-v2.x*v1.y)^v2.y);
+
+            wal++;
+        }
+        while (--wallsleft);
+
+        return cnt>>31;
+    }
+
+    return -1;
+}
 
 
 int32_t getangle(int32_t xvect, int32_t yvect)
diff --git a/source/core/gamefuncs.cpp b/source/core/gamefuncs.cpp
index 9acf67319..09fab5765 100644
--- a/source/core/gamefuncs.cpp
+++ b/source/core/gamefuncs.cpp
@@ -148,48 +148,6 @@ bool calcChaseCamPos(int* px, int* py, int* pz, spritetype* pspr, int *psectnum,
 	return true;
 }
 
-
-//==========================================================================
-//
-// checks if a point is within a given sector
-//
-// Completely redone based on outside information.
-// The math in here is based on this article: https://wrf.ecse.rpi.edu/Research/Short_Notes/pnpoly.html
-// Copyright (c) 1970-2003, Wm. Randolph Franklin , licensed under BSD 3-clause
-// but was transformed to avoid the division it contained and to properly pick the vertices of Build walls.
-//
-//==========================================================================
-
-int inside(int x, int y, int sectnum)
-{
-	if (validSectorIndex(sectnum))
-	{
-		bool c = false;
-		for (auto& wal : wallsofsector(sectnum))
-		{
-			auto& pt1 = wal.pos;
-			auto& pt2 = wal.point2Wall()->pos;
-
-			if ((pt1.y > y) != (pt2.y > y)) // skip if both are on the same side.
-			{
-				// use 64 bit values to avoid overflows in the multiplications below.
-				int64_t deltatx = int64_t(x) - pt1.x;
-				int64_t deltaty = int64_t(y) - pt1.y;
-				int64_t deltax = int64_t(pt2.x) - pt1.x;
-				int64_t deltay = int64_t(pt2.y) - pt1.y;
-				// reformatted to avoid the division - for negative deltay the sign needs to be flipped to give the correct result.
-				//if (x < deltax * (deltaty) / deltay + pt1.x) // this was the original code.
-				if (((deltay * deltatx - deltax * deltaty) ^ deltay) < 0)
-				{
-					c = !c;
-				}
-			}
-		}
-		return int(c);
-	}
-	return -1;
-}
-
 //==========================================================================
 //
 // note that this returns values in renderer coordinate space with inverted sign!