From 3c39edc1ef25003285f0edb01754c6d17a98857b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Gustaf=20Alh=C3=A4ll?= <gustaf@hanicef.me>
Date: Sun, 15 Dec 2024 13:24:54 +0100
Subject: [PATCH] Fall back to old algorithm when on the line

---
 src/r_main.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 src/r_main.h | 13 +++++++++++--
 2 files changed, 60 insertions(+), 2 deletions(-)

diff --git a/src/r_main.c b/src/r_main.c
index c0b97616a..7cacbb610 100644
--- a/src/r_main.c
+++ b/src/r_main.c
@@ -246,6 +246,55 @@ static void FlipCam2_OnChange(void)
 	SendWeaponPref2();
 }
 
+//
+// R_PointOnSide
+// Traverse BSP (sub) tree,
+// check point against partition plane.
+// Returns side 0 (front) or 1 (back).
+//
+// killough 5/2/98: reformatted
+//
+INT32 R_OldPointOnSide(fixed_t x, fixed_t y, node_t *restrict node)
+{
+	if (!node->dx)
+		return x <= node->x ? node->dy > 0 : node->dy < 0;
+
+	if (!node->dy)
+		return y <= node->y ? node->dx < 0 : node->dx > 0;
+
+	fixed_t dx = (x >> 1) - (node->x >> 1);
+	fixed_t dy = (y >> 1) - (node->y >> 1);
+
+	// Try to quickly decide by looking at sign bits.
+	// also use a mask to avoid branch prediction
+	INT32 mask = (node->dy ^ node->dx ^ dx ^ dy) >> 31;
+	return (mask & ((node->dy ^ dx) < 0)) |  // (left is negative)
+		(~mask & (FixedMul(dy, node->dx>>FRACBITS) >= FixedMul(node->dy>>FRACBITS, dx)));
+}
+
+// killough 5/2/98: reformatted
+INT32 R_OldPointOnSegSide(fixed_t x, fixed_t y, seg_t *line)
+{
+	fixed_t lx = line->v1->x;
+	fixed_t ly = line->v1->y;
+	fixed_t ldx = line->v2->x - lx;
+	fixed_t ldy = line->v2->y - ly;
+
+	if (!ldx)
+		return x <= lx ? ldy > 0 : ldy < 0;
+
+	if (!ldy)
+		return y <= ly ? ldx < 0 : ldx > 0;
+
+	fixed_t dx = (x >> 1) - (lx >> 1);
+	fixed_t dy = (y >> 1) - (ly >> 1);
+
+	// Try to quickly decide by looking at sign bits.
+	if ((ldy ^ ldx ^ dx ^ dy) < 0)
+		return (ldy ^ dx) < 0;          // (left is negative)
+	return FixedMul(dy, ldx>>FRACBITS) >= FixedMul(ldy>>FRACBITS, dx);
+}
+
 //
 // R_PointToAngle
 // To get a global angle from cartesian coordinates,
diff --git a/src/r_main.h b/src/r_main.h
index c65154dd2..9767f4b69 100644
--- a/src/r_main.h
+++ b/src/r_main.h
@@ -71,11 +71,17 @@ extern lighttable_t *zlight[LIGHTLEVELS][MAXLIGHTZ];
 // There a 0-31, i.e. 32 LUT in the COLORMAP lump.
 #define NUMCOLORMAPS 32
 
+INT32 R_OldPointOnSide(fixed_t x, fixed_t y, node_t *node);
+INT32 R_OldPointOnSegSide(fixed_t x, fixed_t y, seg_t *line);
+
 // Utility functions.
 static inline INT32 R_PointOnSide(fixed_t x, fixed_t y, node_t *node)
 {
 	// use cross product to determine side quickly
-	return ((INT64)y - node->y) * node->dx - ((INT64)x - node->x) * node->dy > 0;
+	INT64 v = ((INT64)y - node->y) * node->dx - ((INT64)x - node->x) * node->dy;
+	if (v == 0) // if we're on the line, use the old algorithm
+		return R_OldPointOnSide(x, y, node);
+	return v > 0;
 }
 
 static inline INT32 R_PointOnSegSide(fixed_t x, fixed_t y, seg_t *line)
@@ -86,7 +92,10 @@ static inline INT32 R_PointOnSegSide(fixed_t x, fixed_t y, seg_t *line)
 	fixed_t ldy = line->v2->y - ly;
 
 	// use cross product to determine side quickly
-	return ((INT64)y - ly) * ldx - ((INT64)x - lx) * ldy > 0;
+	INT64 v = ((INT64)y - ly) * ldx - ((INT64)x - lx) * ldy > 0;
+	if (v == 0) // if we're on the line, use the old algorithm
+		return R_OldPointOnSegSide(x, y, line);
+	return v > 0;
 }
 
 angle_t R_PointToAngle(fixed_t x, fixed_t y);