From 90cad80422b422038dea28260ffec589d82f6b07 Mon Sep 17 00:00:00 2001
From: Christoph Oelckers <coelckers@zdoom.fake>
Date: Thu, 3 Mar 2016 02:48:05 +0100
Subject: [PATCH] - fixed some issues with FPathTraverse:

 * when starting directly on a blockline the trace was offset by one map unit. Do this only for the internal block trace calculations but not for the variable that's being exposed to the outside because in rare situations that can create incorrect values.
 * using startfrac could lead to an actor whose inside was right at that positon to be missed.
 * when using startfrac the adjusted trace start was not used for all calculations, which could cause the trace to fail.

With these issues fixed, P_AimLineAttack can now successfully navigate line portals.
---
 src/p_maputl.cpp | 61 +++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 48 insertions(+), 13 deletions(-)

diff --git a/src/p_maputl.cpp b/src/p_maputl.cpp
index 11284a7bf8..2a8c0efda1 100644
--- a/src/p_maputl.cpp
+++ b/src/p_maputl.cpp
@@ -1287,9 +1287,33 @@ void FPathTraverse::AddThingIntercepts (int bx, int by, FBlockThingsIterator &it
 						fixed_t frac = P_InterceptVector (&trace, &line);
 						if (frac < startfrac)
 						{ // behind source
+							if (startfrac > 0)
+							{
+								// check if the trace starts within this actor
+								switch (i)
+								{
+								case 0:
+									line.y -= 2 * thing->radius;
+									break;
+
+								case 1:
+									line.x -= 2 * thing->radius;
+									break;
+
+								case 2:
+									line.y += 2 * thing->radius;
+									break;
+
+								case 3:
+									line.x += 2 * thing->radius;
+									break;
+								}
+								fixed_t frac2 = P_InterceptVector(&trace, &line);
+								if (frac2 >= startfrac) goto addit;
+							}
 							continue;
 						}
-
+					addit:
 						intercept_t newintercept;
 						newintercept.frac = frac;
 						newintercept.isaline = false;
@@ -1422,16 +1446,6 @@ void FPathTraverse::init (fixed_t x1, fixed_t y1, fixed_t x2, fixed_t y2, int fl
 	int 		mapystep;
 
 	int 		count;
-				
-	validcount++;
-	intercept_index = intercepts.Size();
-	this->startfrac = startfrac;
-		
-	if ( ((x1-bmaporgx)&(MAPBLOCKSIZE-1)) == 0)
-		x1 += FRACUNIT; // don't side exactly on a line
-	
-	if ( ((y1-bmaporgy)&(MAPBLOCKSIZE-1)) == 0)
-		y1 += FRACUNIT; // don't side exactly on a line
 
 	trace.x = x1;
 	trace.y = y1;
@@ -1445,9 +1459,30 @@ void FPathTraverse::init (fixed_t x1, fixed_t y1, fixed_t x2, fixed_t y2, int fl
 		trace.dx = x2 - x1;
 		trace.dy = y2 - y1;
 	}
+	if (startfrac > 0)
+	{
+		fixed_t startdx = FixedMul(trace.dx, startfrac);
+		fixed_t startdy = FixedMul(trace.dy, startfrac);
 
-	_x1 = (long long)x1 + FixedMul(trace.dx, startfrac) - bmaporgx;
-	_y1 = (long long)y1 + FixedMul(trace.dy, startfrac) - bmaporgy;
+		x1 += startdx;
+		y1 += startdy;
+		x2 = trace.dx - startdx;
+		y2 = trace.dy - startdy;
+		flags |= PT_DELTA;
+	}
+
+	validcount++;
+	intercept_index = intercepts.Size();
+	this->startfrac = startfrac;
+
+	if ( ((x1-bmaporgx)&(MAPBLOCKSIZE-1)) == 0)
+		x1 += FRACUNIT; // don't side exactly on a line
+	
+	if ( ((y1-bmaporgy)&(MAPBLOCKSIZE-1)) == 0)
+		y1 += FRACUNIT; // don't side exactly on a line
+
+	_x1 = (long long)x1 - bmaporgx;
+	_y1 = (long long)y1 - bmaporgy;
 	x1 -= bmaporgx;
 	y1 -= bmaporgy;
 	xt1 = int(_x1 >> MAPBLOCKSHIFT);