Better check of monsters near moving sector

Just check monsters on the periphery of a moving sector instead of all
in bounding box of the sector. Both more accurate and faster. Taken from
PrBoom+ source.

src/p_map.c

@@ -1460,20 +1460,43 @@ boolean PIT_ChangeSector(mobj_t *thing)
 
 
 //
-// P_ChangeSector
+// P_CheckSector
+// jff 3/19/98 added to just check monsters on the periphery
+// of a moving sector instead of all in bounding box of the
+// sector. Both more accurate and faster.
 //
 boolean P_ChangeSector(sector_t *sector, boolean crunch)
 {
-    int         x;
-    int         y;
+    mobj_t *thing;
 
     nofit = false;
     crushchange = crunch;
 
-    // re-check heights for all things near the moving sector
-    for (x = sector->blockbox[BOXLEFT]; x <= sector->blockbox[BOXRIGHT]; x++)
-        for (y = sector->blockbox[BOXBOTTOM]; y <= sector->blockbox[BOXTOP]; y++)
-            P_BlockThingsIterator(x, y, PIT_ChangeSector);
+    // killough 4/4/98: scan list front-to-back until empty or exhausted,
+    // restarting from beginning after each thing is processed. Avoids
+    // crashes, and is sure to examine all things in the sector, and only
+    // the things which are in the sector, until a steady-state is reached.
+    // Things can arbitrarily be inserted and removed and it won't mess up.
+    //
+    // killough 4/7/98: simplified to avoid using complicated counter
+
+    // Mark all things invalid
+
+    for (thing = sector->thinglist; thing; thing = thing->snext)
+        thing->visited = false;
+
+    do
+    {
+        for (thing = sector->thinglist; thing; thing = thing->snext) // go through list
+            if (!thing->visited)                        // unprocessed thing found
+            {
+                thing->visited = true;                  // mark thing as processed
+                if (!(thing->flags & MF_NOBLOCKMAP))    // jff 4/7/98 don't do these
+                    PIT_ChangeSector(thing);            // process it
+                break;                                  // exit and start over
+            }
+    }
+    while (thing);      // repeat from scratch until all things left are marked valid
 
     return nofit;
 }

src/p_mobj.h

@@ -333,6 +333,8 @@ typedef struct mobj_s
     int                bobdirection;
     int                bobcount;
 
+    boolean            visited;
+
 } mobj_t;