observing monster become invisible

Requested by one of the beta testers 13 months ago... when a visible
monster becomes invisible and vanishes, mark its map location with
the remembered, unseen monster glyph.  (When the player zaps a
monster with a wand of make invisible, that only happens if the wand
type is known.  I'm not sure that's right but didn't alter it....)

The request suggested also doing it for a monster who disappears by
teleporting away, but I haven't attempted to implement that.

src/mcastu.c

@@ -443,6 +443,8 @@ int spellnum;
                 pline("%s suddenly %s!", Monnam(mtmp),
                       !See_invisible ? "disappears" : "becomes transparent");
             mon_set_minvis(mtmp);
+            if (cansee(mtmp->mx, mtmp->my) && !canspotmon(mtmp))
+                map_invisible(mtmp->mx, mtmp->my);
             dmg = 0;
         } else
             impossible("no reason for monster to cast disappear spell?");

src/muse.c

@@ -1207,8 +1207,8 @@ register struct monst *mtmp;
 register struct obj *otmp;
 {
     int tmp;
-
     boolean reveal_invis = FALSE;
+
     if (mtmp != &youmonst) {
         mtmp->msleeping = 0;
         if (mtmp->m_ap_type)
@@ -1819,12 +1819,15 @@ struct monst *mtmp;
         Strcpy(nambuf, mon_nam(mtmp));
         mon_set_minvis(mtmp);
         if (vismon && mtmp->minvis) { /* was seen, now invisible */
-            if (canspotmon(mtmp))
+            if (canspotmon(mtmp)) {
                 pline("%s body takes on a %s transparency.",
                       upstart(s_suffix(nambuf)),
                       Hallucination ? "normal" : "strange");
-            else
+            } else {
                 pline("Suddenly you cannot see %s.", nambuf);
+                if (vis)
+                    map_invisible(mtmp->mx, mtmp->my);
+            }
             if (oseen)
                 makeknown(otmp->otyp);
         }

src/potion.c

@@ -1379,10 +1379,15 @@ boolean your_fault;
             if (!resist(mon, POTION_CLASS, 0, NOTELL))
                 mon->mconf = TRUE;
             break;
-        case POT_INVISIBILITY:
+        case POT_INVISIBILITY: {
+            boolean sawit = canspotmon(mon);
+
             angermon = FALSE;
             mon_set_minvis(mon);
+            if (sawit && !canspotmon(mon) && cansee(mon->mx, mon->my))
+                map_invisible(mon->mx, mon->my);
             break;
+        }
         case POT_SLEEPING:
             /* wakeup() doesn't rouse victims of temporary sleep */
             if (sleep_monst(mon, rnd(12), POTION_CLASS)) {

src/teleport.c

@@ -956,11 +956,9 @@ struct monst *mtmp;
  *
  * Pulls a monster from its current position and places a monster at
  * a new x and y.  If oldx is 0, then the monster was not in the
- * levels.monsters
- * array.  However, if oldx is 0, oldy may still have a value because mtmp is
- * a
- * migrating_mon.  Worm tails are always placed randomly around the head of
- * the worm.
+ * levels.monsters array.  However, if oldx is 0, oldy may still have
+ * a value because mtmp is a migrating_mon.  Worm tails are always
+ * placed randomly around the head of the worm.
  */
 void
 rloc_to(mtmp, x, y)
@@ -974,15 +972,15 @@ register int x, y;
         return;
 
     if (oldx) { /* "pick up" monster */
-        if (mtmp->wormno)
+        if (mtmp->wormno) {
             remove_worm(mtmp);
-        else {
+        } else {
             remove_monster(oldx, oldy);
             newsym(oldx, oldy); /* update old location */
         }
     }
 
-    memset(mtmp->mtrack, 0, sizeof(mtmp->mtrack));
+    memset(mtmp->mtrack, 0, sizeof mtmp->mtrack);
     place_monster(mtmp, x, y); /* put monster down */
     update_monster_region(mtmp);
 

src/zap.c

@@ -280,6 +280,7 @@ struct obj *otmp;
         mon_set_minvis(mtmp);
         if (!oldinvis && knowninvisible(mtmp)) {
             pline("%s turns transparent!", nambuf);
+            reveal_invis = TRUE;
             learn_it = TRUE;
         }
         break;