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.

doc/fixes36.1

@@ -314,6 +314,8 @@ for #tip inside shop, credit was incorrectly given for spilled gold if that
 	gold's stale location coordinates didn't happen to be inside the shop
 when confused scroll of light summoned lights, player would be asked what to
 	call the scroll even if scroll of light was already identified
+if a visible monster becomes invisible, mark its spot with the 'remembered,
+	unseen monster' glyph ('I' character or '?' tile)
 
 
 Fixes to Post-3.6.0 Problems that Were Exposed Via git Repository

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;