fix #K3924 - teleport as giant eel

The safe_teleds() change that restored picking random destination
attempts prior to making an exhaustive search contained a typo tjat
accidentally only accepted invalid positions instead of valid ones.
So unless it randomly picked 40 good spots, erroneously rejecting
all of them and then falling back to the try-everywhere situation
(which has its own testing without any typo), it would yield strange
results by placing the hero in walls or solid rock via choosing the
first inappropriate spot it tried.

Not part of that bug but related, sort of:  for rloc(), use
rloc_pos_ok() instead of goodpos() during the exhaustive search as
well as during the random tries, but hang on to the first (after
randomization) position that passes goodpos() for a last resort.

The collect_coords() flag for 'skip-inaccessible' intended to be a
quick way to filter out walls and solid rock was using !ACCESSIBLE()
which also rejects water and lava locations.  So such spots wouldn't
be picked by either safe_teleds() or rloc() when they were finding
a spot for aquatic or lava-tolerant forms.  Instead of duplicating
a bunch of code to decide whether the hero's current form or the
teleporting monster should avoid !ACCESSIBLE() for a reason other
than having Passes_walls, make collect_coords(CC_SKIP_INACCS) use
!ZAP_POS() which rejects walls and rock but allows pools.

src/teleport.c

@@ -1,4 +1,4 @@
-/* NetHack 3.7	teleport.c	$NHDT-Date: 1684140122 2023/05/15 08:42:02 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.200 $ */
+/* NetHack 3.7	teleport.c	$NHDT-Date: 1684374686 2023/05/18 01:51:26 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.202 $ */
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /*-Copyright (c) Robert Patrick Rankin, 2011. */
 /* NetHack may be freely redistributed.  See license for details. */
@@ -493,7 +493,7 @@ collect_coords(
                              * ring_pairs: shuffle pairs of rings together
                              * instead of keeping each ring distinct;
                              * skip_mons: reject occupied spots;
-                             * skip_inaccs: reject !ACCESSIBLE() spots */
+                             * skip_inaccs: reject !ZAP_POS() spots */
     boolean (*filter)(coordxy, coordxy)) /* if Null, no filtering */
 {
     coordxy x, y, lox, hix, loy, hiy;
@@ -508,7 +508,7 @@ collect_coords(
             ring_pairs = (scramble && (cc_flags & CC_RING_PAIRS) != 0),
             /* exclude locations containing monsters from output */
             skip_mons = (cc_flags & CC_SKIP_MONS) != 0,
-            /* exclude !ACCESSIBLE() locations from output */
+            /* exclude !ZAP_POS() locations from output; allows pools+lava */
             skip_inaccessible = (cc_flags & CC_SKIP_INACCS) != 0;
     int result = 0;
 
@@ -589,7 +589,9 @@ collect_coords(
                 if (x != lox && x != hix && y != loy && y != hiy)
                     continue; /* not any edge of ring/square */
                 if ((skip_mons && m_at(x, y))
-                    || (skip_inaccessible && !ACCESSIBLE(levl[x][y].typ)))
+                    /* note: !ACCESSIBLE() would reject water and lava;
+                       !ZAP_POS() accepts them; caller needs to handle such */
+                    || (skip_inaccessible && !ZAP_POS(levl[x][y].typ)))
                     continue; /* quick filters */
                 if (filter && !(*filter)(x, y))
                     continue;
@@ -641,7 +643,7 @@ safe_teleds(int teleds_flags)
     for (tcnt = 0; tcnt < 40; ++tcnt) {
         nux = rnd(COLNO - 1);
         nuy = rn2(ROWNO);
-        if (!teleok(nux, nuy, FALSE)) {
+        if (teleok(nux, nuy, FALSE)) {
             teleds(nux, nuy, teleds_flags);
             return TRUE;
         }
@@ -1630,7 +1632,7 @@ rloc(
     struct monst *mtmp, /* mtmp->mx==0 implies migrating monster arrival */
     unsigned rlocflags)
 {
-    coord cc, candy[ROWNO * (COLNO - 1)]; /* room for entire map */
+    coord cc, backupcc, candy[ROWNO * (COLNO - 1)]; /* room for entire map */
     unsigned cc_flags;
     coordxy x, y;
     int trycount, i, j, candycount;
@@ -1683,6 +1685,7 @@ rloc(
         cc_flags |= CC_SKIP_INACCS;
     candycount = collect_coords(candy, COLNO / 2, ROWNO / 2, 0, cc_flags,
                                 (boolean (*)(coordxy, coordxy)) 0);
+    backupcc.x = backupcc.y = 0;
     for (i = 0; i < candycount; ++i) {
         if ((j = rn2(candycount - i)) > 0) {
             cc = candy[i];
@@ -1690,15 +1693,24 @@ rloc(
             candy[i + j] = cc;
         }
         x = candy[i].x, y = candy[i].y;
-        if (goodpos(x, y, mtmp, NO_MM_FLAGS)) /* accepts 'onscary' */
+        if (rloc_pos_ok(x, y, mtmp))
             goto found_xy;
+        if (!backupcc.x && goodpos(x, y, mtmp, NO_MM_FLAGS))
+            backupcc.x = x, backupcc.y = y;
     }
 
-    /* level either full of monsters or somehow faulty */
-    if ((rlocflags & RLOC_ERR) != 0)
-        impossible("rloc(): couldn't relocate monster");
-    return FALSE;
-
+    /* we didn't find any spot acceptable to rloc_pos_ok() which avoids
+       'onscary' and honors teleport regions, but if we did find a spot
+       that was acceptable to goodpos() (which ignores 'onscary' and
+       teleport regions) we'll use that; otherwise give up */
+    if (!backupcc.x) {
+        /* level either full of monsters or somehow faulty */
+        if ((rlocflags & RLOC_ERR) != 0)
+            impossible("rloc(): couldn't relocate monster");
+        return FALSE;
+    }
+    x = backupcc.x, y = backupcc.y;
+    /*goto found_xy;*/
  found_xy:
     rloc_to_core(mtmp, x, y, rlocflags);
     return TRUE;