Apply dest. limit to monster trap door usage

To prevent monsters from falling past the bottom level or into the
sanctum early, and to maintain consistency between monster and hero hole
usage.

include/extern.h

@@ -2796,6 +2796,7 @@ extern boolean burnarmor(struct monst *);
 extern int erode_obj(struct obj *, const char *, int, int);
 extern boolean grease_protect(struct obj *, const char *, struct monst *);
 extern struct trap *maketrap(coordxy, coordxy, int);
+extern d_level *clamp_hole_destination(d_level *);
 extern void fall_through(boolean, unsigned);
 extern struct monst *animate_statue(struct obj *, coordxy, coordxy, int, int *);
 extern struct monst *activate_statue_trap(struct trap *, coordxy, coordxy,

src/teleport.c

@@ -1540,6 +1540,7 @@ mlevel_tele_trap(
                 return Trap_Effect_Finished;
             } else {
                 assign_level(&tolevel, &trap->dst);
+                (void) clamp_hole_destination(&tolevel);
             }
         } else if (tt == MAGIC_PORTAL) {
             if (In_endgame(&u.uz) && (mon_has_amulet(mtmp)

src/trap.c

@@ -512,6 +512,20 @@ maketrap(coordxy x, coordxy y, int typ)
     return ttmp;
 }
 
+/* limit the destination of a hole or trapdoor to the furthest level you
+   should be able to fall to */
+d_level *
+clamp_hole_destination(d_level *dlev)
+{
+    int bottom = dng_bottom(dlev);
+    dlev->dlevel = min(dlev->dlevel, bottom);
+    if (In_hell(dlev) && !u.uevent.invoked
+        && dlev->dlevel == bottom)
+        dlev->dlevel--;
+
+    return dlev;
+}
+
 void
 fall_through(
     boolean td, /* td == TRUE : trap door or hole */
@@ -585,20 +599,15 @@ fall_through(
         int dist;
 
         if (t) {
-            dtmp.dnum = t->dst.dnum;
+            assign_level(&dtmp, &t->dst);
             /* don't fall beyond the bottom, in case this came from a bones
                file with different dungeon size  */
-            bottom = dng_bottom(&t->dst);
+            (void) clamp_hole_destination(&dtmp);
-            dtmp.dlevel = min(t->dst.dlevel, bottom);
-            if (In_hell(&t->dst) && !u.uevent.invoked
-                && dtmp.dlevel == bottom)
-                dtmp.dlevel--;
         } else {
             dtmp.dnum = u.uz.dnum;
             dtmp.dlevel = newlevel;
         }
-        /* XXX: dist won't be accurate if dtmp.dnum may not match u.uz.dnum */
+        dist = depth(&dtmp) - depth(&u.uz);
-        dist = dtmp.dlevel - dunlev(&u.uz);
         if (dist > 1)
             You("fall down a %s%sshaft!", dist > 3 ? "very " : "",
                 dist > 2 ? "deep " : "");