Rework stairs structure

Use a linked list to store stair and ladder information, instead
of having fixed up/down stairs/ladders and a single "special" (branch)
stair.

Breaks saves and bones.

Adds information to migrating objects and monsters for the dungeon
and level where they are migrating from.

src/dokick.c

@@ -19,7 +19,7 @@ static int FDECL(kick_object, (XCHAR_P, XCHAR_P, char *));
 static int FDECL(really_kick_object, (XCHAR_P, XCHAR_P));
 static char *FDECL(kickstr, (char *, const char *));
 static void FDECL(otransit_msg, (struct obj *, BOOLEAN_P, long));
-static void FDECL(drop_to, (coord *, SCHAR_P));
+static void FDECL(drop_to, (coord *, SCHAR_P, XCHAR_P, XCHAR_P));
 
 static const char kick_passes_thru[] = "kick passes harmlessly through";
 
@@ -1322,10 +1322,13 @@ dokick()
 }
 
 static void
-drop_to(cc, loc)
+drop_to(cc, loc, x,y)
 coord *cc;
 schar loc;
+xchar x,y;
 {
+    stairway *stway = stairway_at(x, y);
+
     /* cover all the MIGR_xxx choices generated by down_gate() */
     switch (loc) {
     case MIGR_RANDOM: /* trap door or hole */
@@ -1340,12 +1343,14 @@ schar loc;
         /*FALLTHRU*/
     case MIGR_STAIRS_UP:
     case MIGR_LADDER_UP:
-        cc->x = u.uz.dnum;
-        cc->y = u.uz.dlevel + 1;
-        break;
     case MIGR_SSTAIRS:
-        cc->x = g.sstairs.tolev.dnum;
-        cc->y = g.sstairs.tolev.dlevel;
+        if (stway) {
+            cc->x = stway->tolev.dnum;
+            cc->y = stway->tolev.dlevel;
+        } else {
+            cc->x = u.uz.dnum;
+            cc->y = u.uz.dlevel + 1;
+        }
         break;
     default:
     case MIGR_NOWHERE:
@@ -1373,7 +1378,7 @@ xchar dlev;          /* if !0 send to dlev near player */
         return;
 
     toloc = down_gate(x, y);
-    drop_to(&cc, toloc);
+    drop_to(&cc, toloc, x, y);
     if (!cc.y)
         return;
 
@@ -1502,7 +1507,7 @@ boolean shop_floor_obj;
         return FALSE;
     if ((toloc = down_gate(x, y)) == MIGR_NOWHERE)
         return FALSE;
-    drop_to(&cc, toloc);
+    drop_to(&cc, toloc, x, y);
     if (!cc.y)
         return FALSE;
 
@@ -1586,6 +1591,7 @@ boolean shop_floor_obj;
     otmp->ox = cc.x;
     otmp->oy = cc.y;
     otmp->owornmask = (long) toloc;
+
     /* boulder from rolling boulder trap, no longer part of the trap */
     if (otmp->otyp == BOULDER)
         otmp->otrapped = 0;
@@ -1614,6 +1620,9 @@ boolean near_hero;
     register int nx, ny;
     int where;
     boolean nobreak, noscatter;
+    stairway *stway;
+    d_level fromdlev;
+    boolean isladder;
 
     for (otmp = g.migrating_objs; otmp; otmp = otmp2) {
         otmp2 = otmp->nobj;
@@ -1633,16 +1642,21 @@ boolean near_hero;
 
         obj_extract_self(otmp);
         otmp->owornmask = 0L;
+        fromdlev.dnum = otmp->omigr_from_dnum;
+        fromdlev.dlevel = otmp->omigr_from_dlevel;
+
+        isladder = FALSE;
 
         switch (where) {
-        case MIGR_STAIRS_UP:
-            nx = xupstair, ny = yupstair;
-            break;
         case MIGR_LADDER_UP:
-            nx = xupladder, ny = yupladder;
-            break;
+            isladder = TRUE;
+        case MIGR_STAIRS_UP:
         case MIGR_SSTAIRS:
-            nx = g.sstairs.sx, ny = g.sstairs.sy;
+            if ((stway = stairway_find_from(&fromdlev, isladder)) != 0) {
+                nx = stway->sx;
+                nx = stway->sy;
+            }
+            break;
             break;
         case MIGR_WITH_HERO:
             nx = u.ux, ny = u.uy;
@@ -1652,6 +1666,8 @@ boolean near_hero;
             nx = ny = 0;
             break;
         }
+        otmp->omigr_from_dnum = 0;
+        otmp->omigr_from_dlevel = 0;
         if (nx > 0) {
             place_object(otmp, nx, ny);
             if (!nobreak && !IS_SOFT(levl[nx][ny].typ)) {
@@ -1727,6 +1743,8 @@ unsigned long deliverflags;
                 free_oname(otmp);
             }
             otmp->migr_species = NON_PM;
+            otmp->omigr_from_dnum = 0;
+            otmp->omigr_from_dlevel = 0;
             (void) add_to_minv(mtmp, otmp);
             cnt++;
             if (maxobj && cnt >= maxobj)
@@ -1773,19 +1791,19 @@ down_gate(x, y)
 xchar x, y;
 {
     struct trap *ttmp;
+    stairway *stway = stairway_at(x, y);
 
     g.gate_str = 0;
     /* this matches the player restriction in goto_level() */
     if (on_level(&u.uz, &qstart_level) && !ok_to_quest())
         return MIGR_NOWHERE;
 
-    if ((xdnstair == x && ydnstair == y)
-        || (g.sstairs.sx == x && g.sstairs.sy == y && !g.sstairs.up)) {
+    if (stway && !stway->up && !stway->isladder) {
         g.gate_str = "down the stairs";
-        return (xdnstair == x && ydnstair == y) ? MIGR_STAIRS_UP
-                                                : MIGR_SSTAIRS;
+        return (stway->tolev.dnum == u.uz.dnum) ? MIGR_STAIRS_UP
+                                : MIGR_SSTAIRS;
     }
-    if (xdnladder == x && ydnladder == y) {
+    if (stway && !stway->up && stway->isladder) {
         g.gate_str = "down the ladder";
         return MIGR_LADDER_UP;
     }