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.

include/decl.h

@@ -75,16 +75,6 @@ struct dgn_topology { /* special dungeon levels for speed */
 #define sokoend_level           (g.dungeon_topology.d_sokoend_level)
 /* clang-format on */
 
-#define xdnstair (g.dnstair.sx)
-#define ydnstair (g.dnstair.sy)
-#define xupstair (g.upstair.sx)
-#define yupstair (g.upstair.sy)
-
-#define xdnladder (g.dnladder.sx)
-#define ydnladder (g.dnladder.sy)
-#define xupladder (g.upladder.sx)
-#define yupladder (g.upladder.sy)
-
 #define dunlev_reached(x) (g.dungeons[(x)->dnum].dunlev_ureached)
 
 #include "quest.h"
@@ -726,10 +716,7 @@ struct instance_globals {
     int y_maze_max;
     int otg_temp; /* used by object_to_glyph() [otg] */
     int in_doagain;
-    stairway dnstair; /* stairs down */
-    stairway upstair; /* stairs up */
-    stairway dnladder; /* ladder down */
-    stairway upladder; /* ladder up */
+    stairway *stairs;
     int smeq[MAXNROFROOMS + 1];
     int doorindex;
     char *save_cm;
@@ -754,7 +741,6 @@ struct instance_globals {
        number of shots, index of current one, validity check, shoot vs throw */
     struct multishot m_shot;
     dungeon dungeons[MAXDUNGEON]; /* ini'ed by init_dungeon() */
-    stairway sstairs;
     dest_area updest;
     dest_area dndest;
     coord inv_pos;
@@ -765,9 +751,6 @@ struct instance_globals {
     boolean mrg_to_wielded; /* weapon picked is merged with wielded one */
     struct plinemsg_type *plinemsg_types;
     char toplines[TBUFSZ];
-    struct mkroom *upstairs_room;
-    struct mkroom *dnstairs_room;
-    struct mkroom *sstairs_room;
     coord bhitpos; /* place where throw or zap hits or stops */
     boolean in_steed_dismounting;
     coord doors[DOORMAX];

include/dungeon.h

@@ -34,7 +34,9 @@ typedef struct s_level { /* special dungeon level element */
 typedef struct stairway { /* basic stairway identifier */
     xchar sx, sy;         /* x / y location of the stair */
     d_level tolev;        /* where does it go */
-    char up;              /* what type of stairway (up/down) */
+    boolean up;           /* up or down? */
+    boolean isladder;     /* ladder or stairway? */
+    struct stairway *next;
 } stairway;
 
 /* level region types */

include/extern.h

@@ -630,6 +630,15 @@ E void FDECL(next_level, (BOOLEAN_P));
 E void FDECL(prev_level, (BOOLEAN_P));
 E void FDECL(u_on_newpos, (int, int));
 E void FDECL(u_on_rndspot, (int));
+E void FDECL(stairway_add, (int,int, BOOLEAN_P, BOOLEAN_P, d_level *));
+E void NDECL(stairway_print);
+E void NDECL(stairway_free_all);
+E stairway *FDECL(stairway_at, (int, int));
+E stairway *FDECL(stairway_find, (d_level *));
+E stairway *FDECL(stairway_find_from, (d_level *, BOOLEAN_P));
+E stairway *FDECL(stairway_find_dir, (BOOLEAN_P));
+E stairway *FDECL(stairway_find_type_dir, (BOOLEAN_P, BOOLEAN_P));
+E stairway *FDECL(stairway_find_special_dir, (BOOLEAN_P));
 E void FDECL(u_on_sstairs, (int));
 E void NDECL(u_on_upstairs);
 E void NDECL(u_on_dnstairs);

include/monst.h

@@ -82,6 +82,7 @@ struct monst {
     xchar mx, my;
     xchar mux, muy;       /* where the monster thinks you are */
 #define MTSZ 4
+    /* mtrack[0..2] is used to keep extra data when migrating the monster */
     coord mtrack[MTSZ];   /* monster track */
     int mhp, mhpmax;
     unsigned mappearance; /* for undetected mimics and the wiz */

include/obj.h

@@ -122,6 +122,8 @@ struct obj {
     long age;               /* creation date */
     long owornmask;
     unsigned lua_ref_cnt;  /* # of lua script references for this object */
+    xchar omigr_from_dnum; /* where obj is migrating from */
+    xchar omigr_from_dlevel; /* where obj is migrating from */
     struct oextra *oextra; /* pointer to oextra struct */
 };
 

include/patchlevel.h

@@ -17,7 +17,7 @@
  * Incrementing EDITLEVEL can be used to force invalidation of old bones
  * and save files.
  */
-#define EDITLEVEL 24
+#define EDITLEVEL 25
 
 /*
  * Development status possibilities.

include/rm.h

@@ -243,6 +243,8 @@ enum screen_symbols {
 #define is_cmap_furniture(i) ((i) >= S_upstair && (i) <= S_fountain)
 #define is_cmap_water(i) ((i) == S_pool || (i) == S_water)
 #define is_cmap_lava(i) ((i) == S_lava)
+#define is_cmap_stairs(i) ((i) == S_upstair || (i) == S_dnstair || \
+                           (i) == S_upladder || (i) == S_dnladder)
 
 
 struct symdef {