adopt github pull request #286 - rndmonst()

Eliminate the cache that was supporting rndmonst() and pick a random
monster in a single pass through mons[] via "weighted reservoir
sampling", a term I'm not familiar with.

It had a couple of bugs:  if the first monster examined happened to
be given a weighting of 0, rn2() would divide by 0.  I didn't try
to figure out how to trigger that.  But the second one was easy to
trigger:  if all eligible monsters were extinct or genocided, it
would issue a warning even though the situation isn't impossible.

Aside from fixing those, the rest is mostly as-is.  I included a bit
of formatting in decl.c, moved some declarations to not require C99,
and changed a couple of macros to not hide and duplicate a call to
level_difficulty().

Fixes #286

include/decl.h

@@ -886,18 +886,19 @@ struct instance_globals {
     boolean chosen_symset_end;
     int symset_which_set;
     /* SAVESIZE, BONESSIZE, LOCKNAMESIZE are defined in "fnamesiz.h" */
-    char SAVEF[SAVESIZE]; /* holds relative path of save file from playground */
+    char SAVEF[SAVESIZE]; /* relative path of save file from playground */
 #ifdef MICRO
     char SAVEP[SAVESIZE]; /* holds path of directory for save file */
 #endif
     char bones[BONESSIZE];
     char lock[LOCKNAMESIZE];
 
-
     /* hack.c */
     anything tmp_anything;
     int wc; /* current weight_cap(); valid after call to inv_weight() */
 
+    /* insight.c */
+
     /* invent.c */
     int lastinvnr;  /* 0 ... 51 (never saved&restored) */
     unsigned sortlootmode; /* set by sortloot() for use by sortloot_cmp(); 
@@ -905,7 +906,7 @@ struct instance_globals {
     char *invbuf;
     unsigned invbufsiz;
     /* for perm_invent when operating on a partial inventory display, so that
-       the persistent one doesn't get shrunk during filtering for item selection
+       persistent one doesn't get shrunk during filtering for item selection
        then regrown to full inventory, possibly being resized in the process */
     winid cached_pickinv_win;
     /* query objlist callback: return TRUE if obj type matches "this_type" */
@@ -916,15 +917,10 @@ struct instance_globals {
     /* light.c */
     light_source *light_base;
 
-
     /* lock.c */
     struct xlock_s xlock;
 
     /* makemon.c */
-    struct {
-        int choice_count;
-        char mchoices[SPECIAL_PM]; /* value range is 0..127 */
-    } rndmonst_state;
 
     /* mhitm.c */
     boolean vis;

include/extern.h

@@ -1216,7 +1216,6 @@ E void FDECL(dealloc_mextra, (struct monst *));
 E struct monst *FDECL(makemon, (struct permonst *, int, int, int));
 E boolean FDECL(create_critters, (int, struct permonst *, BOOLEAN_P));
 E struct permonst *NDECL(rndmonst);
-E void FDECL(reset_rndmonst, (int));
 E struct permonst *FDECL(mkclass, (CHAR_P, int));
 E struct permonst *FDECL(mkclass_aligned, (CHAR_P, int, ALIGNTYP_P));
 E int FDECL(mkclass_poly, (int));

include/monst.h

@@ -201,4 +201,13 @@ struct monst {
 #define is_obj_mappear(mon,otyp) (M_AP_TYPE(mon) == M_AP_OBJECT \
                                   && (mon)->mappearance == (otyp))
 
+/* Get the maximum difficulty monsters that can currently be generated,
+   given the current level difficulty and the hero's level. */
+#define monmax_difficulty(levdif) (((levdif) + u.ulevel) / 2)
+#define monmin_difficulty(levdif) ((levdif) / 6)
+
+/* Macros for whether a type of monster is too strong for a specific level. */
+#define montoostrong(monindx, lev) (mons[monindx].difficulty > lev)
+#define montooweak(monindx, lev) (mons[monindx].difficulty < lev)
+
 #endif /* MONST_H */