Purple worm changes

Shriekers only spawn purple worms when they're appropriate difficulty.
Non-tame Purple worms eat corpses off the ground.
Baby purple worms attack shriekers.
Hero polyed into baby purple worm is warned against shriekers.

Original changes by copperwater <aosdict@gmail.com>, added with some
formatting adjustments and consolidation.

doc/fixes37.0

@@ -103,6 +103,10 @@ for farlook, describe water in the castle moat and in Juiblex's swamp as moat
 make hezrous emit poison clouds when they move
 stepping from one type of terrain to another was triggering an unnecessary
 	status update
+make shriekers summon baby purple worms if purple worms would be too tough
+make non-tame (baby) purple worms eat corpses off the ground
+make baby purple worms attack shriekers
+make hero polymorphed into baby purple worm warned against shriekers
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository

include/extern.h

@@ -1458,6 +1458,7 @@ E int FDECL(minliquid, (struct monst *));
 E int NDECL(movemon);
 E int FDECL(meatmetal, (struct monst *));
 E int FDECL(meatobj, (struct monst *));
+E int FDECL(meatcorpse, (struct monst *));
 E void FDECL(mpickgold, (struct monst *));
 E boolean FDECL(mpickstuff, (struct monst *, const char *));
 E int FDECL(curr_mon_load, (struct monst *));

include/monst.h

@@ -205,6 +205,7 @@ struct monst {
    given the current level difficulty and the hero's level. */
 #define monmax_difficulty(levdif) (((levdif) + u.ulevel) / 2)
 #define monmin_difficulty(levdif) ((levdif) / 6)
+#define monmax_difficulty_lev() (monmax_difficulty(level_difficulty()))
 
 /* Macros for whether a type of monster is too strong for a specific level. */
 #define montoostrong(monindx, lev) (mons[monindx].difficulty > lev)

src/mon.c

@@ -1143,6 +1143,69 @@ struct monst *mtmp;
     return (count > 0 || ecount > 0) ? 1 : 0;
 }
 
+/* Monster eats a corpse off the ground.
+ * Return value is 0 = nothing eaten, 1 = ate a corpse, 2 = died */
+int
+meatcorpse(mtmp) /* for purple worms and other voracious monsters */
+struct monst* mtmp;
+{
+    struct obj *otmp;
+    struct permonst *ptr, *original_ptr = mtmp->data, *corpsepm;
+    boolean poly, grow, heal, eyes = FALSE;
+    int x = mtmp->mx, y = mtmp->my;
+
+    /* if a pet, eating is handled separately, in dog.c */
+    if (mtmp->mtame)
+        return 0;
+
+    for (otmp = g.level.objects[x][y]; otmp; otmp = otmp->nexthere) {
+        if (otmp->otyp != CORPSE)
+            continue;
+
+        corpsepm = &mons[otmp->corpsenm];
+        if (is_rider(corpsepm)) {
+            revive_corpse(otmp);
+            continue;
+        }
+
+        /* don't eat harmful corpses */
+        if (touch_petrifies(corpsepm) && !resists_ston(mtmp))
+            continue;
+
+        if (cansee(x, y) && canseemon(mtmp)) {
+            if (flags.verbose)
+                pline("%s eats %s!", Monnam(mtmp), distant_name(otmp, doname));
+        } else if (flags.verbose)
+            You_hear("a masticating sound.");
+
+        poly = polyfodder(otmp);
+        grow = mlevelgain(otmp);
+        heal = mhealup(otmp);
+        eyes = (otmp->otyp == CARROT);
+        delobj(otmp);
+        if (poly) {
+            if (newcham(mtmp, NULL, FALSE, FALSE))
+                ptr = mtmp->data;
+        } else if (grow) {
+            ptr = grow_up(mtmp, (struct monst *) 0);
+        } else if (heal) {
+            mtmp->mhp = mtmp->mhpmax;
+        }
+        if ((eyes || heal) && !mtmp->mcansee)
+            mcureblindness(mtmp, canseemon(mtmp));
+        /* in case it polymorphed or died */
+        if (ptr != original_ptr)
+            return !ptr ? 2 : 1;
+
+        /* Engulf & devour is instant, so don't set meating */
+        if (mtmp->minvis)
+            newsym(x, y);
+
+        return 1;
+    }
+    return 0;
+}
+
 void
 mpickgold(mtmp)
 register struct monst *mtmp;
@@ -1602,9 +1665,11 @@ mm_aggression(magr, mdef)
 struct monst *magr, /* monster that is currently deciding where to move */
              *mdef; /* another monster which is next to it */
 {
+    int mndx = monsndx(magr->data);
+
     /* supposedly purple worms are attracted to shrieking because they
        like to eat shriekers, so attack the latter when feasible */
-    if (magr->data == &mons[PM_PURPLE_WORM]
+    if ((mndx == PM_PURPLE_WORM || mndx == PM_BABY_PURPLE_WORM)
         && mdef->data == &mons[PM_SHRIEKER])
         return ALLOW_M | ALLOW_TM;
     /* Various other combinations such as dog vs cat, cat vs rat, and
@@ -2930,9 +2995,13 @@ struct monst *mtmp;
             stop_occupation();
         }
         if (!rn2(10)) {
-            if (!rn2(13))
+            if (!rn2(13)) {
-                (void) makemon(&mons[PM_PURPLE_WORM], 0, 0, NO_MM_FLAGS);
+                /* don't generate purple worms if they would be too difficult */
-            else
+                int pm = montoostrong(PM_PURPLE_WORM, monmax_difficulty_lev())
+                    ? PM_BABY_PURPLE_WORM : PM_PURPLE_WORM;
+
+                (void) makemon(&mons[pm], 0, 0, NO_MM_FLAGS);
+            } else
                 (void) makemon((struct permonst *) 0, 0, 0, NO_MM_FLAGS);
         }
         aggravate();

src/monmove.c

@@ -1534,6 +1534,13 @@ register int after;
                     return 2; /* it died */
             }
 
+            /* Maybe a purple worm ate a corpse */
+            if (ptr == &mons[PM_PURPLE_WORM]
+                || ptr == &mons[PM_BABY_PURPLE_WORM]) {
+                if (meatcorpse(mtmp) == 2)
+                    return 2; /* it died */
+            }
+
             if (!*in_rooms(mtmp->mx, mtmp->my, SHOPBASE) || !rn2(25)) {
                 boolean picked = FALSE;
 

src/polyself.c

@@ -1908,6 +1908,7 @@ polysense()
 
     switch (u.umonnum) {
     case PM_PURPLE_WORM:
+    case PM_BABY_PURPLE_WORM:
         warnidx = PM_SHRIEKER;
         break;
     case PM_VAMPIRE: