Allow some monsters to break boulders

... if the boulder is in a position they want to move to.
Shopkeepers, priests, and the quest leader can break one boulder
and then need to take several turns before being able to break
another.  Riders can break a boulder every turn.

doc/fixes3-7-0.txt

@@ -1147,6 +1147,7 @@ prevent hug attacks and touch or engulf attacks for wrap, stick-to, and
 	digestion damage from succeeding against unsolid targets (ghosts,
 	vortices, a few others)
 wand of speed gives temporary speed, potion gives intrinsic
+some monsters (riders, shopkeepers, priests, quest leader) can break boulders
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository

include/extern.h

@@ -1711,6 +1711,8 @@ extern struct monst *find_pmmonst(int);
 extern int bee_eat_jelly(struct monst *, struct obj *);
 extern void monflee(struct monst *, int, boolean, boolean);
 extern void mon_yells(struct monst *, const char *);
+extern boolean m_can_break_boulder(struct monst *);
+extern void m_break_boulder(struct monst *, coordxy, coordxy);
 extern int dochug(struct monst *);
 extern boolean m_digweapon_check(struct monst *, coordxy, coordxy);
 extern int m_move(struct monst *, int);

src/mon.c

@@ -1829,7 +1829,7 @@ mon_allowflags(struct monst* mtmp)
         allowflags |= ALLOW_SSM | ALLOW_SANCT;
     if (passes_walls(mtmp->data))
         allowflags |= (ALLOW_ROCK | ALLOW_WALL);
-    if (throws_rocks(mtmp->data))
+    if (throws_rocks(mtmp->data) || m_can_break_boulder(mtmp))
         allowflags |= ALLOW_ROCK;
     if (can_tunnel)
         allowflags |= ALLOW_DIG;

src/monmove.c

@@ -104,6 +104,36 @@ mon_yells(struct monst* mon, const char* shout)
     }
 }
 
+/* can monster mtmp break boulders? */
+boolean
+m_can_break_boulder(struct monst *mtmp)
+{
+    return (is_rider(mtmp->data)
+            || (!mtmp->mspec_used
+                && (mtmp->isshk || mtmp->ispriest
+                    || (mtmp->data->msound == MS_LEADER))));
+}
+
+/* monster mtmp breaks boulder at x,y */
+void
+m_break_boulder(struct monst *mtmp, coordxy x, coordxy y)
+{
+    struct obj *otmp;
+
+    if (m_can_break_boulder(mtmp) && ((otmp = sobj_at(BOULDER, x, y)) != 0)) {
+        if (!is_rider(mtmp->data)) {
+            if (!Deaf && (mdistu(mtmp) < 4*4))
+                pline("%s mutters %s.",
+                      Monnam(mtmp),
+                      mtmp->ispriest ? "a prayer" : "an incantation");
+            mtmp->mspec_used += rn1(20, 10);
+        }
+        if (cansee(x, y))
+            pline_The("boulder falls apart.");
+        fracture_rock(otmp);
+    }
+}
+
 static void
 watch_on_duty(register struct monst* mtmp)
 {
@@ -1536,6 +1566,11 @@ m_move(register struct monst* mtmp, register int after)
         if (!m_in_out_region(mtmp, nix, niy))
             return MMOVE_DONE;
 
+        if ((info[chi] & ALLOW_ROCK) && m_can_break_boulder(mtmp)) {
+            (void) m_break_boulder(mtmp, nix, niy);
+            return MMOVE_DONE;
+        }
+
         /* move a normal monster; for a long worm, remove_monster() and
            place_monster() only manipulate the head; they leave tail as-is */
         remove_monster(omx, omy);

src/priest.c

@@ -101,7 +101,10 @@ move_special(struct monst *mtmp, boolean in_his_shop, schar appr,
 
     if (nix != omx || niy != omy) {
 
-        if (ninfo & ALLOW_M) {
+        if (ninfo & ALLOW_ROCK) {
+            m_break_boulder(mtmp, nix, niy);
+            return 1;
+        } else if (ninfo & ALLOW_M) {
             /* mtmp is deciding it would like to attack this turn.
              * Returns from m_move_aggress don't correspond to the same things
              * as this function should return, so we need to translate. */