monsters moving other monsters (trunk only)

For now, the code is conditional on BARGETHROUGH
being defined, while it gets tested further. While behavior is
different with and without BARGETHROUGH defined, savefiles
are the same either way.

After this patch is applied, only the riders have the M3_DISPLACES
bit set, but the Wizard and Vlad probably should too. Any others?

src/mhitm.c

@@ -168,10 +168,9 @@ fightm(mtmp)		/* have monsters fight each other */
 	return 0;
 }
 
+#ifdef BARGETHROUGH
 /*
- * mattackm() -- a monster attacks another monster.
- *
- * This function returns a result bitfield:
+ * mattackm() and mdisplacem() below both return a result bitfield:
  *
  *	    --------- aggressor died
  *	   /  ------- defender died
@@ -184,6 +183,112 @@ fightm(mtmp)		/* have monsters fight each other */
  *	0x1	MM_HIT
  *	0x0	MM_MISS
  *
+ */
+
+/*
+ * mdisplacem() -- a monster moves another monster out of the way.
+ */
+int
+mdisplacem(magr, mdef, quietly)
+register struct monst *magr,*mdef;
+boolean quietly;
+{
+	struct permonst *pa, *pd;
+	struct monst *mon;			/* displaced monster */
+	int tx = mdef->mx, ty = mdef->my;	/* destination */
+	int fx = magr->mx, fy = magr->my; 	/* current location */
+	boolean struck = FALSE;
+	
+	pa = magr->data;  pd = mdef->data;
+
+	/* The 1 in 7 failure below matches the chance in attack()
+	 * for pet displacement.
+	 */
+	if (!rn2(7)) return(MM_MISS);
+
+	/* Grid bugs cannot displace at an angle. */
+	if (pa == &mons[PM_GRID_BUG] && magr->mx != mdef->mx
+						&& magr->my != mdef->my)
+		return(MM_MISS);
+
+
+	/* undetected monsters become un-hidden if they are displaced */
+	if (mdef->mundetected)
+		mdef->mundetected = 0;
+
+	/*
+	 * Set up the visibility of action.
+	 * You can observe monster displacement if you can see both of
+	 * the monsters involved.
+	 */
+	vis = (canspotmon(magr) && canspotmon(mdef));
+
+
+	if (touch_petrifies(pd) && !resists_ston(magr)) {
+		if (which_armor(magr, W_ARMG) != 0) {
+			if (poly_when_stoned(pa)) {
+			    mon_to_stone(magr);
+			    return MM_HIT; /* no damage during the polymorph */
+			}
+			if (!quietly && canspotmon(magr))
+				pline("%s turns to stone!", Monnam(magr));
+			monstone(magr);
+			if (magr->mhp > 0) return 0;
+			else if (magr->mtame && !vis)
+			    You(brief_feeling, "peculiarly sad");
+			return MM_AGR_DIED;
+	        }
+	}
+
+	if (m_at(fx, fy) == magr)
+		remove_monster(fx, fy);		/* pick up from orig position */
+	if ((mon = m_at(tx, ty)) == mdef) {
+		if (!quietly && (vis))
+			pline("%s moves %s out of %s way!",
+				Monnam(magr), mon_nam(mdef),
+				is_rider(pa) ? "the" : mhis(magr));
+		remove_monster(tx, ty);
+	}
+	place_monster(magr,tx,ty);	/* put magr down */
+
+	/* Restore original mon */
+	if (mon) {
+	    if ((mon->mx != tx) || (mon->my != ty))
+		place_worm_seg(mon, fx, fy);
+	    else
+		place_monster(mon, fx, fy);
+	    struck = TRUE;
+	} else 
+	    remove_monster(fx, fy);	/* shouldn't happen */
+	newsym(fx,fy);			/* see it */
+	newsym(tx,ty);			/*   all happen */
+	flush_screen(0);		/* make sure it shows up */
+
+	/*
+	 *  Wake up the displaced defender.
+	 */
+	mdef->msleeping = 0;
+	
+	return(struck ? MM_HIT : MM_MISS);
+}
+#endif /* BARGETHROUGH */
+
+/*
+ * mattackm() -- a monster attacks another monster.
+#ifndef BARGETHROUGH
+ *
+ *	    --------- aggressor died
+ *	   /  ------- defender died
+ *	  /  /  ----- defender was hit
+ *	 /  /  /
+ *	x  x  x
+ *
+ *	0x4	MM_AGR_DIED
+ *	0x2	MM_DEF_DIED
+ *	0x1	MM_HIT
+ *	0x0	MM_MISS
+ *
+#endif
  * Each successive attack has a lower probability of hitting.  Some rely on the
  * success of previous attacks.  ** this doen't seem to be implemented -dl **
  *