Add the Amulet of Storms artifact

Note by copperwater: Intent of this is to continue expanding a few pools
of artifacts:
    - non-weapon non-quest artifacts
    - chaotic artifacts
    - less powerful artifacts

while still being useful enough to appreciate when given as a sacrifice
gift.

From the original patch, I adjusted the price down (it's not powerful
enough to be worth 2000 base price) and refactored is_stormy_monster for
clarity.

doc/xnh-changelog-9.0.md

@@ -40,6 +40,9 @@ changes:
   monsters.
 - Monsters stepping on a squeaky board near you can wake you up.
 - Leprechauns have a one-sided grudge against gold golems and gold dragons.
+- New artifact The Amulet of Storms: a chaotic amulet of flying that grants
+  shock resistance when worn and allows you to chat to hostile vortices, air
+  elementals, and storm giants to pacify them.
 
 ### Interface changes
 

include/artilist.h

@@ -189,6 +189,11 @@ static NEARDATA struct artifact artilist[] = {
       NO_CARY, CONFLICT, A_CHAOTIC, NON_PM, NON_PM, 4000L, NO_COLOR, GOLD,
       APPLE_OF_DISCORD),
 
+    A("The Amulet of Storms", AMULET_OF_FLYING,
+      (SPFX_RESTR | SPFX_DEFN), 0, 0,
+      NO_ATTK, DFNS(AD_ELEC), NO_CARY, 0, A_CHAOTIC, NON_PM,
+      NON_PM, 600L, NO_COLOR, DEFAULT_MAT, AMULET_OF_STORMS),
+
     /*
      *      The artifacts for the quest dungeon, all self-willed.
      */

src/objnam.c

@@ -4472,7 +4472,9 @@ readobjnam_postparse1(struct _readobjnam_data *d)
         && strncmpi(d->bp, "light armor", 11)
         && strncmpi(d->bp, "tooled horn", 11)
         && strncmpi(d->bp, "food ration", 11)
-        && strncmpi(d->bp, "meat ring", 9))
+        && strncmpi(d->bp, "meat ring", 9)
+        && strncmpi(d->bp, "amulet of storms", 16)
+        )
         for (i = 0; i < (int) (sizeof wrpsym); i++) {
             register int j = Strlen(wrp[i]);
 

src/sounds.c

@@ -12,6 +12,8 @@ static boolean temple_priest_sound(struct monst *);
 static boolean mon_is_gecko(struct monst *);
 static int domonnoise(struct monst *);
 static int dochat(void);
+static boolean is_stormy_monster(struct monst *);
+static void pacify_with_words(struct monst *);
 static struct monst *responsive_mon_at(int, int);
 static int mon_in_room(struct monst *, int);
 
@@ -1353,6 +1355,11 @@ dochat(void)
         return ECMD_OK;
     }
     if (u.uswallow) {
+        if (uamul && uamul->oartifact == ART_AMULET_OF_STORMS
+            && is_stormy_monster(u.ustuck)) {
+            pacify_with_words(u.ustuck);
+            return ECMD_OK;
+        }
         pline("They won't hear you out there.");
         return ECMD_OK;
     }
@@ -1490,6 +1497,12 @@ dochat(void)
         return ECMD_OK;
     }
 
+    if (uamul && uamul->oartifact == ART_AMULET_OF_STORMS
+        && is_stormy_monster(mtmp)) {
+        pacify_with_words(mtmp);
+        return ECMD_OK;
+    }
+
     /* if this monster is waiting for something, prod it into action */
     mtmp->mstrategy &= ~STRAT_WAITMASK;
 
@@ -1513,6 +1526,32 @@ dochat(void)
     return domonnoise(mtmp);
 }
 
+/* is mtmp a storm-like monster pacifiable by the Amulet of Storms? */
+static boolean
+is_stormy_monster(struct monst *mtmp)
+{
+    /* already peaceful */
+    if (mtmp->mpeaceful)
+        return FALSE;
+
+    /* not a "true" stormy monster (but shapeshifters currently turned into one
+     * are okay) */
+    if (mtmp->iswiz || is_vampshifter(mtmp))
+        return FALSE;
+
+    return (mtmp->data->mlet == S_VORTEX
+            || mtmp->data == &mons[PM_AIR_ELEMENTAL]
+            || mtmp->data == &mons[PM_STORM_GIANT]);
+}
+
+static void
+pacify_with_words(struct monst *mtmp)
+{
+    Your("words calm %s.", mon_nam(mtmp));
+    mtmp->mpeaceful = 1;
+    newsym(mtmp->mx, mtmp->my);
+}
+
 /* is there a monster at <x,y> that can see the hero and react? */
 static struct monst *
 responsive_mon_at(int x, int y)