Changes to the scroll of enchant armor

After discussing with paxed, I decided that instead of reducing the
safe enchantment limit for magical armor, it is instead better to
make the scrolls less effective on it.  So this commit restores the
previous rules for the safely-enchant-from level and changes the
effectiveness of the scrolls to compensate.

Non-elven magical armor now only gets +1 from blessed scrolls of
enchant armor when enchanted from +3, coming to a total of +4. But
scrolls of enchant armor are now more effective on nonmagical and
elven and previously unenchanted armor, giving more of an incentive
to use them in the early game.

Cursed and uncursed scrolls of enchant armor are now also more
powerful than they were (but less powerful than blessed scrolls),
hopefully making it a more interesting decision as to whether to use
scrolls of enchant armor even if you don't have the means to bless
them.

src/read.c

@@ -21,8 +21,6 @@ staticfn void forget(int);
 staticfn int maybe_tame(struct monst *, struct obj *);
 staticfn boolean can_center_cloud(coordxy, coordxy);
 staticfn void display_stinking_cloud_positions(boolean);
-staticfn boolean is_special_armor_enchant(struct obj *);
-staticfn int armor_enchant_limit(struct obj *);
 staticfn void seffect_enchant_armor(struct obj **);
 staticfn void seffect_destroy_armor(struct obj **);
 staticfn void seffect_confuse_monster(struct obj **);
@@ -1112,37 +1110,12 @@ display_stinking_cloud_positions(boolean on_off)
     }
 }
 
-/* some armor vibrates warningly when enchanted beyond a limit,
-   or can be enchanted higher than usual */
-staticfn boolean
-is_special_armor_enchant(struct obj *otmp)
-{
-    return is_elven_armor(otmp)
-        || (Role_if(PM_WIZARD) && otmp->otyp == CORNUTHAUM);
-}
-
-/* return the safe enchantment limit for an armor */
-staticfn int
-armor_enchant_limit(struct obj *otmp)
-{
-    int limit = 3; /* default safe armor enchantment limit */
-
-    /* some armor can take a higher enchantment */
-    if (is_special_armor_enchant(otmp))
-        limit += 2;
-    /* object's internal magic interferes */
-    if (objects[otmp->otyp].oc_magic)
-        limit--;
-
-    return limit;
-}
-
 staticfn void
 seffect_enchant_armor(struct obj **sobjp)
 {
     struct obj *sobj = *sobjp;
     schar s;
-    int safe_spe_limit;
+    boolean special_armor;
     boolean same_color;
     struct obj *otmp = some_armor(&gy.youmonst);
     boolean sblessed = sobj->blessed;
@@ -1187,6 +1160,9 @@ seffect_enchant_armor(struct obj **sobjp)
         otmp->oerodeproof = new_erodeproof ? 1 : 0;
         return;
     }
+    /* elven armor vibrates warningly when enchanted beyond a limit */
+    special_armor = is_elven_armor(otmp)
+        || (Role_if(PM_WIZARD) && otmp->otyp == CORNUTHAUM);
     if (scursed)
         same_color = (otmp->otyp == BLACK_DRAGON_SCALE_MAIL
                       || otmp->otyp == BLACK_DRAGON_SCALES);
@@ -1197,11 +1173,9 @@ seffect_enchant_armor(struct obj **sobjp)
     if (Blind)
         same_color = FALSE;
 
-    safe_spe_limit = armor_enchant_limit(otmp);
-
     /* KMH -- catch underflow */
     s = scursed ? -otmp->spe : otmp->spe;
-    if (s > safe_spe_limit && rn2(s)) {
+    if (s > (special_armor ? 5 : 3) && rn2(s)) {
         otmp->in_use = TRUE;
         pline("%s violently %s%s%s for a while, then %s.", Yname2(otmp),
               otense(otmp, Blind ? "vibrate" : "glow"),
@@ -1213,12 +1187,33 @@ seffect_enchant_armor(struct obj **sobjp)
         useup(otmp);
         return;
     }
-    s = scursed ? -1
-        : (otmp->spe >= 9)
-        ? (rn2(otmp->spe) == 0)
-        : sblessed
-        ? rnd(3 - otmp->spe / 3)
-        : 1;
+    if (s < -100) s = -100; /* avoid integer overflow with very negative armor */
+
+    /* Base power of the enchantment:
+
+       2 for -1 to +0 armor;
+       1 for +1 to +2 armor;
+       0 for +3 to +4 armor, etc.
+
+       When disenchanting, everything is done with reversed signs. */
+    s = (4 - s) / 2;
+
+    /* Elven/artifact and nonmagical armor is easier to enchant;
+       blessed scrolls are more effective. */
+    if (special_armor) ++s;
+    if (!objects[otmp->otyp].oc_magic) ++s;
+    if (sblessed) ++s;
+
+    if (s <= 0) {
+        s = 0;
+        if (otmp->spe > 0 && !rn2(otmp->spe)) s = 1;
+    } else {
+        s = rnd(s);
+    }
+    if (s > 11) s = 11;    /* unlikely but possible: avoids an overflow later */
+
+    if (scursed) s = -s;
+
     if (s >= 0 && Is_dragon_scales(otmp)) {
         unsigned was_lit = otmp->lamplit;
         int old_light = artifact_light(otmp) ? arti_light_radius(otmp) : 0;
@@ -1280,8 +1275,8 @@ seffect_enchant_armor(struct obj **sobjp)
             alter_cost(otmp, 0L);
     }
 
-    if ((otmp->spe > safe_spe_limit)
-        && (is_special_armor_enchant(otmp) || !rn2(7)))
+    if ((otmp->spe > (special_armor ? 5 : 3))
+        && (special_armor || !rn2(7)))
         pline("%s %s.", Yobjnam2(otmp, "suddenly vibrate"),
               Blind ? "again" : "unexpectedly");
 }