Check for box of beasts/sack of spiders destruction after success (Pl…

…easingFungus)

This approach feels better than having the item just fall apart on you, but it's
also considerably more consistent/powerful, so I upped the destruction rate to 1/3.

crawl-ref/source/dat/descript/items.txt

@@ -292,8 +292,8 @@ box of beasts
 A box filled with the bizarre leftovers of a depraved wizard's magical
 experimentations. Opening it might set loose a friendly mutant beast: bizarre
 creatures, each with their own combination of traits. Increased skill with
-Evocations will allow stronger beasts to be released, and reduce the chances
-of accidentally destroying the box when using it.
+Evocations will allow stronger beasts to be released. There is a one in three
+chance of accidentally destroying the box when using it.
 
 There are six known traits:
  * Bat beasts fly and move very quickly, but erratically.
@@ -991,8 +991,8 @@ sack of spiders
 
 A woven silk bag. Opening it will release a number of spiders and other
 insects, as well as creating webs nearby. Evocations skill increases its
-effectiveness and reduces the chances of accidentally destroying the bag when
-it is used.
+effectiveness. There is a one in three chance of accidentally destroying the
+sack when using it.
 %%%%
 sacred scourge
 

crawl-ref/source/evoke.cc

@@ -992,14 +992,6 @@ static bool _box_of_beasts(item_def &box)
     surge_power(surge);
     mpr("You open the lid...");
 
-    if (one_chance_in(4))
-    {
-        mpr("...but the box appears empty, and falls apart.");
-        ASSERT(in_inventory(box));
-        dec_inv_item_quantity(box.link, 1);
-        return false;
-    }
-
     // two rolls to reduce std deviation - +-6 so can get < max even at 27 sk
     const int hd_min = min(27,
                            player_adjust_evoc_power(
@@ -1034,6 +1026,14 @@ static bool _box_of_beasts(item_def &box)
     xom_is_stimulated(10); // dubious
     did_god_conduct(DID_CHAOS, random_range(5,10));
 
+    // After unboxing a beast, chance to break.
+    if (one_chance_in(3))
+    {
+        mpr("The now-empty box falls apart.");
+        ASSERT(in_inventory(box));
+        dec_inv_item_quantity(box.link, 1);
+    }
+
     return true;
 }
 
@@ -1055,14 +1055,6 @@ static bool _sack_of_spiders(item_def &sack)
             1 + random2(2) + random2(div_rand_round(evo_skill * 10, 30)), surge);
     const int power = player_adjust_evoc_power(evo_skill, surge);
 
-    if (one_chance_in(4))
-    {
-        mpr("...but the bag is empty, and unravels at your touch.");
-        ASSERT(in_inventory(sack));
-        dec_inv_item_quantity(sack.link, 1);
-        return false;
-    }
-
     if (x_chance_in_y(5, 10 + power))
     {
         mpr("...but nothing happens.");
@@ -1126,10 +1118,15 @@ static bool _sack_of_spiders(item_def &sack)
                 trap = trap_at((*mi)->pos());
                 trap->trigger(**mi);
             }
+
+        }
+        // After gettin' some bugs, check for destruction.
+        if (one_chance_in(3))
+        {
+            mpr("The now-empty bag unravels in your hand.");
+            ASSERT(in_inventory(sack));
+            dec_inv_item_quantity(sack.link, 1);
         }
-        // Decrease charges
-        sack.charges--;
-        sack.used_count++;
     }
     else
         // Failed to create monster for some reason