Simplify the miscellaneous item acquirement code (Reaver, #8272)

The old code had unclear chances of acquirement due to items' chances
depending on where they were in a list.  This commit clarifies the
weightings and sets these to better values.

It does the following:
  * only gifts the 4 kinds of decks not gifted by nemelex.
  * reduces the weights for decks overall.
  * for elemental evokables not seen before, gift them with higher
    weight compared to the other evokable types.
  * for evokables other than the disc of storms and lantern of
    shadows, allow gifting at a lower weight even if they have been
    seen before.
  * gifts crystal ball of energy with chance (evocations *
    max(spellcasting, invocations)) out of 27^2.

1 files changed, 32 insertions, 22 deletions

diff --git a/crawl-ref/source/acquire.cc b/crawl-ref/source/acquire.cc
index b5c3c7bc62..5e9c0878ee 100644
--- a/crawl-ref/source/acquire.cc
+++ b/crawl-ref/source/acquire.cc
@@ -655,30 +655,40 @@ static int _acquirement_staff_subtype(const has_vector& already_has)
     return result;
 }
 
+/**
+ * Return a miscellaneous evokable item for acquirement.
+ * @returns  The item type chosen.
+ */
 static int _acquirement_misc_subtype()
 {
-    // Note: items listed early are less likely due to chances of being
-    // overwritten.
-    int result = random_range(MISC_FIRST_DECK, MISC_LAST_DECK);
-    if (result == MISC_DECK_OF_SUMMONING && coinflip())
-        result = MISC_SACK_OF_SPIDERS;
-    if (result == MISC_DECK_OF_PUNISHMENT)
-        result = MISC_BOX_OF_BEASTS;
-    if (one_chance_in(4) && !you.seen_misc[MISC_DISC_OF_STORMS])
-        result = MISC_DISC_OF_STORMS;
-    if (one_chance_in(4) && !you.seen_misc[MISC_LAMP_OF_FIRE])
-        result = MISC_LAMP_OF_FIRE;
-    if (one_chance_in(4) && !you.seen_misc[MISC_FAN_OF_GALES])
-        result = MISC_FAN_OF_GALES;
-    if (one_chance_in(4) && !you.seen_misc[MISC_STONE_OF_TREMORS])
-        result = MISC_STONE_OF_TREMORS;
-    if (one_chance_in(4) && !you.seen_misc[MISC_PHIAL_OF_FLOODS])
-        result = MISC_PHIAL_OF_FLOODS;
-    if (one_chance_in(4) && !you.seen_misc[MISC_LANTERN_OF_SHADOWS])
-        result = MISC_LANTERN_OF_SHADOWS;
-    if (x_chance_in_y(you.skills[SK_EVOCATIONS], 27)
-        && (x_chance_in_y(max(you.skills[SK_SPELLCASTING],
-                              you.skills[SK_INVOCATIONS]), 27))
+    // Total weight if none have been seen is 100.
+    int result = random_choose_weighted(              // Decks given lowest weight.
+                                                      2, MISC_DECK_OF_DUNGEONS,
+                                                      2, MISC_DECK_OF_WAR,
+                                                      2, MISC_DECK_OF_CHANGES,
+                                                      2, MISC_DECK_OF_DEFENCE,
+                                                      // The player might want
+                                                      // multiple of these.
+       (you.seen_misc[MISC_LAMP_OF_FIRE] ?       8 : 15), MISC_LAMP_OF_FIRE,
+       (you.seen_misc[MISC_PHIAL_OF_FLOODS] ?    8 : 15), MISC_PHIAL_OF_FLOODS,
+       (you.seen_misc[MISC_FAN_OF_GALES] ?       8 : 15), MISC_FAN_OF_GALES,
+       (you.seen_misc[MISC_STONE_OF_TREMORS] ?   8 : 15), MISC_STONE_OF_TREMORS,
+                                                      // These have charges, so
+                                                      // give them a constant
+                                                      // weight.
+                                                      8, MISC_BOX_OF_BEASTS,
+                                                      8, MISC_SACK_OF_SPIDERS,
+                                                      // The player never needs
+                                                      // more than one.
+       (you.seen_misc[MISC_DISC_OF_STORMS] ?     0 :  8), MISC_DISC_OF_STORMS,
+       (you.seen_misc[MISC_LANTERN_OF_SHADOWS] ? 0 :  8), MISC_LANTERN_OF_SHADOWS,
+                                                      0);
+
+    // Give a crystal ball based on both evocations and either spellcasting or
+    // invocations if we haven't seen one.
+    int skills = you.skills[SK_EVOCATIONS]
+        * max(you.skills[SK_SPELLCASTING], you.skills[SK_INVOCATIONS]);
+    if (x_chance_in_y(skills, MAX_SKILL_LEVEL * MAX_SKILL_LEVEL)
         && !you.seen_misc[MISC_CRYSTAL_BALL_OF_ENERGY])
     {
         result = MISC_CRYSTAL_BALL_OF_ENERGY;