Fix randart books being much too cheap. (Yes, this was actually a bug.)

A randart book's rarity is now defined as the average of its three
rarest spells.


git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@9101 c06c8d41-db1a-0410-9941-cceddc491573

2 files changed, 25 insertions, 8 deletions

diff --git a/crawl-ref/source/effects.cc b/crawl-ref/source/effects.cc
index f704f4243c..f13b2d70d1 100644
--- a/crawl-ref/source/effects.cc
+++ b/crawl-ref/source/effects.cc
@@ -1431,7 +1431,7 @@ static void _do_book_acquirement(item_def &book, int agent)
                     60, BOOK_RANDART_THEME,
            agent == GOD_SIF_MUNA ? 24 : 34, book.sub_type,
                 level == -1      ? 0 :
-           agent == GOD_SIF_MUNA ? 3 : 1, BOOK_RANDART_LEVEL,
+           agent == GOD_SIF_MUNA ? 4 : 1, BOOK_RANDART_LEVEL,
                     0);
     }
 
@@ -1481,6 +1481,9 @@ static void _do_book_acquirement(item_def &book, int agent)
             int w = (skill < 12) ? skill + 3
                                  : 25 - skill;
 
+            if (w < 0)
+                w = 0;
+
             // If we don't know any magic skills, make non-magic skills
             // more likely.
             if (!knows_magic && i < SK_SPELLCASTING)
diff --git a/crawl-ref/source/shopping.cc b/crawl-ref/source/shopping.cc
index 3df185d1b6..61c5a41ca0 100644
--- a/crawl-ref/source/shopping.cc
+++ b/crawl-ref/source/shopping.cc
@@ -1666,26 +1666,40 @@ unsigned int item_value( item_def item, bool ident )
         valued = 150;
         if (item_type_known(item))
         {
-            int rarity = 0;
+            double rarity = 0;
             if (is_random_artefact(item))
             {
-                // Consider spellbook as rare as its rarest spell.
-                // NOTE: This probably undervalues a book if it contains
-                // lots of rare spells.
+                // Consider spellbook as rare as the average of its
+                // three rarest spells.
+                int rarities[SPELLBOOK_SIZE];
+                int count_valid = 0;
                 for (int i = 0; i < SPELLBOOK_SIZE; i++)
                 {
                     spell_type spell = which_spell_in_book(item, i);
                     if (spell == SPELL_NO_SPELL)
+                    {
+                        rarities[i] = 0;
                         continue;
+                    }
 
-                    if (rarity > spell_rarity(spell))
-                        rarity = spell_rarity(spell);
+                    rarities[i] = spell_rarity(spell);
+                    count_valid++;
                 }
+                ASSERT(count_valid > 0);
+
+                std::sort(rarities, rarities + SPELLBOOK_SIZE);
+                for (int i = SPELLBOOK_SIZE - 1; i >= SPELLBOOK_SIZE - 3; i--)
+                    rarity += rarities[i];
+
+                if (count_valid > 3)
+                    count_valid = 3;
+
+                rarity /= count_valid;
             }
             else
                 rarity = book_rarity(item.sub_type);
 
-            valued += book_rarity(item.sub_type) * 50;
+            valued += rarity * 50;
         }
         break;