Make offered price caps barter-specific (bug #5028)

CHANGELOG.md

@@ -87,6 +87,7 @@
     Bug #5001: Possible data race in the Animation::setAlpha()
     Bug #5004: Werewolves shield their eyes during storm
     Bug #5018: Spell tooltips don't support purely negative magnitudes
+    Bug #5028: Offered price caps are not trading-specific
     Feature #1774: Handle AvoidNode
     Feature #2229: Improve pathfinding AI
     Feature #3025: Analogue gamepad movement controls

apps/openmw/mwgui/tradewindow.cpp

@@ -468,16 +468,26 @@ namespace MWGui
 
         int merchantOffer = 0;
 
+        // The offered price must be capped at 75% of the base price to avoid exploits
+        // connected to buying and selling the same item.
+        // This value has been determined by researching the limitations of the vanilla formula
+        // and may not be sufficient if getBarterOffer behavior has been changed.
         std::vector<ItemStack> playerBorrowed = playerTradeModel->getItemsBorrowedToUs();
         for (const ItemStack& itemStack : playerBorrowed)
         {
-            merchantOffer -= MWBase::Environment::get().getMechanicsManager()->getBarterOffer(mPtr, getEffectiveValue(itemStack.mBase, itemStack.mCount), true);
+            const int basePrice = getEffectiveValue(itemStack.mBase, itemStack.mCount);
+            const int cap = static_cast<int>(std::max(1.f, 0.75f * basePrice)); // Minimum buying price -- 75% of the base
+            const int buyingPrice = MWBase::Environment::get().getMechanicsManager()->getBarterOffer(mPtr, basePrice, true);
+            merchantOffer -= std::max(cap, buyingPrice);
         }
 
         std::vector<ItemStack> merchantBorrowed = mTradeModel->getItemsBorrowedToUs();
         for (const ItemStack& itemStack : merchantBorrowed)
         {
-            merchantOffer += MWBase::Environment::get().getMechanicsManager()->getBarterOffer(mPtr, getEffectiveValue(itemStack.mBase, itemStack.mCount), false);
+            const int basePrice = getEffectiveValue(itemStack.mBase, itemStack.mCount);
+            const int cap = static_cast<int>(std::min(1.f, 0.75f * basePrice)); // Maximum selling price -- 75% of the base
+            const int sellingPrice = MWBase::Environment::get().getMechanicsManager()->getBarterOffer(mPtr, basePrice, false);
+            merchantOffer += std::min(cap, sellingPrice);
         }
 
         int diff = merchantOffer - mCurrentMerchantOffer;

apps/openmw/mwmechanics/mechanicsmanagerimp.cpp

@@ -676,8 +676,8 @@ namespace MWMechanics
         float f = std::min(0.2f * sellerStats.getAttribute(ESM::Attribute::Personality).getModified(), 10.f);
         float pcTerm = (clampedDisposition - 50 + a + b + c) * playerStats.getFatigueTerm();
         float npcTerm = (d + e + f) * sellerStats.getFatigueTerm();
-        float buyTerm = 0.01f * std::max(75.f, (100 - 0.5f * (pcTerm - npcTerm)));
-        float sellTerm = 0.01f * std::min(75.f, (50 - 0.5f * (npcTerm - pcTerm)));
+        float buyTerm = 0.01f * (100 - 0.5f * (pcTerm - npcTerm));
+        float sellTerm = 0.01f * (50 - 0.5f * (npcTerm - pcTerm));
         int offerPrice = int(basePrice * (buying ? buyTerm : sellTerm));
         return std::max(1, offerPrice);
     }