fix(catalog): bound target offer purchases

Emulator/src/main/java/com/eu/habbo/messages/incoming/catalog/PurchaseTargetOfferEvent.java

@@ -31,14 +31,17 @@ public class PurchaseTargetOfferEvent extends MessageHandler {
             HabboOfferPurchase purchase = HabboOfferPurchase.getOrCreate(this.client.getHabbo(), offerId);
 
             if (purchase != null) {
-                amount = Math.min(offer.getPurchaseLimit() - purchase.getAmount(), amount);
+                amount = TargetOfferPurchaseGuard.purchasableAmount(amount, offer.getPurchaseLimit(), purchase.getAmount());
+                if (amount <= 0) return;
+
                 int now = Emulator.getIntUnixTimestamp();
                 if (offer.getExpirationTime() > now) {
-                    purchase.update(amount, now);
                     CatalogItem item = Emulator.getGameEnvironment().getCatalogManager().getCatalogItem(offer.getCatalogItem());
+                    if (item == null) return;
                     if (item.isLimited()) {
                         amount = 1;
                     }
+                    purchase.update(amount, now);
                     Emulator.getGameEnvironment().getCatalogManager().purchaseItem(null, item, this.client.getHabbo(), amount, "", false);
 
                 }

Emulator/src/main/java/com/eu/habbo/messages/incoming/catalog/TargetOfferPurchaseGuard.java

@@ -0,0 +1,19 @@
+package com.eu.habbo.messages.incoming.catalog;
+
+final class TargetOfferPurchaseGuard {
+    private TargetOfferPurchaseGuard() {
+    }
+
+    static int purchasableAmount(int requestedAmount, int purchaseLimit, int alreadyPurchased) {
+        if (requestedAmount <= 0 || purchaseLimit <= 0) {
+            return 0;
+        }
+
+        int remaining = purchaseLimit - Math.max(alreadyPurchased, 0);
+        if (remaining <= 0) {
+            return 0;
+        }
+
+        return Math.min(requestedAmount, remaining);
+    }
+}

Emulator/src/test/java/com/eu/habbo/messages/incoming/catalog/TargetOfferPurchaseGuardTest.java

@@ -0,0 +1,35 @@
+package com.eu.habbo.messages.incoming.catalog;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class TargetOfferPurchaseGuardTest {
+    @Test
+    void rejectsInvalidRequestedAmounts() {
+        assertEquals(0, TargetOfferPurchaseGuard.purchasableAmount(0, 10, 0));
+        assertEquals(0, TargetOfferPurchaseGuard.purchasableAmount(-5, 10, 0));
+    }
+
+    @Test
+    void rejectsOffersWithoutPositiveLimits() {
+        assertEquals(0, TargetOfferPurchaseGuard.purchasableAmount(1, 0, 0));
+        assertEquals(0, TargetOfferPurchaseGuard.purchasableAmount(1, -10, 0));
+    }
+
+    @Test
+    void capsAmountToRemainingLimit() {
+        assertEquals(3, TargetOfferPurchaseGuard.purchasableAmount(10, 5, 2));
+    }
+
+    @Test
+    void rejectsWhenLimitIsAlreadyConsumed() {
+        assertEquals(0, TargetOfferPurchaseGuard.purchasableAmount(1, 5, 5));
+        assertEquals(0, TargetOfferPurchaseGuard.purchasableAmount(1, 5, 8));
+    }
+
+    @Test
+    void doesNotLetNegativePurchaseHistoryIncreaseRemainingLimit() {
+        assertEquals(5, TargetOfferPurchaseGuard.purchasableAmount(10, 5, -4));
+    }
+}