Minor refresh of RagfairRequiredItemsService

Adding a new set + lookup is now atomic
Returns a readonly set, caller shouldn't be able to ever modify this data
Removed memory allocation when calling `GetRequiredOffersById` with a tpl that doesn't have any offerIds

Libraries/SPTarkov.Server.Core/Services/RagfairRequiredItemsService.cs

@@ -11,43 +11,46 @@ public class RagfairRequiredItemsService(RagfairOfferService ragfairOfferService
     /// <summary>
     /// Key = tpl
     /// </summary>
-    protected readonly ConcurrentDictionary<MongoId, HashSet<MongoId>> _requiredItemsCache = new();
+    protected readonly ConcurrentDictionary<MongoId, HashSet<MongoId>> RequiredItemsCache = new();
+
+    /// <summary>
+    /// Empty hashset to be returned when no keys found by GetRequiredOffersById (reduces memory allocations)
+    /// </summary>
+    protected readonly IReadOnlySet<MongoId> EmptyOfferIdSet = new HashSet<MongoId>();
 
     /// <summary>
     /// Get the offerId of offers that require the supplied tpl
     /// </summary>
     /// <param name="tpl">Tpl to find offers ids for</param>
-    /// <returns></returns>
-    public HashSet<MongoId> GetRequiredOffersById(MongoId tpl)
+    /// <returns>Set of OfferIds</returns>
+    public IReadOnlySet<MongoId> GetRequiredOffersById(MongoId tpl)
     {
-        if (_requiredItemsCache.TryGetValue(tpl, out var offerIds))
-        {
-            return offerIds;
-        }
-
-        return [];
+        return RequiredItemsCache.TryGetValue(tpl, out var offerIds) ? offerIds : EmptyOfferIdSet;
     }
 
     /// <summary>
-    /// Create a cache of requirements to purchase item
+    /// Create a cache of offer Ids keyed against the item tpl they require
     /// </summary>
     public void BuildRequiredItemTable()
     {
-        _requiredItemsCache.Clear();
+        RequiredItemsCache.Clear();
         foreach (var offer in ragfairOfferService.GetOffers())
-        foreach (var requirement in offer.Requirements)
         {
-            if (paymentHelper.IsMoneyTpl(requirement.TemplateId))
-            // This would just be too noisy
+            foreach (var requirement in offer.Requirements ?? [])
             {
-                continue;
+                // Skip offers for currency, it's too expensive to process
+                // Only process barter offers
+                if (paymentHelper.IsMoneyTpl(requirement.TemplateId))
+                {
+                    continue;
+                }
+
+                // Ensure cache has Hashset init for this tpl
+                var offerIds = RequiredItemsCache.GetOrAdd(requirement.TemplateId, _ => []);
+
+                // Add offer id against the tpl key
+                offerIds.Add(offer.Id);
             }
-
-            // Ensure key is init
-            _requiredItemsCache.TryAdd(requirement.TemplateId, []);
-
-            // Add matching offer
-            _requiredItemsCache.GetValueOrDefault(requirement.TemplateId)?.Add(offer.Id);
         }
     }
 }