Inventory refuses non-positive counts; unknown saved items kept; factories follow reloads

tests/InventoryTests.cs

@@ -0,0 +1,36 @@
+namespace Cthangover.Mods.Items.Tests
+{
+    using System.Linq;
+    using Cthangover.Core.Items;
+    using Cthangover.Core.Mods.Resolvers;
+    using Cthangover.Core.Mods.Testing;
+
+    /// <summary>
+    /// Stack arithmetic of <see cref="Inventory"/>: a count that is not
+    /// positive is no operation at all — a negative removal must not grow a
+    /// stack and an addition must not create an empty or negative one.
+    /// </summary>
+    public class InventoryTests
+    {
+        [ModTest("inventory/non_positive_counts_change_nothing")]
+        static void NonPositiveCountsChangeNothing(ModTestContext ctx)
+        {
+            var id = Entities.Collect<ItemInfo>().Keys.First();
+            var inventory = new Inventory();
+            inventory.Add(id, 2);
+            var before = inventory.CheckCount(id);
+
+            ModAssert.False(inventory.Remove(id, -5), "negative removal is refused");
+            ModAssert.False(inventory.Remove(id, 0), "zero removal is refused");
+            inventory.Add(id, -3);
+            inventory.Add(id, 0);
+            ModAssert.Equal(before, inventory.CheckCount(id), "stack count after non-positive operations");
+
+            var other = Entities.Collect<ItemInfo>().Keys.FirstOrDefault(k => !inventory.HasItem(k));
+            if (other == null)
+                return;
+            inventory.Add(other, 0);
+            ModAssert.False(inventory.HasItem(other), "zero addition creates no stack");
+        }
+    }
+}