Rewrote ItemContainers from scratch

- ItemContainers have been entirely rewritten
  - ItemSlots are now wrappers, not owners
- Added InventoryOwners
  - Not yet used
- Added ItemDataWithContainers
- Added some basic windowing functionality to WindowedHUD

src/main/java/ru/windcorp/progressia/client/graphics/world/hud/HUDManager.java

@@ -104,7 +104,7 @@ public class HUDManager implements HUDWorkspace {
 			InventoryComponent component = render.createComponent(inventory, this);
 			openInventory(component);
 		} catch (Exception e) {
-			throw CrashReports.report(null, "Could not open inventory %s", inventory.getId());
+			throw CrashReports.report(e, "Could not open inventory %s", inventory.getId());
 		}
 	}
 	

src/main/java/ru/windcorp/progressia/client/graphics/world/hud/InteractiveSlotComponent.java

@@ -25,10 +25,9 @@ import ru.windcorp.progressia.client.graphics.input.KeyMatcher;
 import ru.windcorp.progressia.client.graphics.input.WheelScrollEvent;
 import ru.windcorp.progressia.common.Units;
 import ru.windcorp.progressia.common.world.item.ItemDataContainer;
-import ru.windcorp.progressia.common.world.item.Items;
 import ru.windcorp.progressia.common.world.item.inventory.ItemContainer;
-import ru.windcorp.progressia.common.world.item.inventory.ItemContainerMixed;
 import ru.windcorp.progressia.common.world.item.inventory.ItemSlot;
+import ru.windcorp.progressia.common.world.item.inventory.Items;
 
 public class InteractiveSlotComponent extends Button {
 
@@ -77,7 +76,7 @@ public class InteractiveSlotComponent extends Button {
 
 	private void onMainAction() {
 		ItemSlot handSlot = workspace.getHand().slot();
-		ItemSlot invSlot = getSlotOrAllocate();
+		ItemSlot invSlot = getSlot();
 		if (invSlot == null) {
 			return;
 		}
@@ -108,13 +107,12 @@ public class InteractiveSlotComponent extends Button {
 		if (success) {
 			requestReassembly();
 		}
-		
-		cleanContainerUpIfNecessary();
 	}
 
 	private void pickAll(ItemSlot handSlot) {
-		for (ItemSlot invSlot : getSlot().getContainer()) {
-			Items.pour(invSlot, handSlot);
+		int maxIndex = getSlot().getContainer().getMaxIndex();
+		for (int index = 0; index < maxIndex; ++index) {
+			Items.pour(new ItemSlot(getSlot().getContainer(), index), handSlot);
 			if (handSlot.isEmpty()) {
 				break;
 			}
@@ -123,7 +121,7 @@ public class InteractiveSlotComponent extends Button {
 
 	private void onAltAction() {
 		ItemSlot handSlot = workspace.getHand().slot();
-		ItemSlot invSlot = getSlotOrAllocate();
+		ItemSlot invSlot = getSlot();
 		if (invSlot == null) {
 			return;
 		}
@@ -135,7 +133,7 @@ public class InteractiveSlotComponent extends Button {
 		}
 
 		if (!success && handSlot.isEmpty()) {
-			success = Items.pour(invSlot, handSlot, invSlot.getAmount() / 2) != 0;
+			success = Items.pour(invSlot, handSlot, invSlot.getCount() / 2) != 0;
 		}
 
 		if (!success) {
@@ -149,19 +147,17 @@ public class InteractiveSlotComponent extends Button {
 		if (success) {
 			requestReassembly();
 		}
-
-		cleanContainerUpIfNecessary();
 	}
 
 	private boolean tryToOpen(ItemSlot invSlot) {
-		if (invSlot.getAmount() != 1) {
+		if (invSlot.getCount() != 1) {
 			return false;
 		}
-		if (!(invSlot.getContents() instanceof ItemDataContainer)) {
+		if (!(invSlot.getItem() instanceof ItemDataContainer)) {
 			return false;
 		}
 
-		ItemDataContainer item = (ItemDataContainer) invSlot.getContents();
+		ItemDataContainer item = (ItemDataContainer) invSlot.getItem();
 		return item.open(workspace.getPlayerEntity()) != null;
 	}
 
@@ -181,38 +177,4 @@ public class InteractiveSlotComponent extends Button {
 		return slotComponent.getSlot();
 	}
 
-	private ItemSlot getSlotOrAllocate() {
-		ItemSlot slot = slotComponent.getSlot();
-		
-		if (slot == null) {
-
-			int slotIndex = slotComponent.getSlotIndex();
-			ItemContainer uncastContainer = slotComponent.getSlotContainer();
-
-			assert slotIndex >= 0 : "Slot index is negative: " + slotIndex;
-			assert slotIndex >= uncastContainer.getSlotCount()
-				: "Slot index is valid (" + slotIndex + ") but container does not provide a slot";
-
-			if (uncastContainer instanceof ItemContainerMixed) {
-				ItemContainerMixed container = (ItemContainerMixed) uncastContainer;
-				container.addSlots(slotIndex - container.getSlotCount() + 1);
-				
-				slot = slotComponent.getSlot();
-				assert slot != null : "Could not allocate slot for index " + slotIndex;
-			} else {
-				// Leave slot null
-			}
-			
-		}
-		
-		return slot;
-	}
-
-	private void cleanContainerUpIfNecessary() {
-		ItemContainer container = slotComponent.getSlotContainer();
-		if (container instanceof ItemContainerMixed) {
-			((ItemContainerMixed) container).cleanUpSlots();
-		}
-	}
-
 }

src/main/java/ru/windcorp/progressia/client/graphics/world/hud/InventoryWindow.java

@@ -43,6 +43,8 @@ public class InventoryWindow extends Panel {
 	
 	private final InventoryComponent content;
 	private final HUDWorkspace workspace;
+	
+	Object layoutCookie;
 
 	public InventoryWindow(String name, InventoryComponent component, HUDWorkspace workspace) {
 		super(name, new LayoutVertical(15, 15));

src/main/java/ru/windcorp/progressia/client/graphics/world/hud/SlotComponent.java

@@ -51,8 +51,7 @@ public class SlotComponent extends Component {
 	private static Renderable containerOpenDecoration = null;
 	private static Renderable containerOpenableDecoration = null;
 
-	private final ItemContainer container;
-	private final int index;
+	private final ItemSlot slot;
 
 	private float scale = 2;
 
@@ -66,8 +65,7 @@ public class SlotComponent extends Component {
 
 	public SlotComponent(String name, ItemContainer container, int index) {
 		super(name);
-		this.container = container;
-		this.index = index;
+		this.slot = new ItemSlot(container, index);
 
 		setScale(2);
 
@@ -95,18 +93,18 @@ public class SlotComponent extends Component {
 	 * @return the container
 	 */
 	public ItemContainer getSlotContainer() {
-		return container;
+		return slot.getContainer();
 	}
 	
 	/**
 	 * @return the index
 	 */
 	public int getSlotIndex() {
-		return index;
+		return slot.getIndex();
 	}
 
 	public ItemSlot getSlot() {
-		return container.getSlot(index);
+		return slot;
 	}
 
 	public SlotComponent setScale(float scale) {
@@ -142,14 +140,7 @@ public class SlotComponent extends Component {
 
 	private void updateItemRenderer() {
 		ItemData contents;
-		
-		ItemSlot slot = getSlot();
-		
-		if (slot == null) {
-			contents = null;
-		} else {
-			contents = slot.getContents();
-		}
+		contents = slot.getItem();
 
 		if (contents == null) {
 			itemRenderer = null;
@@ -160,7 +151,7 @@ public class SlotComponent extends Component {
 				itemRenderer = ItemRenderRegistry.getInstance().get(contents.getId()).createRenderable(contents);
 			}
 
-			int newAmount = slot.getAmount();
+			int newAmount = slot.getCount();
 			if (newAmount != amountDisplayInt) {
 				amountDisplayInt = newAmount;
 				amountDisplayString = newAmount == 1 ? "" : Integer.toString(newAmount);
@@ -195,7 +186,7 @@ public class SlotComponent extends Component {
 	}
 
 	private void renderDecorations(ShapeRenderHelper renderer) {
-		ItemData contents = getSlot().getContents();
+		ItemData contents = getSlot().getItem();
 
 		if (contents instanceof ItemDataContainer) {
 			ItemDataContainer asContainer = (ItemDataContainer) contents;

src/main/java/ru/windcorp/progressia/client/graphics/world/hud/WindowedHUD.java

@@ -17,13 +17,49 @@
  */
 package ru.windcorp.progressia.client.graphics.world.hud;
 
+import glm.vec._2.Vec2;
+import glm.vec._2.i.Vec2i;
 import ru.windcorp.progressia.client.graphics.gui.Component;
+import ru.windcorp.progressia.client.graphics.gui.Layout;
 
 public class WindowedHUD extends Component {
+	
+	private static class Cookie {
+		private final Vec2 relPos = new Vec2();
+	}
 
 	public WindowedHUD(String name) {
 		super(name);
-		setLayout(null);
+		setLayout(new Layout() {
+			@Override
+			public void layout(Component c) {
+				for (Component component : c.getChildren()) {
+					InventoryWindow window = (InventoryWindow) component;
+					
+					window.setSize(window.getPreferredSize());
+					
+					Cookie cookie = (Cookie) window.layoutCookie;
+					
+					if (cookie == null) {
+						window.layoutCookie = cookie = new Cookie();
+						cookie.relPos.x = 0.5f;
+						cookie.relPos.y = 2 / 3.0f;
+					}
+					
+					cookie.relPos.clamp(0, 1);
+					
+					window.setPosition(
+						(int) (cookie.relPos.x * c.getWidth() - window.getWidth() / 2.0f),
+						(int) (cookie.relPos.y * c.getHeight() - window.getHeight())
+					);
+				}
+			}
+			
+			@Override
+			public Vec2i calculatePreferredSize(Component c) {
+				throw new AssertionError("welp this wasnt supposed to hapen :(");
+			}
+		});
 	}
 
 	public void addWindow(InventoryWindow window) {

src/main/java/ru/windcorp/progressia/client/world/item/inventory/ContainerComponentSimple.java

@@ -19,6 +19,7 @@ package ru.windcorp.progressia.client.world.item.inventory;
 
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.List;
 
 import com.google.common.eventbus.Subscribe;
 
@@ -33,19 +34,24 @@ import ru.windcorp.progressia.client.graphics.world.hud.Bar;
 import ru.windcorp.progressia.client.graphics.world.hud.HUDWorkspace;
 import ru.windcorp.progressia.client.graphics.world.hud.InteractiveSlotComponent;
 import ru.windcorp.progressia.common.world.item.inventory.ItemContainer;
-import ru.windcorp.progressia.common.world.item.inventory.event.ItemSlotAddedEvent;
-import ru.windcorp.progressia.common.world.item.inventory.event.ItemSlotRemovedEvent;
+import ru.windcorp.progressia.common.world.item.inventory.event.ItemSlotChangedEvent;
 
 public class ContainerComponentSimple extends ContainerComponent {
 
 	private final Group slots = new Group("Inventory.Slots", new LayoutGrid(0, 15));
-	private final Collection<InteractiveSlotComponent> slotCollection = new ArrayList<>();
+	private final List<InteractiveSlotComponent> slotCollection = new ArrayList<>();
 	
 	private final ItemContainer container;
+	private final HUDWorkspace workspace;
+	
+	private int tmp__getSlotsPerRow() {
+		return 6;
+	}
 
 	public ContainerComponentSimple(ItemContainer container, HUDWorkspace workspace) {
 		super("Inventory");
 		this.container = container;
+		this.workspace = workspace;
 		
 		if (container.getInventory() != null) {
 			container.getInventory().subscribe(this);
@@ -77,14 +83,12 @@ public class ContainerComponentSimple extends ContainerComponent {
 
 		addChild(slotsAndVolumeBar.setLayoutHint(LayoutBorderHorizontal.CENTER));
 		addChild(massBar.setLayoutHint(LayoutBorderHorizontal.LEFT));
-
-		for (int i = 0; i < container.getSlotCount(); ++i) {
-			addSlot(container, i, workspace);
-		}
+		
+		onSlotChanged(null);
 	}
 
-	private void addSlot(ItemContainer container, int index, HUDWorkspace workspace) {
-		final int maxX = 6;
+	private void addSlot(int index) {
+		final int maxX = tmp__getSlotsPerRow();
 
 		InteractiveSlotComponent component = new InteractiveSlotComponent("Inventory.Slot." + index, container, index, workspace);
 
@@ -92,6 +96,10 @@ public class ContainerComponentSimple extends ContainerComponent {
 		slots.addChild(component.setLayoutHint(pos));
 		slotCollection.add(component);
 	}
+	
+	private void removeSlot(int index) {
+		slots.removeChild(slotCollection.remove(index));
+	}
 
 	@Override
 	public ItemContainer getContainer() {
@@ -104,13 +112,26 @@ public class ContainerComponentSimple extends ContainerComponent {
 	}
 	
 	@Subscribe
-	private void onSlotAdded(ItemSlotAddedEvent e) {
+	private void onSlotChanged(ItemSlotChangedEvent e) {
+		if (e != null && e.getContainer() != container) {
+			return;
+		}
 		
-	}
-	
-	@Subscribe
-	private void onSlotAdded(ItemSlotRemovedEvent e) {
+		int wantSlots = container.getLastFilledSlot();
+		int slotsPerRow = tmp__getSlotsPerRow();
 		
+		wantSlots = (wantSlots / slotsPerRow + 2) * slotsPerRow;
+		if (wantSlots < slotsPerRow) {
+			wantSlots = slotsPerRow;
+		}
+		
+		while (wantSlots > slotCollection.size()) {
+			addSlot(slotCollection.size());
+		}
+		
+		while (wantSlots < slotCollection.size()) {
+			removeSlot(slotCollection.size() - 1);
+		}
 	}
 
 }

src/main/java/ru/windcorp/progressia/common/world/item/ItemDataContainer.java

@@ -17,12 +17,18 @@
  */
 package ru.windcorp.progressia.common.world.item;
 
+import java.util.Iterator;
+
+import com.google.common.collect.Iterators;
+
 import ru.windcorp.progressia.common.state.ObjectStateField;
+import ru.windcorp.progressia.common.world.item.inventory.InventoryOwner;
 import ru.windcorp.progressia.common.world.item.inventory.InventorySimple;
 import ru.windcorp.progressia.common.world.item.inventory.InventoryUser;
+import ru.windcorp.progressia.common.world.item.inventory.ItemContainer;
 import ru.windcorp.progressia.common.world.item.inventory.ItemContainerMixedSimple;
 
-public class ItemDataContainer extends ItemData {
+public class ItemDataContainer extends ItemData implements InventoryOwner, ItemDataWithContainers {
 
 	private final ObjectStateField<InventorySimple> inventory = field("Core:Contents").setShared().def(this::createInventory)
 		.build();
@@ -53,13 +59,19 @@ public class ItemDataContainer extends ItemData {
 	protected InventorySimple createInventory() {
 		return new InventorySimple(
 			getId(),
-			new ItemContainerMixedSimple(getId(), containerMassLimit, containerVolumeLimit, 10)
+			this,
+			new ItemContainerMixedSimple(getId(), containerMassLimit, containerVolumeLimit)
 		);
 	}
 	
 	public InventorySimple getInventory() {
 		return inventory.get(this);
 	}
+	
+	@Override
+	public Iterator<? extends ItemContainer> getAllContainers() {
+		return Iterators.forArray(getInventory().getContainers());
+	}
 
 	public boolean isOpen() {
 		return !getInventory().getUsers().isEmpty();

src/main/java/ru/windcorp/progressia/common/world/item/ItemDataWithContainers.java

@@ -15,14 +15,14 @@
  * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
-package ru.windcorp.progressia.common.world.item.inventory.event;
+package ru.windcorp.progressia.common.world.item;
 
-import ru.windcorp.progressia.common.world.item.inventory.ItemSlot;
+import java.util.Iterator;
 
-public class ItemSlotAddedEvent extends ItemSlotEvent {
+import ru.windcorp.progressia.common.world.item.inventory.ItemContainer;
 
-	public ItemSlotAddedEvent(ItemSlot slot) {
-		super(slot);
-	}
+public interface ItemDataWithContainers {
+	
+	Iterator<? extends ItemContainer> getAllContainers();
 
 }

src/main/java/ru/windcorp/progressia/common/world/item/Items.java

@@ -1,120 +0,0 @@
-/*
- * Progressia
- * Copyright (C)  2020-2021  Wind Corporation and contributors
- *
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program.  If not, see <https://www.gnu.org/licenses/>.
- */
-package ru.windcorp.progressia.common.world.item;
-
-import ru.windcorp.progressia.common.world.item.inventory.ItemSlot;
-
-public class Items {
-
-	/**
-	 * Attempts to transfer as many items as possible, but no more than
-	 * {@code max}, between two slots. The origin of the items is {@code from},
-	 * the destination is {@code into}. This method does nothing if the items
-	 * could not be removed from origin or could not be inserted into
-	 * destination, such as when {@code into} contains different items.
-	 * 
-	 * @param from the slot to transfer item from
-	 * @param into the destination of the items
-	 * @param max  the maximum amount of items to transfer. Use
-	 *             {@code Integer.MAX_VALUE} to remove the limit
-	 * @return the actual amount of items moved between slots
-	 */
-	public static int pour(ItemSlot from, ItemSlot into, int max) {
-		synchronized (from) {
-			synchronized (into) {
-
-				ItemData item = from.getContents();
-
-				int originalAmount = from.getAmount();
-				int transferAmount = Math.min(originalAmount, max);
-
-				while (transferAmount > 0 && !into.canInsert(item, transferAmount)) {
-					transferAmount--;
-				}
-				
-				if (transferAmount == 0) {
-					return 0;
-				}
-
-				if (!from.canRemove(transferAmount)) {
-					return 0;
-				}
-
-				into.setContents(item, into.getAmount() + transferAmount);
-				from.setAmount(originalAmount - transferAmount);
-
-				return transferAmount;
-
-			}
-		}
-	}
-
-	/**
-	 * Attempts to transfer as many items as possible between two slots. The
-	 * origin of the items is {@code from}, the destination is {@code into}.
-	 * This method does nothing if the items could not be removed from origin or
-	 * could not be inserted into destination, such as when {@code into}
-	 * contains different items.
-	 * 
-	 * @param from the slot to transfer item from
-	 * @param into the destination of the items
-	 * @return the actual amount of items moved between slots
-	 */
-	public static int pour(ItemSlot from, ItemSlot into) {
-		return pour(from, into, Integer.MAX_VALUE);
-	}
-	
-	/**
-	 * Attempts to swap the contents of the two slots.
-	 * 
-	 * @param a one of the slots
-	 * @param b the other slot
-	 * 
-	 * @return whether the swap succeeded
-	 */
-	public static boolean swap(ItemSlot a, ItemSlot b) {
-		synchronized (a) {
-			synchronized (b) {
-
-				ItemData aItem = a.getContents();
-				int aAmount = a.getAmount();
-				
-				ItemData bItem = b.getContents();
-				int bAmount = b.getAmount();
-				
-				a.clear();
-				b.clear();
-				
-				if (a.canInsert(bItem, bAmount) && b.canInsert(aItem, aAmount)) {
-					a.setContents(bItem, bAmount);
-					b.setContents(aItem, aAmount);
-					return true;
-				} else {
-					a.setContents(aItem, aAmount);
-					b.setContents(bItem, bAmount);
-					return false;
-				}
-				
-			}
-		}
-	}
-
-	private Items() {
-	}
-
-}

src/main/java/ru/windcorp/progressia/common/world/item/LinearItemProperty.java

@@ -0,0 +1,75 @@
+/*
+ * Progressia
+ * Copyright (C)  2020-2021  Wind Corporation and contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package ru.windcorp.progressia.common.world.item;
+
+import ru.windcorp.progressia.common.world.item.inventory.ItemContainer;
+import ru.windcorp.progressia.common.world.item.inventory.ItemSlot;
+
+/**
+ * A generalization of mass and volume. Not to be extended by mods.
+ */
+public enum LinearItemProperty {
+	
+	MASS,
+	VOLUME;
+	
+	public float get(ItemData item) {
+		switch (this) {
+		case MASS:
+			return item.getMass();
+		case VOLUME:
+			return item.getVolume();
+		default:
+			throw new AssertionError();
+		}
+	}
+	
+	public float get(ItemSlot slot) {
+		switch (this) {
+		case MASS:
+			return slot.getMass();
+		case VOLUME:
+			return slot.getVolume();
+		default:
+			throw new AssertionError();
+		}
+	}
+	
+	public float get(ItemContainer container) {
+		switch (this) {
+		case MASS:
+			return container.getMass();
+		case VOLUME:
+			return container.getVolume();
+		default:
+			throw new AssertionError();
+		}
+	}
+	
+	public float getLimit(ItemContainer container) {
+		switch (this) {
+		case MASS:
+			return container.getMassLimit();
+		case VOLUME:
+			return container.getVolumeLimit();
+		default:
+			throw new AssertionError();
+		}
+	}
+
+}

src/main/java/ru/windcorp/progressia/common/world/item/inventory/Inventory.java

@@ -36,19 +36,25 @@ import ru.windcorp.progressia.common.world.item.inventory.event.InventoryOpenedE
 
 public class Inventory extends Namespaced implements Encodable {
 	
+	private final InventoryOwner owner;
 	private final ItemContainer[] containers;
 	private final List<InventoryUser> users = new ArrayList<>();
 	
 	private EventBus eventBus = null;
 
-	public Inventory(String id, ItemContainer... containers) {
+	public Inventory(String id, InventoryOwner owner, ItemContainer... containers) {
 		super(id);
+		this.owner = owner;
 		this.containers = containers;
 		for (ItemContainer container : containers) {
 			container.setInventory(this);
 		}
 	}
 	
+	public InventoryOwner getOwner() {
+		return owner;
+	}
+	
 	public synchronized void open(InventoryUser user) {
 		users.add(user);
 		subscribe(user);

src/main/java/ru/windcorp/progressia/common/world/item/inventory/InventoryOwner.java

@@ -15,14 +15,8 @@
  * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
-package ru.windcorp.progressia.common.world.item.inventory.event;
+package ru.windcorp.progressia.common.world.item.inventory;
 
-import ru.windcorp.progressia.common.world.item.inventory.ItemSlot;
-
-public class ItemSlotRemovedEvent extends ItemSlotEvent {
-
-	public ItemSlotRemovedEvent(ItemSlot slot) {
-		super(slot);
-	}
+public interface InventoryOwner {
 
 }

src/main/java/ru/windcorp/progressia/common/world/item/inventory/InventorySimple.java

@@ -19,8 +19,8 @@ package ru.windcorp.progressia.common.world.item.inventory;
 
 public class InventorySimple extends Inventory {
 
-	public InventorySimple(String id, ItemContainer container) {
-		super(id, container);
+	public InventorySimple(String id, InventoryOwner owner, ItemContainer container) {
+		super(id, owner, container);
 	}
 	
 	public ItemContainer getContainer() {

src/main/java/ru/windcorp/progressia/common/world/item/inventory/ItemContainer.java

@@ -18,79 +18,335 @@
 package ru.windcorp.progressia.common.world.item.inventory;
 
 import java.util.Iterator;
-import java.util.NoSuchElementException;
-import java.util.function.Consumer;
 
+import gnu.trove.iterator.TIntIterator;
+import gnu.trove.set.TIntSet;
+import gnu.trove.set.hash.TIntHashSet;
 import ru.windcorp.progressia.common.state.Encodable;
 import ru.windcorp.progressia.common.util.namespaces.Namespaced;
+import ru.windcorp.progressia.common.world.item.ItemData;
+import ru.windcorp.progressia.common.world.item.ItemDataWithContainers;
+import ru.windcorp.progressia.common.world.item.inventory.event.ItemSlotChangedEvent;
 
 /**
- * A collection of {@link ItemSlot}s representing a single storage unit. A
- * container may impose limits on the maximum total mass and volume of its
- * contents, although this is not enforced on data structure level.
+ * The base class of item containers. An item container is a set of slots with
+ * defined limits on mass and volume. Containers are typically grouped into an
+ * {@link Inventory} by in-game function.
  * <p>
- * At any moment container has a definite amount of slots, each identified by a
- * unique index. If a container has <i>n</i> slots, its slots are numbered 0
- * through <i>n</i> - 1.
+ * A container contains some number of slots indexed from 0. Users may inspect
+ * the contents of any slot. Slots may contain an {@link ItemData} and an item
+ * count. Item count is zero iff the {@link ItemData} is {@code null}, in which
+ * case the slot is considered empty.
+ * <p>
+ * Item containers also implement mass and volume calculation. Both properties
+ * are determined by the corresponding {@link ItemData} methods, and combine
+ * linearly:
+ * {@code mass(2 * stick + 5 * apple) = 2 * mass(stick) + 5 * mass(apple)},
+ * empty slots do not contribute any mass of volume. Users may query these
+ * computed values. In addition, a limit may be imposed on both properties.
+ * Containers will refuse operations which would lead to the violation of these
+ * limits. However, containers cannot detect excess mass or volume resulting
+ * from
+ * a implementation-triggered change of limits.
+ * <p>
+ * As a safeguard against memory leaks, all containers must be provided with a
+ * maximum possible size. Item containers will refuse operations on slots with
+ * indices exceeding or equal to the maximum possible size.
+ * <p>
+ * Implementors will typically find the following subclasses useful:
+ * <ul>
+ * <li>{@link ItemContainerMixed} for containers allowing an arbitrary amount of
+ * slots
+ * <ul>
+ * <li>see {@link ItemContainerMixedSimple} for a complete implementation</li>
+ * </ul>
+ * </li>
+ * <li>{@link ItemContainerSingle} for containers providing only one slot</li>
+ * </ul>
  */
-public abstract class ItemContainer extends Namespaced implements Encodable, Iterable<ItemSlot> {
-	
-	private Inventory inventory;
+public abstract class ItemContainer extends Namespaced implements Encodable {
 
-	public ItemContainer(String id) {
+	private Inventory inventory;
+	private final int maxPossibleSize;
+
+	private final TIntSet subContainersCache = new TIntHashSet();
+
+	public ItemContainer(String id, int maxPossibleSize) {
 		super(id);
+		this.maxPossibleSize = maxPossibleSize;
 	}
-	
+
+	/**
+	 * @return the inventory
+	 */
 	public Inventory getInventory() {
 		return inventory;
 	}
-	
-	protected void setInventory(Inventory inventory) {
+
+	/**
+	 * @param inventory the inventory to set
+	 */
+	void setInventory(Inventory inventory) {
 		this.inventory = inventory;
 	}
 
 	/**
-	 * Retrieves the slot with the given index.
+	 * Retrieves the item in the slot at the specified index.
 	 * 
-	 * @param index the index of the slot to retrieve
-	 * @return the slot or {@code null} if the slot does not exist
+	 * @param index the index of the slot to query
+	 * @return the item or {@code null} if the slot is empty.
 	 */
-	public abstract ItemSlot getSlot(int index);
+	public abstract ItemData getItem(int index);
 
 	/**
-	 * Returns the current slot count of this container.
+	 * Retrieves the item count in the slot at the specified index.
 	 * 
-	 * @return the number of slots in this container
+	 * @param index the index of the slot to query
+	 * @return the item count or {@code 0} if the slot is empty
 	 */
-	public abstract int getSlotCount();
-	
-	@Override
-	public abstract void forEach(Consumer<? super ItemSlot> action);
-	
-	@Override
-	public Iterator<ItemSlot> iterator() {
-		return new Iterator<ItemSlot>() {
-			
-			private int index = 0;
+	public abstract int getCount(int index);
 
-			@Override
-			public boolean hasNext() {
-				return index < getSlotCount();
-			}
+	/**
+	 * Returns a slot index strictly greater than all indices containing
+	 * items. Use this index as upper bound when iterating the container's
+	 * items. The slot at this index is empty.
+	 * 
+	 * @return a strict upper bound on the indices of filled slots
+	 */
+	public abstract int getMaxIndex();
 
-			@Override
-			public ItemSlot next() {
-				if (!hasNext()) {
-					throw new NoSuchElementException("index = " + index + ", size = " + getSlotCount());
-				}
-				ItemSlot slot = getSlot(index);
-				index++;
-				return slot;
-			}
-			
-		};
+	/**
+	 * Computes and returns the slot index of the last non-empty slot in this
+	 * container. This operation is potentially slower than
+	 * {@link #getMaxIndex()}, which is less precise.
+	 * 
+	 * @return the index of the last filled slot, or -1 if no such slot exists
+	 */
+	public int getLastFilledSlot() {
+		int index = getMaxIndex();
+		while (isEmpty(index) && index >= 0) {
+			index--;
+		}
+		return index;
 	}
 
+	/**
+	 * Determines whether items of the specified kind could be inserted into
+	 * this container assuming a slot is available and mass and volume
+	 * requirements of the container are satisfied.
+	 * <p>
+	 * This method should only be a function of the configuration of the
+	 * container and of the item; it must not depend on the contents of the
+	 * container.
+	 * 
+	 * @param item the item to check, not null
+	 * @return {@code true} iff the item is allowed in this container
+	 */
+	protected boolean canAdd(ItemData item) {
+		if (item == null) {
+			return false;
+		}
+		
+		if (containsRecursively(item, this)) {
+			return false;
+		}
+
+		return true;
+	}
+
+	private boolean containsRecursively(ItemData haystack, ItemContainer needle) {
+		if (!(haystack instanceof ItemDataWithContainers)) {
+			return false;
+		}
+		
+		ItemDataWithContainers haystackWithTastyHay = (ItemDataWithContainers) haystack;
+		
+		Iterator<? extends ItemContainer> iterator = haystackWithTastyHay.getAllContainers();
+		while (iterator.hasNext()) {
+			ItemContainer container = iterator.next();
+			
+			if (container == needle) {
+				return true;
+			}
+			
+			TIntIterator indexIterator = container.subContainersCache.iterator();
+			
+			while (indexIterator.hasNext()) {
+				
+				int subHaystackIndex = indexIterator.next();
+				ItemData subHaystackOrMaybeNot = container.getItem(subHaystackIndex);
+				if (containsRecursively(subHaystackOrMaybeNot, needle)) {
+					return true;
+				}
+				
+			}
+		}
+		
+		return false;
+	}
+
+	/**
+	 * Determines whether inventory users are allowed to manually remove items
+	 * from this container.
+	 * <p>
+	 * This method should only be a function of the configuration of the
+	 * container; it must not depend on the contents of the container.
+	 * 
+	 * @return {@code true} iff items could be removed from this container
+	 */
+	protected boolean isRemovingAllowed() {
+		return true;
+	}
+
+	/**
+	 * Determines whether the items can be added at the specified index. The
+	 * slot must already contain items equal to {@code item} parameter or be
+	 * empty. Neither total mass nor volume of the added items may exceed the
+	 * available mass and volume of the container. Additional restrictions may
+	 * apply.
+	 * <p>
+	 * When index is valid, {@code item == null} and {@code count == 0}, this
+	 * method returns {@code true}. Otherwise, when {@code count <= 0}, this
+	 * method returns {@code false}.
+	 * 
+	 * @param index the index of the slot to query
+	 * @param item  the item type
+	 * @param count the amount of items to add
+	 * @return {@code true} iff the operation is possible
+	 */
+	public boolean canAdd(int index, ItemData item, int count) {
+		if (index < 0 || index >= maxPossibleSize) {
+			return false;
+		}
+
+		if (item == null && count == 0) {
+			return true;
+		}
+
+		if (count < 0) {
+			return false;
+		}
+
+		ItemData currentItem = getItem(index);
+		if (currentItem == null) {
+			// Pass
+		} else if (currentItem.equals(item)) {
+			// Pass
+		} else {
+			return false;
+		}
+
+		if (!canAdd(item)) {
+			return false;
+		}
+
+		float addedMass = item.getMass() * count;
+		if (getMass() + addedMass > getMassLimit()) {
+			return false;
+		}
+
+		float addedVolume = item.getVolume() * count;
+		if (getVolume() + addedVolume > getVolumeLimit()) {
+			return false;
+		}
+
+		return true;
+	}
+
+	/**
+	 * Determines whether the items can be removed from the specified index. The
+	 * slot must already contain items equal to {@code item} parameter. The item
+	 * count must be no lower than {@code count} parameter. Additional
+	 * restrictions may apply.
+	 * <p>
+	 * When index is valid, {@code item == null} and {@code count == 0}, this
+	 * method returns {@code true}. Otherwise, when {@code count <= 0}, this
+	 * method returns {@code false}.
+	 * 
+	 * @param index the index of the slot to query
+	 * @param item  the item type
+	 * @param count the amount of items to remove
+	 * @return {@code true} iff the operation is possible
+	 */
+	public boolean canRemove(int index, ItemData item, int count) {
+		if (index < 0 || index >= maxPossibleSize) {
+			return false;
+		}
+
+		if (item == null && count == 0) {
+			return true;
+		}
+
+		if (count < 0) {
+			return false;
+		}
+
+		if (!isRemovingAllowed()) {
+			return false;
+		}
+
+		ItemData currentItem = getItem(index);
+		if (currentItem == null) {
+			return false;
+		} else if (currentItem.equals(item)) {
+			// Pass
+		} else {
+			return false;
+		}
+
+		if (getCount(index) < count) {
+			return false;
+		}
+
+		return true;
+	}
+
+	/**
+	 * Attempts to add the provided items to the specified slot in the
+	 * container. This method modifies the data structure directly; use an
+	 * appropriate {@link Items} method to add items in a safe and convenient
+	 * way.
+	 * <p>
+	 * A {@link #canAdd(int, ItemData, int)} check is performed. If the check
+	 * fails, the method does not alter the contents of the container and
+	 * returns {@code false}. If the check succeeds, the item type and item
+	 * count of the referenced slot are altered appropriately and {@code true}
+	 * is returned.
+	 * <p>
+	 * When {@code item == null} or {@code count <= 0}, this method returns
+	 * {@code false}.
+	 * 
+	 * @param index the index of the slot to alter
+	 * @param item  the item type
+	 * @param count the amount of items to add
+	 * @return {@code true} iff the container was changed as the result of this
+	 *         operation
+	 */
+	protected abstract boolean add(int index, ItemData item, int count);
+
+	/**
+	 * Attempts to remove the provided items from the specified slot in the
+	 * container. This method modifies the data structure directly; use an
+	 * appropriate {@link Items} method to remove items in a safe and convenient
+	 * way.
+	 * <p>
+	 * A {@link #canRemove(int, ItemData, int)} check is performed. If the check
+	 * fails, the method does not alter the contents of the container and
+	 * returns {@code false}. If the check succeeds, the item type and item
+	 * count of the referenced slot are altered appropriately and {@code true}
+	 * is returned.
+	 * <p>
+	 * When {@code item == null} or {@code count <= 0}, this method returns
+	 * {@code false}.
+	 * 
+	 * @param index the index of the slot to alter
+	 * @param item  the item type
+	 * @param count the amount of items to remove
+	 * @return {@code true} iff the container was changed as the result of this
+	 *         operation
+	 */
+	protected abstract boolean remove(int index, ItemData item, int count);
+
 	/**
 	 * Computes and returns the mass limit that the container imposes.
 	 * 
@@ -108,33 +364,162 @@ public abstract class ItemContainer extends Namespaced implements Encodable, Ite
 	 *         boundary is set
 	 */
 	public abstract float getVolumeLimit();
-	
+
 	public synchronized float getMass() {
 		float sum = 0;
-		for (int i = 0; i < getSlotCount(); ++i) {
-			ItemSlot slot = getSlot(i);
-			
-			if (slot.isEmpty()) {
+		for (int i = 0; i < getMaxIndex(); ++i) {
+			ItemData data = getItem(i);
+
+			if (data == null) {
 				continue;
 			}
-			
-			sum += slot.getContents().getMass() * slot.getAmount();
-		}
-		return sum;
-	}
-	
-	public synchronized float getVolume() {
-		float sum = 0;
-		for (int i = 0; i < getSlotCount(); ++i) {
-			ItemSlot slot = getSlot(i);
-			
-			if (slot.isEmpty()) {
-				continue;
-			}
-			
-			sum += slot.getContents().getVolume() * slot.getAmount();
+
+			sum += data.getMass() * getCount(i);
 		}
 		return sum;
 	}
 
+	public synchronized float getVolume() {
+		float sum = 0;
+		for (int i = 0; i < getMaxIndex(); ++i) {
+			ItemData data = getItem(i);
+
+			if (data == null) {
+				continue;
+			}
+
+			sum += data.getVolume() * getCount(i);
+		}
+		return sum;
+	}
+
+	protected void fireSlotChangeEvent(int index) {
+		if (getItem(index) instanceof ItemDataWithContainers) {
+			subContainersCache.add(index);
+		} else {
+			subContainersCache.remove(index);
+		}
+		
+		Inventory inventory = this.inventory;
+		if (inventory == null || inventory.getEventBus() == null) {
+			return;
+		}
+
+		inventory.getEventBus().post(new ItemSlotChangedEvent(this, index));
+	}
+
+	/**
+	 * Checks class invariants and throws an {@link IllegalStateException} in
+	 * case of discrepancies.
+	 */
+	protected synchronized void checkState() {
+		int maxIndex = getMaxIndex();
+		for (int index = 0; index < maxIndex; ++index) {
+
+			ItemData item = getItem(index);
+			int count = getCount(index);
+
+			if ((item == null) != (count == 0)) {
+				if (item == null) {
+					throw new IllegalStateException("Item is null but count (" + count + ") != 0 in slot " + index);
+				} else {
+					throw new IllegalStateException("Item is " + item + " but count is zero in slot " + index);
+				}
+			}
+
+			if (count < 0) {
+				throw new IllegalStateException("count is negative: " + count + " in slot " + index);
+			}
+
+			boolean isContainer = item instanceof ItemDataWithContainers;
+			if (isContainer != subContainersCache.contains(index)) {
+				if (!isContainer) {
+					throw new IllegalStateException(
+						"subContainersCache is invalid: item in slot " + index + " (" + item
+							+ ") is cached as a container"
+					);
+				} else {
+					throw new IllegalStateException(
+						"subContainersCache is invalid: item in slot " + index + " (" + item
+							+ ") is not cached as a container"
+					);
+				}
+			}
+			
+			if (isContainer) {
+				if (containsRecursively(item, this)) {
+					throw new IllegalStateException("Recursion detected in slot " + index);
+				}
+			}
+
+		}
+
+		// Using negation in following checks to trigger errors if any value is
+		// NaN
+		// (since all comparisons return false if any operand is NaN)
+
+		float mass = getMass();
+		if (!(mass >= 0)) {
+			throw new IllegalStateException("Mass is negative: " + mass);
+		}
+
+		float massLimit = getMassLimit();
+		if (!(mass <= massLimit)) {
+			throw new IllegalStateException("Mass is greater than mass limit: " + mass + " > " + massLimit);
+		}
+
+		float volume = getVolume();
+		if (!(volume >= 0)) {
+			throw new IllegalStateException("Volume is negative: " + volume);
+		}
+
+		float volumeLimit = getVolumeLimit();
+		if (!(volume <= volumeLimit)) {
+			throw new IllegalStateException("Volume is greater than volume limit: " + volume + " > " + volumeLimit);
+		}
+	}
+
+	@FunctionalInterface
+	public interface SlotConsumer {
+		void accept(ItemData item, int count);
+	}
+
+	/**
+	 * Invokes the provided action for each slot in this container. The action
+	 * is run for empty slots, in which case call is invoked with parameters
+	 * {@code (null, 0)} for each empty slot. The exact amount of invocations is
+	 * determined by {@link #getMaxIndex()}.
+	 * 
+	 * @param action the action to run
+	 * @see #forEachItem(SlotConsumer)
+	 */
+	public synchronized void forEachSlot(SlotConsumer action) {
+		int maxIndex = getMaxIndex();
+		for (int i = 0; i < maxIndex; ++i) {
+			action.accept(getItem(i), getCount(i));
+		}
+	}
+
+	/**
+	 * Invokes the provided action for each non-empty slot in this container.
+	 * 
+	 * @param action the action to run
+	 * @see #forEachSlot(SlotConsumer)
+	 */
+	public synchronized void forEachItem(SlotConsumer action) {
+		int maxIndex = getMaxIndex();
+		for (int i = 0; i < maxIndex; ++i) {
+
+			int count = getCount(i);
+			if (count != 0) {
+				action.accept(getItem(i), count);
+			}
+
+		}
+	}
+
+	public boolean isEmpty(int index) {
+		return getCount(index) == 0;
+	}
+
 }

src/main/java/ru/windcorp/progressia/common/world/item/inventory/ItemContainerEquipment.java

@@ -28,10 +28,20 @@ public class ItemContainerEquipment extends ItemContainerSingle {
 	private final SpeciesData.EquipmentSlot equipmentSlot;
 
 	public ItemContainerEquipment(String id, SpeciesData.EquipmentSlot equipmentSlot) {
-		super(id, EQUIP_MASS_LIMIT, EQUIP_VOLUME_LIMIT);
+		super(id);
 		this.equipmentSlot = equipmentSlot;
 	}
 	
+	@Override
+	public float getMassLimit() {
+		return EQUIP_MASS_LIMIT;
+	}
+	
+	@Override
+	public float getVolumeLimit() {
+		return EQUIP_VOLUME_LIMIT;
+	}
+	
 	public SpeciesData.EquipmentSlot getEquipmentSlot() {
 		return equipmentSlot;
 	}

src/main/java/ru/windcorp/progressia/common/world/item/inventory/ItemContainerHand.java

@@ -29,10 +29,20 @@ public class ItemContainerHand extends ItemContainerSingle {
 	private final SpeciesData.Hand hand;
 
 	public ItemContainerHand(String id, Hand hand) {
-		super(id, HAND_MASS_LIMIT, HAND_VOLUME_LIMIT);
+		super(id);
 		this.hand = hand;
 	}
 	
+	@Override
+	public float getMassLimit() {
+		return HAND_MASS_LIMIT;
+	}
+	
+	@Override
+	public float getVolumeLimit() {
+		return HAND_VOLUME_LIMIT;
+	}
+	
 	public SpeciesData.Hand getHand() {
 		return hand;
 	}

src/main/java/ru/windcorp/progressia/common/world/item/inventory/ItemContainerMixed.java

@@ -20,198 +20,214 @@ package ru.windcorp.progressia.common.world.item.inventory;
 import java.io.DataInput;
 import java.io.DataOutput;
 import java.io.IOException;
-import java.util.ArrayList;
-import java.util.Collection;
-import java.util.List;
-import java.util.function.Consumer;
-
-import com.google.common.eventbus.Subscribe;
-
 import ru.windcorp.progressia.common.state.Encodable;
 import ru.windcorp.progressia.common.state.IOContext;
-import ru.windcorp.progressia.common.world.item.inventory.event.ItemSlotAddedEvent;
-import ru.windcorp.progressia.common.world.item.inventory.event.ItemSlotChangedEvent;
-import ru.windcorp.progressia.common.world.item.inventory.event.ItemSlotRemovedEvent;
+import ru.windcorp.progressia.common.world.item.ItemData;
+import ru.windcorp.progressia.common.world.item.ItemDataRegistry;
 
-/**
- * An {@link ItemContainer} capable of storing multiple item stacks. The set
- * of slots is dynamic: new slots may be added and existing slots may be removed
- * at will.
- */
 public abstract class ItemContainerMixed extends ItemContainer {
+
+	public static final int MAX_SLOTS = 10000;
 	
-	private final List<ItemSlot> slots = new ArrayList<>();
+	private static final int GROWTH_STEP = 10;
+	private static final int MINIMUM_CAPACITY = 10;
+	
+	private ItemData[] items = new ItemData[MINIMUM_CAPACITY];
+	private int[] counts = new int[MINIMUM_CAPACITY];
 
 	public ItemContainerMixed(String id) {
-		super(id);
+		super(id, MAX_SLOTS);
 	}
 	
-	@Override
-	protected void setInventory(Inventory inventory) {
-		if (getInventory() != null) {
-			getInventory().unsubscribe(this);
+	protected void setCapacity(int minimumCapacity) {
+		if (minimumCapacity < 0) {
+			return;
 		}
 		
-		super.setInventory(inventory);
+		int newCapacity = ((minimumCapacity - MINIMUM_CAPACITY - 1) / GROWTH_STEP + 1) * GROWTH_STEP + MINIMUM_CAPACITY;
 		
-		if (getInventory() != null) {
-			getInventory().subscribe(this);
-		}
-	}
-
-	/**
-	 * Appends additional empty slots to the end of this container.
-	 * 
-	 * @param amount the amount of slots to add
-	 */
-	public synchronized void addSlots(int amount) {
-		Inventory inventory = getInventory();
+		ItemData[] newItems = new ItemData[newCapacity];
+		int[] newCounts = new int[newCapacity];
 		
-		for (int i = 0; i < amount; ++i) {
-			ItemSlot slot = createSlot(slots.size());
-			slots.add(slot);
-			slot.setContainer(this);
-			
-			if (inventory != null) {
-				inventory.getEventBus().post(new ItemSlotAddedEvent(slot));
-			}
-		}
+		int length = Math.min(this.items.length, newItems.length);
+		System.arraycopy(this.items, 0, newItems, 0, length);
+		System.arraycopy(this.counts, 0, newCounts, 0, length);
+		
+		this.items = newItems;
+		this.counts = newCounts;
 	}
 	
-	/**
-	 * Instantiates a new slot object that will be appended to the container. 
-	 * 
-	 * @param index the index that the new slot will receive
-	 * @return the new slot
-	 */
-	protected abstract ItemSlot createSlot(int index);
-	
-	@Subscribe
-	private void onSlotChanged(ItemSlotChangedEvent e) {
-		cleanUpSlots();
-	}
-	
-	public void cleanUpSlots() {
-		Inventory inventory = getInventory();
-		Collection<ItemSlotRemovedEvent> events = null;
-		
-		// Do not remove slot 0
-		for (int i = slots.size() - 1; i > 0; --i) {
-			ItemSlot slot = slots.get(i);
-			if (slot.isEmpty()) {
-				slots.remove(i);
-				
-				if (inventory != null) {
-					if (events == null) {
-						events = new ArrayList<>(slots.size() - i);
-					}
-					events.add(new ItemSlotRemovedEvent(slot));
-				}
-			} else {
-				break;
-			}
+	protected void ensureCapacity(int minimumCapacity) {
+		if (items.length >= minimumCapacity) {
+			return;
 		}
 		
-		if (events != null) {
-			// events != null only if inventory != null 
-			events.forEach(inventory.getEventBus()::post);
-		}
-	}
-	
-	@Override
-	public ItemSlot getSlot(int index) {
-		if (index < 0 || index >= slots.size()) {
-			return null;
-		}
-		return slots.get(index);
-	}
-	
-	@Override
-	public int getSlotCount() {
-		return slots.size();
-	}
-	
-	@Override
-	public void forEach(Consumer<? super ItemSlot> action) {
-		slots.forEach(action);
+		setCapacity(minimumCapacity);
 	}
 
 	@Override
 	public synchronized void read(DataInput input, IOContext context) throws IOException {
-		synchronized (slots) {
+		int size = input.readInt();
+		
+		ensureCapacity(size);
 
-			int needSlots = input.readInt();
-			int hasSlots = slots.size();
+		for (int index = 0; index < size; ++index) {
 
-			int costOfResetting = needSlots;
-			int costOfEditing = Math.abs(needSlots - hasSlots);
+			ItemData item;
+			int count = input.readInt();
 
-			if (costOfResetting < costOfEditing) {
-				slots.clear();
-				addSlots(needSlots);
+			if (count != 0) {
+				String id = input.readUTF();
+				item = ItemDataRegistry.getInstance().create(id);
+				item.read(input, context);
 			} else {
-				while (slots.size() > needSlots) {
-					slots.remove(slots.size() - 1);
-				}
-
-				if (slots.size() < needSlots) {
-					addSlots(needSlots - slots.size());
-				}
+				item = null;
 			}
 
-			for (int i = 0; i < needSlots; ++i) {
-				slots.get(i).read(input, context);
-			}
+			items[index] = item;
+			counts[index] = count;
+			
+			fireSlotChangeEvent(index);
 
 		}
+
+		checkState();
 	}
 
 	@Override
 	public synchronized void write(DataOutput output, IOContext context) throws IOException {
-		synchronized (slots) {
 
-			output.writeInt(slots.size());
-			for (int i = 0; i < slots.size(); ++i) {
-				slots.get(i).write(output, context);
+		int size = items.length;
+		output.writeInt(size);
+
+		for (int index = 0; index < size; ++index) {
+			output.writeInt(counts[index]);
+			ItemData item = items[index];
+
+			if (item != null) {
+				output.writeUTF(item.getId());
+				item.write(output, context);
 			}
-
 		}
+
 	}
 
 	@Override
 	public void copy(Encodable destination) {
-		ItemContainerMixed container = (ItemContainerMixed) destination;
-		List<ItemSlot> mySlots = this.slots;
-		List<ItemSlot> containerSlots = container.slots;
+		ItemContainerMixed other = (ItemContainerMixed) destination;
+		int myLength = this.items.length;
 
-		synchronized (mySlots) {
-			synchronized (containerSlots) {
+		synchronized (this) {
+			synchronized (other) {
+				
+				other.setCapacity(myLength);
+				System.arraycopy(this.counts, 0, other.counts, 0, myLength);
 
-				int needSlots = mySlots.size();
-				int hasSlots = containerSlots.size();
+				for (int i = 0; i < myLength; ++i) {
+					ItemData myItem = this.items[i];
+					ItemData otherItem;
 
-				int costOfResetting = needSlots;
-				int costOfEditing = Math.abs(needSlots - hasSlots);
-
-				if (costOfResetting < costOfEditing) {
-					containerSlots.clear();
-					container.addSlots(needSlots);
-				} else {
-					while (containerSlots.size() > needSlots) {
-						slots.remove(containerSlots.size() - 1);
+					if (myItem == null) {
+						otherItem = null;
+					} else {
+						otherItem = ItemDataRegistry.getInstance().create(myItem.getId());
+						myItem.copy(otherItem);
 					}
 
-					if (containerSlots.size() < needSlots) {
-						addSlots(needSlots - containerSlots.size());
-					}
-				}
-
-				for (int i = 0; i < needSlots; ++i) {
-					mySlots.get(i).copy(containerSlots.get(i));
+					other.items[i] = otherItem;
+					other.fireSlotChangeEvent(i);
 				}
 
 			}
 		}
 	}
 
+	@Override
+	public ItemData getItem(int index) {
+		if (index < 0 || index >= items.length) {
+			return null;
+		}
+		return items[index];
+	}
+
+	@Override
+	public int getCount(int index) {
+		if (index < 0 || index >= counts.length) {
+			return 0;
+		}
+		return counts[index];
+	}
+
+	@Override
+	public int getMaxIndex() {
+		return items.length;
+	}
+
+	@Override
+	protected boolean add(int index, ItemData item, int count) {
+		if (!canAdd(index, item, count)) {
+			return false;
+		}
+		
+		if (item != null) {
+			ensureCapacity(index + 1);
+			this.items[index] = item;
+			this.counts[index] += count;
+			fireSlotChangeEvent(index);
+			checkState();
+		}
+		
+		return true;
+	}
+
+	@Override
+	protected boolean remove(int index, ItemData item, int count) {
+		if (!canRemove(index, item, count)) {
+			return false;
+		}
+
+		if (count != 0) {
+			this.counts[index] -= count;
+			
+			if (this.counts[index] == 0) {
+				this.items[index] = null;
+				shrinkIfPossible();
+			}
+	
+			fireSlotChangeEvent(index);
+			checkState();
+		}
+		
+		return true;
+	}
+
+	protected void shrinkIfPossible() {
+		int upperBound;
+		
+		for (upperBound = counts.length; upperBound > MINIMUM_CAPACITY; --upperBound) {
+			if (counts[upperBound - 1] != 0) {
+				break;
+			}
+		}
+		
+		if (upperBound != counts.length) {
+			setCapacity(upperBound);
+		}
+	}
+
+	@Override
+	protected synchronized void checkState() {
+		super.checkState();
+		
+		if (items.length > MAX_SLOTS) {
+			throw new IllegalStateException("Container has more than " + MAX_SLOTS + " slots (items): " + items.length);
+		}
+
+		if (counts.length > MAX_SLOTS) {
+			throw new IllegalStateException(
+				"Container has more than " + MAX_SLOTS + " slots (counts): " + counts.length
+			);
+		}
+	}
+
 }

src/main/java/ru/windcorp/progressia/common/world/item/inventory/ItemContainerMixedSimple.java

@@ -21,22 +21,11 @@ public class ItemContainerMixedSimple extends ItemContainerMixed {
 	
 	private final float massLimit;
 	private final float volumeLimit;
-
-	public ItemContainerMixedSimple(String id, float massLimit, float volumeLimit) {
-		this(id, massLimit, volumeLimit, 1);
-	}
 	
-	public ItemContainerMixedSimple(String id, float massLimit, float volumeLimit, int startingSlots) {
+	public ItemContainerMixedSimple(String id, float massLimit, float volumeLimit) {
 		super(id);
 		this.massLimit = massLimit;
 		this.volumeLimit = volumeLimit;
-		
-		addSlots(startingSlots);
-	}
-
-	@Override
-	public ItemSlot createSlot(int index) {
-		return new ItemSlot();
 	}
 	
 	@Override

src/main/java/ru/windcorp/progressia/common/world/item/inventory/ItemContainerSingle.java

@@ -20,48 +20,89 @@ package ru.windcorp.progressia.common.world.item.inventory;
 import java.io.DataInput;
 import java.io.DataOutput;
 import java.io.IOException;
-import java.util.function.Consumer;
 
 import ru.windcorp.progressia.common.state.Encodable;
 import ru.windcorp.progressia.common.state.IOContext;
+import ru.windcorp.progressia.common.world.item.ItemData;
+import ru.windcorp.progressia.common.world.item.ItemDataRegistry;
 
 public abstract class ItemContainerSingle extends ItemContainer {
 	
-	private final ItemSlot slot = new ItemSlot();
-	
-	private final float massLimit;
-	private final float volumeLimit;
+	private ItemData item;
+	private int count;
+	private ItemSlot slot = new ItemSlot(this, 0);
 
-	public ItemContainerSingle(String id, float massLimit, float volumeLimit) {
-		super(id);
-		this.massLimit = massLimit;
-		this.volumeLimit = volumeLimit;
+	public ItemContainerSingle(String id) {
+		super(id, 1);
+	}
+
+	@Override
+	public synchronized void read(DataInput input, IOContext context) throws IOException {
+		count = input.readInt();
+		if (count != 0) {
+			String id = input.readUTF();
+			item = ItemDataRegistry.getInstance().create(id);
+			item.read(input, context);
+		} else {
+			item = null;
+		}
 		
-		slot.setContainer(this);
+		fireSlotChangeEvent(0);
+		checkState();
 	}
 
 	@Override
-	public void read(DataInput input, IOContext context) throws IOException {
-		slot.read(input, context);
-	}
-
-	@Override
-	public void write(DataOutput output, IOContext context) throws IOException {
-		slot.write(output, context);
+	public synchronized void write(DataOutput output, IOContext context) throws IOException {
+		output.writeInt(count);
+		if (item != null) {
+			output.writeUTF(item.getId());
+			item.write(output, context);
+		}
 	}
 
 	@Override
 	public void copy(Encodable destination) {
-		slot.copy(((ItemContainerSingle) destination).slot);
+		ItemContainerSingle other = (ItemContainerSingle) destination;
+
+		synchronized (this) {
+			synchronized (other) {
+				other.count = this.count;
+				
+				if (this.item == null) {
+					other.item = null;
+				} else {
+					if (other.item == null || !other.item.isLike(this.item)) {
+						other.item = ItemDataRegistry.getInstance().create(this.item.getId());
+					}
+					this.item.copy(other.item);
+					other.fireSlotChangeEvent(0);
+				}
+			}
+		}
 	}
 
 	@Override
-	public ItemSlot getSlot(int index) {
-		if (index == 0) {
-			return slot;
-		} else {
+	public ItemData getItem(int index) {
+		if (index != 0) {
 			return null;
 		}
+		return item;
+	}
+	
+	public ItemData getItem() {
+		return item;
+	}
+
+	@Override
+	public int getCount(int index) {
+		if (index != 0) {
+			return 0;
+		}
+		return count;
+	}
+	
+	public int getCount() {
+		return count;
 	}
 	
 	public ItemSlot slot() {
@@ -69,23 +110,42 @@ public abstract class ItemContainerSingle extends ItemContainer {
 	}
 
 	@Override
-	public int getSlotCount() {
+	public int getMaxIndex() {
 		return 1;
 	}
-
+	
 	@Override
-	public void forEach(Consumer<? super ItemSlot> action) {
-		action.accept(slot);
+	protected boolean add(int index, ItemData item, int count) {
+		if (!canAdd(index, item, count)) {
+			return false;
+		}
+		
+		if (item != null) {
+			this.item = item;
+			this.count += count;
+			fireSlotChangeEvent(0);
+			checkState();
+		}
+		
+		return true;
 	}
-
+	
 	@Override
-	public float getMassLimit() {
-		return massLimit;
-	}
-
-	@Override
-	public float getVolumeLimit() {
-		return volumeLimit;
+	protected boolean remove(int index, ItemData item, int count) {
+		if (!canRemove(index, item, count)) {
+			return false;
+		}
+		
+		if (count != 0) {
+			this.count -= count;
+			if (this.count == 0) {
+				this.item = null;
+			}
+			fireSlotChangeEvent(0);
+			checkState();
+		}
+		
+		return true;
 	}
 
 }

src/main/java/ru/windcorp/progressia/common/world/item/inventory/ItemSlot.java

@@ -17,182 +17,124 @@
  */
 package ru.windcorp.progressia.common.world.item.inventory;
 
-import java.io.DataInput;
-import java.io.DataOutput;
-import java.io.IOException;
-
-import ru.windcorp.progressia.common.state.Encodable;
-import ru.windcorp.progressia.common.state.IOContext;
 import ru.windcorp.progressia.common.world.item.ItemData;
-import ru.windcorp.progressia.common.world.item.ItemDataRegistry;
-import ru.windcorp.progressia.common.world.item.inventory.event.ItemSlotChangedEvent;
 
 /**
- * An entity optionally containing an {@link ItemData}. Item slots are typically
- * found in {@link ItemContainerMixed}s.
+ * A reference to a slot in a container. The container and the index of an
+ * {@code ItemSlot} cannot be changed.
+ * <p>
+ * {@code ItemSlot}s are wrapper objects; there may be multiple objects
+ * referencing a single slot. Slot objects are considered
+ * {@linkplain #equals(Object) equal} iff their indices are equal and they refer
+ * to the same container.
+ * <p>
+ * This class provides public methods for fetching slot contents but not for
+ * changing them. To alter a slot, use an appropriate method from {@link Items}.
  */
-public class ItemSlot implements Encodable {
+public class ItemSlot {
+
+	private final ItemContainer container;
+	private final int index;
+
+	public ItemSlot(ItemContainer container, int index) {
+		this.container = container;
+		this.index = index;
+	}
 
-	private ItemData contents;
-	private int amount;
-	
-	private ItemContainer container;
-	
 	/**
 	 * @return the container
 	 */
 	public ItemContainer getContainer() {
 		return container;
 	}
-	
+
+	/**
+	 * @return the index
+	 */
+	public int getIndex() {
+		return index;
+	}
+
 	public Inventory getInventory() {
-		return getContainer().getInventory();
-	}
-	
-	/**
-	 * @param container the container to set
-	 */
-	void setContainer(ItemContainer container) {
-		this.container = container;
+		return container.getInventory();
 	}
 
-	/**
-	 * Retrieves the contents of this slot.
-	 * 
-	 * @return the stored {@link ItemData} or {@code null}
-	 */
-	public synchronized final ItemData getContents() {
-		return contents;
+	public ItemData getItem() {
+		return container.getItem(index);
 	}
 
-	/**
-	 * Sets the new contents of this slot. If an item stack was present
-	 * previously, it is discarded. If the contents are {@code null}, the slot
-	 * is emptied.
-	 * <p>
-	 * When the slot receives non-null contents, the new amount must be a
-	 * positive integer. When the slot is emptied, {@code amount} must be 0.
-	 * 
-	 * @param contents the new contents of this slot or {@code null} to clear
-	 *                 the slot
-	 * @param amount   the amount of items to set.
-	 *                 {@code (amount == 0) == (contents == null)} must be true.
-	 */
-	public synchronized final void setContents(ItemData contents, int amount) {
-		this.contents = contents;
-		this.amount = amount;
-		
-		checkState();
-		
-		Inventory inventory = getInventory();
-		if (inventory != null) {
-			inventory.getEventBus().post(new ItemSlotChangedEvent(this));
-		}
+	public int getCount() {
+		return container.getCount(index);
 	}
 
-	/**
-	 * Sets the amount of items stored in this slot.
-	 * <p>
-	 * Setting the amount to zero also erases the slot's contents.
-	 * 
-	 * @param amount the new amount
-	 */
-	public synchronized void setAmount(int amount) {
-		setContents(amount == 0 ? null : contents, amount);
-	}
-	
-	/**
-	 * Clears this slot
-	 */
-	public synchronized void clear() {
-		setContents(null, 0);
+	public boolean isEmpty() {
+		return container.isEmpty(index);
 	}
 
-	/**
-	 * Retrieves the amount of items stored in this slot. If not items are
-	 * present, this returns 0.
-	 * 
-	 * @return the amount of items stored
-	 */
-	public synchronized int getAmount() {
-		return amount;
+	public boolean canAdd(ItemData item, int count) {
+		return container.canAdd(index, item, count);
 	}
-	
-	public synchronized boolean isEmpty() {
-		return amount == 0;
+
+	public boolean canRemove(ItemData item, int count) {
+		return container.canRemove(index, item, count);
 	}
-	
-	public synchronized boolean canInsert(ItemData contents, int amount) {
-		if (contents == null) {
-			return false;
-		}
-		
-		if (container.getMass() + contents.getMass() * amount > container.getMassLimit()) {
-			return false;
-		}
-		
-		if (container.getVolume() + contents.getVolume() * amount > container.getVolumeLimit()) {
-			return false;
-		}
-		
-		return this.contents == null || this.contents.equals(contents);
+
+	protected boolean add(ItemData item, int count) {
+		return container.add(index, item, count);
 	}
-	
-	public synchronized boolean canRemove(int amount) {
-		return this.amount >= amount;
+
+	protected boolean remove(ItemData item, int count) {
+		return container.remove(index, item, count);
 	}
-	
-	private synchronized void checkState() {
-		if ((contents == null) != (amount == 0)) {
-			if (contents == null) {
-				throw new IllegalArgumentException("Contents is null but amount (" + amount + ") != 0");
-			} else {
-				throw new IllegalArgumentException("Contents is " + contents + " but amount is zero");
+
+	public float getMass() {
+		synchronized (container) {
+			int count = getCount();
+			if (count == 0) {
+				return 0;
 			}
-		}
-		
-		if (amount < 0) {
-			throw new IllegalArgumentException("amount is negative: " + amount);
+			return count * getItem().getMass();
 		}
 	}
 
-	@Override
-	public synchronized void read(DataInput input, IOContext context) throws IOException {
-		amount = input.readInt();
-		if (amount != 0) {
-			String id = input.readUTF();
-			contents = ItemDataRegistry.getInstance().create(id);
-			contents.read(input, context);
-		} else {
-			contents = null;
-		}
-		
-		checkState();
-	}
-
-	@Override
-	public synchronized void write(DataOutput output, IOContext context) throws IOException {
-		output.writeInt(amount);
-		if (contents != null) {
-			output.writeUTF(contents.getId());
-			contents.write(output, context);
-		}
-	}
-
-	@Override
-	public void copy(Encodable destination) {
-		ItemSlot slot = (ItemSlot) destination;
-
-		slot.amount = this.amount;
-		
-		if (this.contents == null) {
-			slot.contents = null;
-		} else {
-			if (slot.contents == null || !slot.contents.isLike(this.contents)) {
-				slot.contents = ItemDataRegistry.getInstance().create(this.contents.getId());
+	public float getVolume() {
+		synchronized (container) {
+			int count = getCount();
+			if (count == 0) {
+				return 0;
 			}
-			this.contents.copy(slot.contents);
+			return count * getItem().getVolume();
 		}
 	}
 
+	/*
+	 * For purposes of equality checking, all container instances are considered
+	 * different
+	 */
+
+	@Override
+	public int hashCode() {
+		final int prime = 31;
+		int result = 1;
+		result = prime * result + System.identityHashCode(container);
+		result = prime * result + index;
+		return result;
+	}
+
+	@Override
+	public boolean equals(Object obj) {
+		if (this == obj)
+			return true;
+		if (obj == null)
+			return false;
+		if (getClass() != obj.getClass())
+			return false;
+		ItemSlot other = (ItemSlot) obj;
+		if (container != other.container)
+			return false;
+		if (index != other.index)
+			return false;
+		return true;
+	}
+
 }

src/main/java/ru/windcorp/progressia/common/world/item/inventory/Items.java

@@ -0,0 +1,264 @@
+/*
+ * Progressia
+ * Copyright (C)  2020-2021  Wind Corporation and contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package ru.windcorp.progressia.common.world.item.inventory;
+
+import ru.windcorp.progressia.common.world.item.ItemData;
+import ru.windcorp.progressia.common.world.item.LinearItemProperty;
+
+public class Items {
+
+	private static int pour(ItemContainer from, int fromIndex, ItemContainer into, int intoIndex, int max) {
+		synchronized (from) {
+			synchronized (into) {
+
+				if (!from.isRemovingAllowed()) {
+					return 0;
+				}
+
+				ItemData item = from.getItem(fromIndex);
+				
+				if (item == null) {
+					return 0;
+				}
+
+				if (!into.canAdd(item)) {
+					return 0;
+				}
+
+				if (!into.isEmpty(intoIndex) && !into.getItem(intoIndex).equals(item)) {
+					return 0;
+				}
+
+				int originalCount = from.getCount(fromIndex);
+				int transferCount = Math.min(originalCount, max);
+
+				for (LinearItemProperty prop : LinearItemProperty.values()) {
+					int canFitPropwise = (int) Math.floor((prop.getLimit(into) - prop.get(into)) / prop.get(item));
+					if (canFitPropwise < transferCount) {
+						transferCount = canFitPropwise;
+					}
+				}
+
+				if (transferCount == 0) {
+					return 0;
+				}
+
+				if (!into.canAdd(intoIndex, item, transferCount)) {
+					return 0;
+				}
+
+				if (!from.remove(fromIndex, item, transferCount)) {
+					return 0;
+				}
+
+				boolean success = into.add(intoIndex, item, transferCount);
+				assert success : "Wait, canAdd and canRemove promised the operation would be safe!";
+
+				return transferCount;
+
+			}
+		}
+	}
+
+	/**
+	 * Attempts to transfer as many items as possible, but no more than
+	 * {@code max}, between two slots. The origin of the items is {@code from},
+	 * the destination is {@code into}. This method does nothing if the items
+	 * could not be removed from origin or could not be inserted into
+	 * destination, such as when {@code into} contains different items.
+	 * 
+	 * @param from the slot to transfer item from
+	 * @param into the destination of the items
+	 * @param max  the maximum amount of items to transfer. Use
+	 *             {@code Integer.MAX_VALUE} to remove the limit
+	 * @return the actual amount of items moved between slots
+	 */
+	public static int pour(ItemSlot from, ItemSlot into, int max) {
+		return pour(from.getContainer(), from.getIndex(), into.getContainer(), into.getIndex(), max);
+	}
+
+	/**
+	 * Attempts to transfer as many items as possible between two slots. The
+	 * origin of the items is {@code from}, the destination is {@code into}.
+	 * This method does nothing if the items could not be removed from origin or
+	 * could not be inserted into destination, such as when {@code into}
+	 * contains different items.
+	 * 
+	 * @param from the slot to transfer item from
+	 * @param into the destination of the items
+	 * @return the actual amount of items moved between slots
+	 */
+	public static int pour(ItemSlot from, ItemSlot into) {
+		return pour(from.getContainer(), from.getIndex(), into.getContainer(), into.getIndex(), Integer.MAX_VALUE);
+	}
+
+	/**
+	 * Attempts to swap the contents of the two slots. Swapping contents of two
+	 * empty slots results in a no-op and is considered a success.
+	 * 
+	 * @param a one of the slots
+	 * @param b the other slot
+	 * @return whether the swap succeeded
+	 */
+	public static boolean swap(ItemSlot a, ItemSlot b) {
+		synchronized (a.getContainer()) {
+			synchronized (b.getContainer()) {
+
+				if (a.isEmpty() && b.isEmpty()) {
+					return true;
+				}
+
+				if (!a.isEmpty() && !a.getContainer().isRemovingAllowed()) {
+					return false;
+				}
+
+				if (!b.isEmpty() && !b.getContainer().isRemovingAllowed()) {
+					return false;
+				}
+
+				ItemData aItem = a.getItem();
+				int aCount = a.getCount();
+
+				ItemData bItem = b.getItem();
+				int bCount = b.getCount();
+
+				a.remove(aItem, aCount);
+				b.remove(bItem, bCount);
+
+				if (a.canAdd(bItem, bCount) && b.canAdd(aItem, aCount)) {
+					a.add(bItem, bCount);
+					b.add(aItem, aCount);
+					return true;
+				} else {
+					a.add(aItem, aCount);
+					b.add(bItem, bCount);
+					return false;
+				}
+
+			}
+		}
+	}
+
+	/**
+	 * Attempts to place new items into the specified slot. Either all or none
+	 * of the requested items will be spawned.
+	 * 
+	 * @param into  destination slot
+	 * @param item  the item to add
+	 * @param count the item count
+	 * @return whether the addition succeeded
+	 */
+	public static boolean spawn(ItemSlot into, ItemData item, int count) {
+		synchronized (into.getContainer()) {
+			return into.add(item, count);
+		}
+	}
+
+	/**
+	 * Attempts to remove items from the specified slot. Either all or none of
+	 * the requested items will be destroyed.
+	 * 
+	 * @param from  the slot
+	 * @param item  the item to remove
+	 * @param count the item count
+	 * @return whether the removal succeeded
+	 */
+	public static boolean destroy(ItemSlot from, ItemData item, int count) {
+		synchronized (from.getContainer()) {
+			return from.remove(item, count);
+		}
+	}
+
+	public static int pour(ItemSlot from, ItemContainer into, int max) {
+		synchronized (from.getContainer()) {
+			synchronized (into) {
+
+				int totalPoured = 0;
+
+				for (int index = 0; max > 0 && index <= into.getMaxIndex(); ++index) {
+					int poured = pour(from.getContainer(), from.getIndex(), into, index, max);
+					max -= poured;
+					totalPoured += poured;
+				}
+
+				return totalPoured;
+
+			}
+		}
+	}
+
+	public static int pour(ItemSlot from, ItemContainer into) {
+		return pour(from, into, Integer.MAX_VALUE);
+	}
+
+	public static boolean spawn(ItemContainer into, ItemData item, int count) {
+		synchronized (into) {
+			
+			if (item == null && count == 0) {
+				return true;
+			}
+			
+			if (count < 0) {
+				return false;
+			}
+			
+			if (!into.canAdd(item)) {
+				return false;
+			}
+			
+			for (LinearItemProperty prop : LinearItemProperty.values()) {
+				float requested = prop.get(item) * count;
+				float available = prop.getLimit(into) - prop.get(into);
+				if (requested > available) {
+					return false;
+				}
+			}
+
+			int compatibleSlot = -1;
+			int firstEmptySlot = -1;
+			
+			for (int index = 0; index <= into.getMaxIndex(); ++index) {
+				ItemData inSlot = into.getItem(index);
+				if (inSlot == null) {
+					if (firstEmptySlot == -1) {
+						firstEmptySlot = index;
+					}
+				} else if (inSlot.equals(item)) {
+					compatibleSlot = index;
+					break;
+				}
+			}
+			
+			if (compatibleSlot == -1) {
+				compatibleSlot = firstEmptySlot;
+			}
+			
+			if (compatibleSlot == -1) {
+				// Means the inventory is full due to slot limit
+				return false;
+			}
+			
+			return into.add(compatibleSlot, item, count);
+			
+		}
+	}
+
+	private Items() {
+	}
+
+}

src/main/java/ru/windcorp/progressia/common/world/item/inventory/event/ItemSlotChangedEvent.java

@@ -17,6 +17,7 @@
  */
 package ru.windcorp.progressia.common.world.item.inventory.event;
 
+import ru.windcorp.progressia.common.world.item.inventory.ItemContainer;
 import ru.windcorp.progressia.common.world.item.inventory.ItemSlot;
 
 public class ItemSlotChangedEvent extends ItemSlotEvent {
@@ -25,4 +26,8 @@ public class ItemSlotChangedEvent extends ItemSlotEvent {
 		super(slot);
 	}
 
+	public ItemSlotChangedEvent(ItemContainer container, int index) {
+		super(container, index);
+	}
+
 }

src/main/java/ru/windcorp/progressia/common/world/item/inventory/event/ItemSlotEvent.java

@@ -17,21 +17,38 @@
  */
 package ru.windcorp.progressia.common.world.item.inventory.event;
 
+import ru.windcorp.progressia.common.world.item.inventory.ItemContainer;
 import ru.windcorp.progressia.common.world.item.inventory.ItemSlot;
 
 public abstract class ItemSlotEvent extends ItemContainerEvent {
 	
-	private final ItemSlot slot;
+	private ItemSlot slot = null;
+	private final int index;
 
+	public ItemSlotEvent(ItemContainer container, int index) {
+		super(container);
+		this.index = index;
+	}
+	
 	public ItemSlotEvent(ItemSlot slot) {
-		super(slot.getContainer());
+		this(slot.getContainer(), slot.getIndex());
 		this.slot = slot;
 	}
 	
+	/**
+	 * @return the index
+	 */
+	public int getIndex() {
+		return index;
+	}
+	
 	/**
 	 * @return the slot
 	 */
 	public ItemSlot getSlot() {
+		if (slot == null) {
+			slot = new ItemSlot(getContainer(), index);
+		}
 		return slot;
 	}
 

src/main/java/ru/windcorp/progressia/server/PlayerManager.java

@@ -28,6 +28,7 @@ import ru.windcorp.progressia.common.world.entity.EntityData;
 import ru.windcorp.progressia.common.world.entity.EntityDataPlayer;
 import ru.windcorp.progressia.common.world.entity.EntityDataRegistry;
 import ru.windcorp.progressia.common.world.item.ItemDataRegistry;
+import ru.windcorp.progressia.common.world.item.inventory.Items;
 import ru.windcorp.progressia.server.events.PlayerJoinedEvent;
 import ru.windcorp.progressia.test.TestContent;
 
@@ -63,8 +64,8 @@ public class PlayerManager {
 	private EntityDataPlayer spawnPlayerEntity(String login) {
 		EntityDataPlayer player = (EntityDataPlayer) EntityDataRegistry.getInstance().create("Core:Player");
 
-		player.getHand(0).slot().setContents(ItemDataRegistry.getInstance().create("Test:Stick"), 20);
-		player.getEquipmentSlot(0).slot().setContents(ItemDataRegistry.getInstance().create("Test:CardboardBackpack"), 1);
+		Items.spawn(player.getHand(0).slot(), ItemDataRegistry.getInstance().create("Test:Stick"), 5);
+		Items.spawn(player.getEquipmentSlot(0).slot(), ItemDataRegistry.getInstance().create("Test:CardboardBackpack"), 1);
 
 		player.setPosition(getServer().getWorld().getGenerator().suggestSpawnLocation());
 		player.setUpVector(new Vec3(0, 0, 1));