src/main/java/Inventory/Inventory.java

@@ -6,6 +6,7 @@ import Item.ItemStack;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.stream.IntStream;
 
 public class Inventory {
     private final List<ItemStack> items;
@@ -25,12 +26,44 @@ public class Inventory {
         return Collections.unmodifiableList(items);
     }
 
-    public void addItem(ItemStack item) {
-        int totalWeight = calculateStackWeight(item);
-        if (totalWeight > weightLimit) {
-            return;
+    public void addItem(ItemStack stack) {
+        boolean isSingleItem = stack.getQuantity() == 1;
+        if (isSingleItem) {
+            if(stack.getItem().getWeight() > weightLimit) return;
+            items.add(stack);
         }
-        items.add(item);
+        if (stack.getWeight() + getCurrentWeight() > weightLimit) addSubStack(stack);
+        items.add(stack);
+    }
+
+    private void addSubStack(ItemStack stack) {
+        int maxQuantity = calculateMaxQuantity(stack);
+        items.add(new ItemStack(stack.getItem(), maxQuantity));
+    }
+
+    private int calculateMaxQuantity(ItemStack stack) {
+        int maxQuantity = 1;
+        int weight = stack.getItem().getWeight();
+
+
+        for (int i = maxQuantity; i <= stack.getQuantity(); i++) {
+            int currentWeightWithStack = (weight * maxQuantity) + getCurrentWeight();
+            boolean weightOutsideLimits = currentWeightWithStack >= weightLimit;
+            if (weightOutsideLimits) {
+                break;
+            } else {
+                maxQuantity++;
+            }
+        }
+        return maxQuantity;
+    }
+
+    private int getCurrentWeight() {
+        int  currentWeight = 0;
+        for (ItemStack item : items) {
+            currentWeight += item.getItem().getWeight();
+        }
+        return currentWeight;
     }
 
     private int calculateStackWeight(ItemStack stack) {

src/main/java/Item/ItemStack.java

@@ -13,4 +13,8 @@ public record ItemStack(Item item, int quantity) {
     public int getQuantity() {
         return quantity;
     }
+
+    public int getWeight() {
+        return item.getWeight() * quantity;
+    }
 }

src/test/java/InventoryTest.java

@@ -35,7 +35,7 @@ public class InventoryTest {
     }
 
     @Test
-    void item_can_not_be_added_if_it_exeeds_weight_limit() {
+    void item_can_not_be_added_if_above_weight_limit() {
         var inventory = new Inventory(5);
         var item = new BasicItem("iron_sword", "Iron Sword", 10);
         var stack = new ItemStack(item, 1);
@@ -43,4 +43,21 @@ public class InventoryTest {
         assertThat(inventory.getItems().size(), is(0));
     }
 
+    @Test
+    void stack_can_be_added_up_to_capacity() {
+        var inventory = new Inventory(5);
+        var item = new BasicItem("sand_grain", "Sand Grain", 1);
+        var stack = new ItemStack(item, 8);
+        inventory.addItem(stack);
+        assertThat(inventory.getItems(), hasItem(allOf(
+                hasProperty("item", (
+                        hasProperty("id", equalTo("sand_grain"))
+
+                )),
+                hasProperty("quantity", equalTo(5)))
+
+        ));
+
+    }
+
 }