Tweaks

fabric-transfer-api-v1/src/main/java/net/fabricmc/fabric/api/transfer/v1/storage/SlottedStorage.java

@@ -24,8 +24,10 @@
 
 /**
  * A {@link Storage} implementation made of indexed slots.
- * Please note that some storages may not implement this interface:
- * checking whether a storage is slotted can be done using {@code instanceof}.
+ *
+ * <p>Please note that some storages may not implement this interface.
+ * It is up to the storage implementation to decide whether to implement this interface or not.
+ * Checking whether a storage is slotted can be done using {@code instanceof}.
  *
  * @param <T> The type of the stored resources.
  *

fabric-transfer-api-v1/src/main/java/net/fabricmc/fabric/api/transfer/v1/storage/Storage.java

@@ -18,7 +18,6 @@
 
 import java.util.Iterator;
 
-import com.google.common.collect.Iterators;
 import org.jetbrains.annotations.ApiStatus;
 import org.jetbrains.annotations.Nullable;
 
@@ -159,10 +158,17 @@ default long simulateExtract(T resource, long maxAmount, @Nullable TransactionCo
 	 * <p>This can provide a large performance benefit over {@link #iterator()} if the caller is only interested in non-empty views,
 	 * for example because it is trying to extract resources from the storage.
 	 *
+	 * <p>This function should only be overridden if the storage is able to provide an optimized iterator over non-empty views,
+	 * for example because it is keeping an index of non-empty views.
+	 * Otherwise, the default implementation simply calls {@link #iterator()} and filters out empty views.
+	 *
+	 * <p>When implementing this function, note that the guarantees of {@link #iterator()} still apply.
+	 * In particular, {@link #insert} and {@link #extract} may be called safely during iteration.
+	 *
 	 * @return An iterator over the non-empty views of this storage. Calling remove on the iterator is not allowed.
 	 */
 	default Iterator<StorageView<T>> nonEmptyIterator() {
-		return Iterators.filter(iterator(), view -> view.getAmount() > 0 && !view.isResourceBlank());
+		return TransferApiImpl.filterEmptyViews(iterator());
 	}
 
 	/**

fabric-transfer-api-v1/src/main/java/net/fabricmc/fabric/api/transfer/v1/storage/base/SingleSlotStorage.java

@@ -20,7 +20,7 @@
 
 import org.jetbrains.annotations.ApiStatus;
 
-import net.fabricmc.fabric.api.transfer.v1.storage.Storage;
+import net.fabricmc.fabric.api.transfer.v1.storage.SlottedStorage;
 import net.fabricmc.fabric.api.transfer.v1.storage.StorageView;
 import net.fabricmc.fabric.impl.transfer.TransferApiImpl;
 
@@ -34,9 +34,23 @@
  * The transfer API is a complex addition, and we want to be able to correct possible design mistakes.
  */
 @ApiStatus.Experimental
-public interface SingleSlotStorage<T> extends Storage<T>, StorageView<T> {
+public interface SingleSlotStorage<T> extends SlottedStorage<T>, StorageView<T> {
 	@Override
 	default Iterator<StorageView<T>> iterator() {
 		return TransferApiImpl.singletonIterator(this);
 	}
+
+	@Override
+	default int getSlotCount() {
+		return 1;
+	}
+
+	@Override
+	default SingleSlotStorage<T> getSlot(int slot) {
+		if (slot != 0) {
+			throw new IndexOutOfBoundsException("Slot " + slot + " does not exist in a single-slot storage.");
+		}
+
+		return this;
+	}
 }

fabric-transfer-api-v1/src/main/java/net/fabricmc/fabric/impl/transfer/TransferApiImpl.java

@@ -89,4 +89,40 @@ public T next() {
 			}
 		};
 	}
+
+	public static <T> Iterator<StorageView<T>> filterEmptyViews(Iterator<StorageView<T>> iterator) {
+		return new Iterator<>() {
+			StorageView<T> next;
+
+			{
+				findNext();
+			}
+
+			private void findNext() {
+				while (iterator.hasNext()) {
+					next = iterator.next();
+
+					if (next.getAmount() > 0 && !next.isResourceBlank()) {
+						return;
+					}
+				}
+
+				next = null;
+			}
+
+			@Override
+			public boolean hasNext() {
+				return next != null;
+			}
+
+			@Override
+			public StorageView<T> next() {
+				if (!hasNext()) {
+					throw new NoSuchElementException();
+				}
+
+				return next;
+			}
+		};
+	}
 }