Inventory. Get empty slots or contains not full ItemStack

Plugins Plugin Development
1

How to get the number of empty slots in the player’s inventory?
I’m trying to calculate how many specific items can be given to the player.
I currently have this code.

private Integer calculateMaxItems(Player player, ItemStack itemStack) {
	int value = 0;
	MainPlayerInventory mainPlayerInventory = player.getInventory().query(QueryOperationTypes.INVENTORY_TYPE.of(MainPlayerInventory.class));
	Iterable<Slot> slots = mainPlayerInventory.slots();
	for(Slot playerSlot : slots) {
		int difference = itemStack.getMaxStackQuantity() - playerSlot.query(QueryOperationTypes.ITEM_STACK_IGNORE_QUANTITY.of(itemStack)).totalItems();
		if(playerSlot.totalItems() == 0) {
			value = value + itemStack.getMaxStackQuantity();
		}
		if(playerSlot.contains(itemStack)) {
			if(playerSlot.query(QueryOperationTypes.ITEM_STACK_IGNORE_QUANTITY.of(itemStack)).totalItems() != itemStack.getMaxStackQuantity()) {
				value = value + difference;
			}
		}
	}
	return value;
}

It works, but I would like to know how to do better if possible.

2

slot.peek().isEmpty() should be able to tell you whether any specific slot is empty.

3

I need to consider not full stacks of items.
I found another way to get the allowable amount of items, but this does not work with MainPlayerInventory.
And if I use player.getInventory(), then I also consider my left hand, which is also not acceptable, since I give out items in MainPlayerInventory.
The method consists in accessing the inventory with a request for things of a certain type in it. Next, we calculate the maximum possible volume and subtract from it what the player has. But I can’t correctly get how many items the player has if I check MainPlayerInventory.

4

I would use this:

Inventory main = player.getInventory().query(QueryOperationTypes.INVENTORY_TYPE.of(MainPlayerInventory.class));
Inventory freeSlots = main.query(QueryOperationTypes.ITEM_STACK_CUSTOM.of(stack -> stack.isEmpty() || stack.getType().equals(DIAMOND)));
int count = freeSlots.capacity() * DIAMOND.getMaxStackQuantity() - freeSlots.totalItems();

Or use 2 queries (one for empty and one for non-empty) and union the results together.