Inventory. Get empty slots or contains not full ItemStack

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.

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

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.

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.