# Returns the first valid free identifier which correspond to this mask
	private fun first_valid_id(mask: Int): Int
	do
		# Search for the first valid free identifier
		var inter = interval.iterator
		var found = false
		var id = 0
		while inter.is_ok and not found do
			var i = inter.item.first.as(not null)
			while i != inter.item.second and not found do
				# Tests if this id is free for this mask
				var hv = i & mask

				# If the hashtable if full, push an empty item
				if hv >= tempht.length then
					tempht.push(null)
				end

				if tempht[hv] != mask then
					found = true
					id = i
				end
				i = i + 1
			end

			if not found then
				inter.next
			end
		end

		# Updates the structure of union of intervals
		if id == inter.item.first then
			if inter.item.first == inter.item.second then
				inter.delete
			else
				inter.item.first += 1
			end
		else
			if id != inter.item.first and id != inter.item.second then
				# We need to split in two this interval
				var last = inter.item.second
				inter.item.second = id - 1
				inter.next

				var p = new Couple[nullable Int, nullable Int](id + 1, last)
				if inter.is_ok then
					inter.insert_before(p)
				else
					interval.push(p)
				end
			else
				inter.item.second = id - 1
			end
		end

		return id
	end