stack/queue tdd

interface MyStack<T> {
    fun pop(): T
    fun push(item: T)

    class LIFO<T : Any>(private val maxCount: Int) : MyStack<T> {
        init {
            if (maxCount <= 0) throw IllegalStateException()
        }

        private var currentPosition = -1
        private val stack = Array<Any>(maxCount) { }

        override fun pop(): T {
            if (currentPosition < 0) throw IllegalStateException()
            val item = stack[currentPosition]
            stack[currentPosition--] = Unit
            return item as T
        }

        override fun push(item: T) {
            if (currentPosition == stack.lastIndex)
                throw IllegalStateException("Stack overflow exception, maximum is $maxCount")
            stack[++currentPosition] = item
        }
    }

    class FIFO<T : Any>(private val maxCount: Int) : MyStack<T> {
        init {
            if (maxCount <= 0) throw IllegalStateException()
        }

        private var endPosition = -1
        private var elementsInQueue = 0
        private val queue = Array<Any>(maxCount) { }

        override fun pop(): T {
            if (elementsInQueue <= 0) throw IllegalStateException()
            val position = (endPosition + 1) - elementsInQueue--
            val item = queue[position]
            queue[position] = Unit
            return item as T
        }

        override fun push(item: T) {
            if (elementsInQueue >= maxCount)
                throw IllegalStateException("Stack overflow exception, maximum is $maxCount")
            endPosition = ++endPosition % maxCount
            elementsInQueue++
            queue[endPosition] = item
        }
    }
}