interface IndexedEntity<T : IndexedEntity<T>> {

    var index: EntityIndexReference<T>?

}

class EntityIndexReference<T : IndexedEntity<T>> internal constructor(val container: EntityList<T>, val entity: T, val index: Int)

class EntityList<T : IndexedEntity<T>>(private val capacity: Int) : AbstractCollection<T>() {

    private val entities = HashMap<Int, EntityIndexReference<T>>(capacity)

    private val indicies = ArrayDeque<Int>((1 .. capacity).toList())

    override val size get() = capacity - indicies.size

    override fun add(element: T): Boolean {

        if (isExhausted()) {
            return false
        }

        assert(element.index == null)

        val index = indicies.poll()

        assert(entities[index] == null)

        val reference = EntityIndexReference(this, element, index)

        entities[index] = reference

        element.index = reference

        return true
    }

    override fun remove(element: T): Boolean {
        assert(contains(element))

        val reference = element.index!!

        entities.remove(reference.index)

        indicies.add(reference.index)

        element.index = null

        return true
    }

    fun remove(index: Int): T? {
        val reference = entities.remove(index) ?: return null

        val entity = reference.entity

        indicies.add(reference.index)

        entity.index = null

        return entity
    }

    fun get(index: Int): T? = entities[index]?.entity

    override fun contains(element: T): Boolean {
        val reference = element.index ?: return false

        if (reference.container != this) {
            return false
        }

        return entities[reference.index]?.entity == element
    }

    override fun iterator(): MutableIterator<T> = EntityListIterator(entities.values.iterator())

    private fun isExhausted() = indicies.isEmpty()

    private inner class EntityListIterator(private val itr: MutableIterator<EntityIndexReference<T>>) : MutableIterator<T> {

        override fun hasNext() = itr.hasNext()

        override fun next() = itr.next().entity

        override fun remove() = itr.remove()

    }

}

data class Player(val name: String, override var index: EntityIndexReference<Player>? = null) : IndexedEntity<Player>

abstract class AbstractEntityManager {

    companion object {

        const val MAX_PLAYERS = 2048

    }

    abstract val players: Collection<Player>

}

class EntityManager : AbstractEntityManager() {

    override val players = EntityList<Player>(MAX_PLAYERS)

}

class ConcurrentEntityManager : AbstractEntityManager() {

    override val players = Collections.synchronizedCollection(EntityList<Player>(MAX_PLAYERS))

}

object Main {

    @JvmStatic
    fun main(args: Array<String>) {

        val manager = EntityManager() //or ConcurrentEntityManager

        var index = 1

        do {

            val player = Player("P${index++}")

        } while(manager.players.add(player))

        println("Players Size: ${manager.players.size}")

        manager.players.forEach { println("Player: $it") }

    }

}