Zero-garbage fast iteration collection

Indexer.kt

package com.acelta.util

import com.acelta.world.mob.Mob

class Indexer<T : Mob>(val capacity: Int) : MutableIterable<T> {

	private inner class Iterator : MutableIterator<T> {

		internal var cursor = 0

		override fun hasNext() = size > 0 && cursor <= highest

		override tailrec fun next(): T = get(cursor++) ?: next()

		override fun remove() {
			set(cursor, null)
		}

	}

	private var reusableIterator = Iterator()

	override fun iterator(): MutableIterator<T> {
		reusableIterator.cursor = 0
		return reusableIterator
	}

	private val arr = arrayOfNulls<Any>(capacity)

	var size = 0
	var highest = 0

	operator fun get(id: Int) = arr[id] as? T

	operator fun set(id: Int, mob: T?): T? {
		val last = arr[id]
		arr[id] = mob
		if (last == null && mob != null) {
			size++
			if (highest < id) highest = id
		} else if (last != null && mob == null) {
			size--
			if (highest == id) highest--
		}
		return last as? T
	}

	fun nextIndex(): Int {
		for (i in 0..size - 1) if (arr[i] != null) return i
		throw IllegalStateException("There is no next index because the indexer is filled to capacity!")
	}

}