package fifteen

class RambunctiousRecitation(private val startingNumbers: List<Int>) {

    fun play(stopAt: Int = 2020): Int {
        val spokenNumbers =
            startingNumbers.dropLast(1).mapIndexed { index, it -> Pair(it, index + 1) }.toMap().toMutableMap()
        var turn = startingNumbers.size + 1
        var lastSpokenNumber = startingNumbers.last()

        while (turn <= stopAt) {
            val currentNumber = if (lastSpokenNumber in spokenNumbers.keys)
                (turn - 1) - (spokenNumbers[lastSpokenNumber]!!) else 0
            spokenNumbers[lastSpokenNumber] = turn - 1
            lastSpokenNumber = currentNumber
            turn++
        }
        return lastSpokenNumber
    }
}