15th December solution - Advent of code 2020 - swift

december15.swift

let startingNumbers = [12,1,16,3,11,0]
var numbers = [Int : Int]()
startingNumbers.enumerated().dropLast().forEach { numbers[$0.element] = $0.offset }
var roundCount = startingNumbers.count
var currentNumber = startingNumbers.last!

//let roundEnd = 2020
let roundEnd = 30000000

while roundCount < roundEnd {
    let lastRound = numbers[currentNumber]
    var newNumber = -1
    if let lastRound = lastRound {
        newNumber = roundCount - 1 - lastRound
    } else {
        newNumber = 0
    }
    numbers[currentNumber] = roundCount - 1
    currentNumber = newNumber
    roundCount += 1
}
print(currentNumber)