AoC 2020 - Day 15

day15.swift

import Foundation

final class Day1520: Day {
    override func perform() {
        let numbers = String.input(forDay: 15, year: 2020)
            .split(separator: ",")
            .map(String.init)
            .map(Int.init)
            .compactMap { $0 }

        var lastSpoken: Int = numbers.last!
        var numbersAndTurns = zip(numbers.dropLast().indices, numbers.dropLast())
            .reduce(into: [:]) { $0[$1.1] = $1.0 + 1 }
        
        for i in numbers.count+1 ... 30000000 {
            if let last = numbersAndTurns[lastSpoken] {
                numbersAndTurns[lastSpoken] = i - 1
                lastSpoken = (i - 1) - last
            } else {
                numbersAndTurns[lastSpoken] = i - 1
                lastSpoken = 0
            }
        }

        stageTwoOutput = String(lastSpoken)
    }
}