AoC 2020 Day 10

day10.swift

import Foundation

final class Day1020: Day {

    override func perform() {
        let input = String.input(forDay: 10, year: 2020)
        var joltages = input
            .split(separator: "\n")
            .map(String.init)
            .map(Int.init)
            .compactMap { $0 }
            .sorted()

        joltages = [0] + joltages + [joltages.max()! + 3]

        print(part1(joltages))
        print(part2(joltages))
    }

    func part1(_ joltages: [Int]) -> Int {
        let (ones, threes) = zip(joltages, joltages.dropFirst())
            .reduce((ones: 0, threes: 0)) { (result, next) in
                switch next.1 - next.0 {
                case 1: return (result.0+1, result.1)
                case 3: return (result.0, result.1+1)
                default: return result
                }
            }
        return ones * threes
    }

    func part2(_ joltagesIncludingBounds: [Int]) -> Int {
        var initialCache = [joltagesIncludingBounds.max()!: 1]
        return countJoltageArrangements(joltagesIncludingBounds.dropLast(), cache: &initialCache)
    }

    func countJoltageArrangements(_ joltages: [Int], cache: inout [Int: Int]) -> Int {
        guard let last = joltages.last else { return cache[0, default: 0] }
        cache[last] = (1...3)
            .map { cache[last+$0, default: 0] }
            .reduce(0, +)
        return countJoltageArrangements(joltages.dropLast(), cache: &cache)
    }
 }