13th December solution - Advent of Code 2020 - swift

december13_1.swift

let departureTime = Int(lines[0])!
let busIds = lines[1].components(separatedBy: ",").compactMap{ Int($0) }

let minTimeToWait = busIds.map { busId in (busId, busId - (departureTime % busId)) }.min { first, second in first.1 < second.1 }!

print(minTimeToWait.0 * minTimeToWait.1)

december13_2.swift

// Chinese remainder theorem
let busIds = lines[1].components(separatedBy: ",").map{ Int($0) }
let busIdsWithOffset = busIds.enumerated().filter { (element)  in
    element.element != nil
}
let N = busIdsWithOffset.compactMap { $0.element }.reduce(1, *)
let result = busIdsWithOffset.map { ele in
    let ni = ele.element!
    let Ni = N / ni
    let bi = (ni - ele.offset) % ni
    if bi == 0 {
        return 0
    }
    var x = 0
    while( (Ni * x) % ni != 1) {
        x += 1
    }
    return bi * Ni * x
}.reduce(0, +)
print(result % N)

december13_shared.swift

let filename = "/Users/felix/xCodeProjects/AdventOfCode2020.playground/Resources/december13.txt"
let contents = try! String(contentsOfFile: filename)
let lines = contents.components(separatedBy: CharacterSet.newlines).filter { !$0.isEmpty }