25th December solution - Advent of code - swift

december25_1.swift

let cardsPublicKey = 9033205
let doorsPublicKey = 9281649

let subjectNumber = 7

let cardLoopSize = findLoopSizeFor(publicKey: cardsPublicKey, subjectNumber: subjectNumber)
let doorLoopSize = findLoopSizeFor(publicKey: doorsPublicKey, subjectNumber: subjectNumber)

print(cardLoopSize, doorLoopSize)

let encryptionKey = transformWithLoopSize(subjectNumber: cardsPublicKey, loopSize: doorLoopSize)

print(encryptionKey)

func findLoopSizeFor(publicKey: Int, subjectNumber: Int) -> Int {
    var loopSize = 0
    var value = 1
    while value != publicKey {
        value = transform(value: value, subjectNumber: subjectNumber)
        loopSize += 1
    }
    return loopSize
}

func transform(value: Int, subjectNumber: Int) -> Int {
    var returnValue = value
    returnValue *= subjectNumber
    returnValue %= 20201227
    return returnValue
}

func transformWithLoopSize(subjectNumber: Int, loopSize: Int) -> Int {
    var value = 1
    for _ in 0 ..< loopSize {
        value = transform(value: value, subjectNumber: subjectNumber)
    }
    return value
}