What's the best way to get updated Character by moving a given Integer - swift

Question

Swift character is so hard to manipulate..
I have a simple request:
For a given string, I'd like to encode it with a "moving integer" for all the alphabetical digits.
For example: "abc", move 1: would become "bcd"; "ABC", move 1 -> "BCD".
One thing to note is, if after moving, the range it larger than "z" or "Z", it should loop back and calculate from "a" or "A" again. e.g "XYZ", move 1 -> "YZA"
This would be very easy to do in Java. But could anyone show me what would be the cleanest way to do it swift?
I've done something like:
let arr: [Character] = Array(s)
for i in arr {
let curAsciiValue = i.asciiValue!
var updatedVal = (curAsciiValue + UInt8(withRotationFactor))
if i >= "A" && i <= "Z" {
newAsciiVal = 65
updatedVal = updatedVal - 65 >= 26 ? 65 + (updatedVal - 65) % 26 : updatedVal
} else if i >= "a" && i <= "z" {
newAsciiVal = 97
updatedVal = updatedVal - 97 >= 26 ? 97 + (updatedVal - 97) % 26 : updatedVal
}
}
What should be best way to do this?

Answer 1

Better to bring all your logic together extending the Character type:
extension Character {
var isAlphabet: Bool { isLowercaseAlphabet || isUppercaseAlphabet }
var isLowercaseAlphabet: Bool { "a"..."z" ~= self }
var isUppercaseAlphabet: Bool { "A"..."Z" ~= self }
func rotateAlphabetLetter(_ factor: Int) -> Self? {
precondition(factor > 0, "rotation must be positive")
guard let asciiValue = asciiValue, isAlphabet else { return nil }
let addition = asciiValue + UInt8(factor % 26)
return .init(UnicodeScalar(addition > (isLowercaseAlphabet ? 122 : 90) ? addition - 26 : addition))
}
}
Character("a").rotateAlphabetLetter(2) // "c"
Character("A").rotateAlphabetLetter(2) // "C"
Character("j").rotateAlphabetLetter(2) // "l"
Character("J").rotateAlphabetLetter(2) // "L"
Character("z").rotateAlphabetLetter(2) // "b"
Character("Z").rotateAlphabetLetter(2) // "B"
Character("ç").rotateAlphabetLetter(2) // nil
Character("Ç").rotateAlphabetLetter(2) // nil
Expanding on that you can map your string elements using compactMap:
let string = "XYZ"
let rotatedAlphabetLetters = string.compactMap { $0.rotateAlphabetLetter(2) } // ["Z", "A", "B"]
And to convert the sequence of characters back to string:
extension Sequence where Element == Character {
var string: String { .init(self) }
}
let result = rotatedAlphabetLetters.string // "ZAB"
or simply in one liner:
let result = "XYZ".compactMap { $0.rotateAlphabetLetter(2) }.string // "ZAB"