randomString

randomString.swift

import Foundation
import UIKit

private func arrayBetween(_ c1: String, _ c2: String) -> [UInt32] {
    guard let s1 = c1.unicodeScalars.first?.value,
        let s2 = c2.unicodeScalars.first?.value else {
            return []
    }
    return (s1 < s2 ? (s1...s2) : (s2...s1)).map { $0 }
}

private func character(from code: UInt32) -> Character? {
    return UnicodeScalar(code).map { Character($0) }
}

private func randomCharacter(array: [UInt32]) -> Character? {
    let index = Int(arc4random_uniform(UInt32(array.count)))
    let code = array[index]
    return character(from: code)
}

func randomString(_ length: Int = 1) -> String {
    let candidates = arrayBetween("a", "z") + arrayBetween("A", "Z") + arrayBetween("0", "9")
    return String((0..<length).flatMap { _ in randomCharacter(array: candidates) })
}

print(randomString(10))