Swift 2.0 strings-- I don't understand

random_characters.swift

private let ValidCharacters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789\n".utf8

private func RandomString() -> String {
    var str = ""
    
    // 1-140
    let len = 1+(Int(rand())%140)
    for _ in 0...len {
        let offset = Int(rand())%Int(ValidCharacters.count)
        let index = advance(ValidCharacters.startIndex, offset)
        let ch = UnicodeScalar(UInt8(ValidCharacters[index].value))
        str.append(ch)
    }

    return str
}

import Darwin

private let ValidCharacters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".characters

private func RandomString() -> String {
    var result = String.CharacterView()

    for _ in 1...(1 + Int(arc4random_uniform(140))) {
        let n = Int(arc4random_uniform(UInt32(ValidCharacters.count)))
        result.append(ValidCharacters[advance(ValidCharacters.startIndex, n)])
    }

    return String(result)
}

RandomString()