NumberStringsGenerating.swift

func randomString(length: Int) -> String {
    return (0..<length).map { _ in String(Int.random(in: 0...9)) }.joined()
}

func randomStringBuffer(length: Int) -> String {
    var buffer = ""
    (0..<length).forEach { _ in buffer += String(Int.random(in: 0...9)) }
    return buffer
}

func randomStringB(length: Int) -> String {
  let letters = "0123456789"
  return String((0..<length).map{ _ in letters.randomElement()! })
}

func test(_ funcToRun: () -> Void) {
    let a = Date().timeIntervalSince1970
    _ = funcToRun()
    let b = Date().timeIntervalSince1970
    print(b - a)
}

test {
    let g = randomString(length: 10000)
}

test {
    let f = randomStringBuffer(length: 10000)
}

test {
    let g = randomStringB(length: 10000)
}

/**
several test results:

0.13645076751708984
0.34798574447631836
0.22037696838378906

0.134080171585083
0.2427217960357666
0.1422710418701172

0.13546204566955566
0.28347301483154297
0.2760488986968994

0.13517093658447266
0.347398042678833
0.197998046875

Seems better approach is to use joined()
*/