RelativeCount.swift

extension Int {

  var relativeCount: String {
    return relativeCount(toDigit: 1)
  }

  func relativeCount(toDigit digit: Int) -> String {
    // should not round to negative digit, its impossible
    guard digit >= 0 else { fatalError() }
    // if is negative, just return self
    guard self >= 0 else { return "\(self)" }

    let factor = NSDecimalNumber(decimal: pow(10, digit)).doubleValue

    switch self {
    case 1...999:
      return "\(self)"
    case 1000...999999:
      let number = ((Double(self) / 1000.0) * factor).rounded(.down) / factor
      return String(format: "%.\(digit)f", number) + "k"
    case 1000000...999999999:
      let number = ((Double(self) / 1000000.0) * factor).rounded(.down) / factor
      return String(format: "%.\(digit)f", number) + "m"
    default:
      let number = ((Double(self) / 1000000000.0) * factor).rounded(.down) / factor
      return String(format: "%.\(digit)f", number) + "b"
    }
  }

}

21039023.relativeCount // "21.0m"
7866.relativeCount // "7.8k"
832832983928923.relativeCount // "832832.9b"

(-1).relativeCount(toDigit: 2) // "-1"
(-2189).relativeCount // "-2189"

0.relativeCount(toDigit: 1) // "0"