AbbreviatedNumber.swift

protocol AbbreviatedNumber {
   var doubleValue: Double { get }
}

extension Int: AbbreviatedNumber {
   var doubleValue: Double {
      return Double(self)
   }
}

extension Double: AbbreviatedNumber {
   var doubleValue: Double {
      return self
   }
}

typealias AbbreviatedNumberFormatter = (Double, String) -> String

private let abbreviatedFormatter: NSNumberFormatter = {
   let formatter = NSNumberFormatter()
   formatter.maximumFractionDigits = 1
   formatter.roundingMode = .RoundDown
   return formatter
}()

extension AbbreviatedNumber {
   private func abbreviatedStringWithFormatter(formatter: AbbreviatedNumberFormatter) -> String {
      let unit: Double = 1000
      let value = self.doubleValue
      if value < unit {
         return "\(self)"
      }
      let suffixes = ["K", "M", "B"]
      let exp = min(Int(log(value) / log(unit)), suffixes.count)
      
      let suffix = suffixes[exp - 1]
      let prefixValue = value / pow(unit, Double(exp))
      return formatter(prefixValue, suffix)
   }

   func abbreviatedWithPrintfString() -> String {
      return self.abbreviatedStringWithFormatter {
         prefix, suffix in
         return String(format: "%.1f%@", arguments: [prefix, suffix])
      }
   }

   func abbreviatedString() -> String {
      return self.abbreviatedStringWithFormatter {
         prefix, suffix in
         return abbreviatedFormatter.stringFromNumber(NSNumber(double: prefix))!.stringByAppendingString(suffix)
      }
   }
}

1051.9.abbreviatedString()

123.abbreviatedString()
1234.abbreviatedString()
12345678.abbreviatedString()
123456789.abbreviatedString()
100_000_000_000_000.abbreviatedString()

1051.9.abbreviatedWithPrintfString()

123.abbreviatedWithPrintfString()
1234.abbreviatedWithPrintfString()
12345678.abbreviatedWithPrintfString()
123456789.abbreviatedWithPrintfString()
100_000_000_000_000.abbreviatedWithPrintfString()