Range<Int> に distance 作って Int に指定範囲で乱数を作成させる extension ref: http://qiita.com/lovee/items/942de12e9edb5ad01f48

file0.swift

//普通に distanceTo を使うからこの方法やめる 
//extension Range where Element: IntegerType {
extension Range {

    //普通に distanceTo を使うからこの方法やめる 
    //var distance: Element {
    //  return self.endIndex - self.startIndex
    //}

    var distance: Element.Distance {
        return self.startIndex.distanceTo(self.endIndex)
    }

}

extension Int {

    static var random = {(range: Range<Int>) -> Int in
        let distance = {(distance: Int) -> UInt32 in
            guard distance <= Int(UInt32.max) else {
                return UInt32.max
            }
            return UInt32(distance)
        }(range.distance)
        let randomUInt = arc4random_uniform(distance)
        return Int(randomUInt) + range.startIndex
    }

}


let a = Int.random(-127 ..< 128)
let b = Int.random(-127 ..< 128)