NSRange intersection

struct NSRange {
  location : Int;
    length : Int;
}
typealias NSNotFound = Int.max

//**证明相交最简单的办法就是 -> 证明不是不相交**

func intersection(_ r1 : NSRange, _ r2 : NSRange) -> NSRange{
    guard r1.location != NSNotFound && r1.length != NSNotFound
    && r2.location != NSNotFound && r2.length != NSNotFound  else{
        return NSRange(location: NSNotFound, length: NSNotFound)
    }
    //check if intersection
    if r1.location + r1.length - 1 < r2.location || r2.location + r2.length - 1 < r1.location {
        return NSRange(location: NSNotFound, length: NSNotFound)
    }

    //get intersection
    let startIndex = max(r1.location, r2.location)
    let endIndex = min(r1.location + r1.length - 1, r2.location + r2.length - 1)
    return NSRange(location: startIndex, length: endIndex - startIndex + 1)
}