Swift.String truncate

StringExtensions.swift

import Foundation

public enum TruncateMode: Int {
    case Head = 0
    case Middle = 1
    case Tail = 2
}

public extension String {
    func nilIfEmpty() -> String? {
        return self.isEmpty ? nil : self
    }

    /// Truncates a string to the max length using the specified mode
    /// :param: maxLength The maximum length (not including any ellipses)
    /// :param: mode The truncation mode; if Middle then ellipses are always added
    /// :param: addEllipses If mode is not middle, controls whether ellipses are added to the truncated end of the string
    func truncateToLength(maxLength: Int, mode: TruncateMode = TruncateMode.Tail, addEllipses: Bool = false) -> String {
        if maxLength <= 0 {
            return ""
        }

        let ellipses = addEllipses || mode == .Middle ? "\u{2026}" : ""
        var headIndex = self.startIndex
        var tailIndex = self.endIndex

        if mode == .Head {
            tailIndex = advance(self.endIndex, -maxLength, self.startIndex)
        }
        else if mode == .Tail {
            headIndex = advance(self.startIndex, maxLength, self.endIndex)
        }
        else {
            let tailLength = maxLength / 2
            let headLength = maxLength - tailLength
            headIndex = advance(self.startIndex, headLength, self.endIndex)
            tailIndex = advance(self.endIndex, -tailLength, headIndex)
        }

        if headIndex == self.startIndex && tailIndex == self.endIndex {
            return self
        }

        return self.substringToIndex(headIndex) + ellipses + self.substringFromIndex(tailIndex)
    }
}

Second revision owes to kballard for his comments; see his take here https://gist.github.com/kballard/f6844e35755e6a04c8f2

advance(self.startIndex, headLength, self.endIndex) --> this method is depricated in swift 3 and 4 can you please modify this code.