StringExtensions.swift

public enum TruncateMode {
    case Head, Middle, Tail
}

public extension String {
    /// Truncates a string to a maximum length in grapheme clusters using the
    /// specified mode. If `addEllipsis` is `true`, the resulting string
    /// including the ellipsis will be no longer than `maxLength`.
    func truncateToLength(maxLength: Int, mode: TruncateMode = .Tail, addEllipsis: Bool = false) -> String {
        if maxLength <= 0 {
            return ""
        }
        switch mode {
        case .Head:
            let start = advance(endIndex, -maxLength, startIndex)
            if start == startIndex {
                // no truncation necessary
                return self
            }
            if addEllipsis {
                return "…\(self[start.successor()..<endIndex])"
            }
            return self[start..<endIndex]
        case .Middle:
            let tailLength = maxLength / 2
            let headLength = maxLength - tailLength
            let headEnd = advance(startIndex, headLength, endIndex)
            if headEnd == endIndex {
                // no truncation necessary
                return self
            }
            let tailStart = advance(endIndex, -tailLength, headEnd)
            if tailStart == headEnd {
                // no truncation necessary
                return self
            }
            // always add the ellipsis for middle truncation
            return "\(self[startIndex..<headEnd.predecessor()])…\(self[tailStart..<endIndex])"
        case .Tail:
            let end = advance(startIndex, maxLength, endIndex)
            if end == endIndex {
                // no truncation necessary
                return self
            }
            if addEllipsis {
                return "\(self[startIndex..<end.predecessor()])…"
            }
            return self[startIndex..<end]
        }
    }
}