extension String

extension String {
    subscript(i: Int) -> String {
        guard i >= 0 && i < characters.count else { return "" }
        return String(self[index(startIndex, offsetBy: i)])
    }
    subscript(range: Range<Int>) -> String {
        let lowerIndex = index(startIndex, offsetBy: max(0,range.lowerBound), limitedBy: endIndex) ?? endIndex
        return substring(with: lowerIndex..<(index(lowerIndex, offsetBy: range.upperBound - range.lowerBound, limitedBy: endIndex) ?? endIndex))
    }
    subscript(range: ClosedRange<Int>) -> String {
        let lowerIndex = index(startIndex, offsetBy: max(0,range.lowerBound), limitedBy: endIndex) ?? endIndex
        return substring(with: lowerIndex..<(index(lowerIndex, offsetBy: range.upperBound - range.lowerBound + 1, limitedBy: endIndex) ?? endIndex))
    }
}