String parser using XML like formatter

StringTagParser.swift

import UIKit
import PlaygroundSupport

class Stack: CustomStringConvertible {
    var tag: String
    var open: Int
    var close: Int

    init(tag: String, open: Int, close: Int) {
        self.tag = tag
        self.open = open
        self.close = close
    }

    var description: String {
        return "\((tag, open, close))"
    }
}

class StringFormatter {
    private var bufferText = ""
    private var stack: [Stack] = []

    enum Tag: String {
        case a = "a"
        case b = "b"
        case c = "c"

        func attKey() -> (NSAttributedStringKey, Any) {
            switch self {
            case .a:
                return (.backgroundColor, UIColor.red)
            case .b:
                return (.foregroundColor, UIColor.blue)
            case .c:
                return (.font, UIFont.italicSystemFont(ofSize: 15))
            }
        }
    }

    func transformText(initalText: String) -> NSMutableAttributedString {
        formatString(text: initalText, idx: 0, bufferText: "", stack: [])
        let att = NSMutableAttributedString(string: bufferText)
        for s in stack {
            if let tag = Tag(rawValue: s.tag) {
                let r = NSRange(location: s.open, length: s.close - s.open)
                att.addAttribute(tag.attKey().0, value: tag.attKey().1, range: r)
            }
        }
        return att
    }

    private func formatString(text: String, idx: Int, bufferText: String, stack: [Stack]) {
        if text.count == 0 {
            self.bufferText = bufferText
            self.stack = stack
            return
        }

        let index = text.index(text.startIndex, offsetBy: 2)
        let mySubstring = text[..<index]

        if "\(mySubstring)" == "</" {
            //1.3
            var txt = String(text.dropFirst(2))

            var tag = ""
            while txt.count > 0 {
                if txt.first! != ">" {
                    //1.1.1
                    tag.append(txt.first!)
                } else {
                    //1.1.2
                    break
                }
                txt = String(txt.dropFirst())
            }

            for t in stack.reversed() {
                if t.tag == tag {
                    t.close = bufferText.count
                }
            }

            formatString(text: String(txt.dropFirst()), idx: idx + 1, bufferText: bufferText, stack: stack)

        } else if "\(text.first!)" == "<" {
            //1.1
            var txt = String(text.dropFirst())
            var tag = ""
            while txt.count > 0 {
                if txt.first! != ">" {
                    //1.1.1
                    tag.append(txt.first!)
                } else {
                    //1.1.2
                    break
                }
                txt = String(txt.dropFirst())
            }
            var stk = stack
            stk.append(Stack(tag: tag, open: bufferText.count, close: bufferText.count))
            formatString(text: String(txt.dropFirst()), idx: idx + 1, bufferText: bufferText, stack: stk)
        } else {
            //1.2
            var txt = bufferText
            txt.append(text.first!)
            formatString(text: String(text.dropFirst()), idx: idx + 1, bufferText: txt, stack: stack)
        }
    }
}

class MyViewController : UIViewController {
    override func loadView() {
        let view = UIView()
        view.backgroundColor = .white

        let label = UILabel()
        label.frame = CGRect(x: 150, y: 200, width: 200, height: 20)
        label.textColor = .black

        let txt = "<c>hello <a>world</a></c>"
        let str = StringFormatter()
        print(str.transformText(initalText: txt))
        label.attributedText = str.transformText(initalText: txt)

        view.addSubview(label)
        self.view = view
    }
}
// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()