Learning Swift -- some string manipulation in a playground

MyPlayground.swift

import UIKit

extension StringProtocol {
    func ranges(of: String) -> [Range<Index>] {
        var ranges: [Range<Index>] = []
        var range = self.range(of: of)
        
        while let r = range {
            ranges.append(r)
            range = self[r.upperBound...].range(of: of)
        }
        
        return ranges
    }
    
    func substring(_ start: Index, _ end: Index? = nil) -> String {
        guard let e = end else {
            return String(self[start...])
        }
        return String(self[start..<e])
    }
}

func getAttributes(isBold: Bool) -> [NSAttributedString.Key: Any] {
    let font = isBold ? UIFont.boldSystemFont(ofSize: 12.0) : UIFont.systemFont(ofSize: 12.0)
    return [NSAttributedString.Key.font: font]
}

func boldText(src: String, toBold: String) -> NSAttributedString {
    let ranges = src.lowercased().ranges(of: toBold.lowercased())
    
    if ranges.isEmpty {
        return NSAttributedString(string: src, attributes: getAttributes(isBold: false))
    }
    
    let text = NSMutableAttributedString(string: String(src[..<ranges[0].lowerBound]), attributes: getAttributes(isBold: false))
    
    for i in 0..<ranges.count {
        if i > 0 && ranges[i-1].upperBound < ranges[i].lowerBound {
            text.append(NSAttributedString(string: src.substring(ranges[i-1].upperBound, ranges[i].lowerBound), attributes: getAttributes(isBold: false)))
        }
        text.append(NSAttributedString(string: src.substring(ranges[i].lowerBound, ranges[i].upperBound), attributes: getAttributes(isBold: true)))
    }
    text.append(NSAttributedString(string: src.substring(ranges[ranges.count-1].upperBound), attributes: getAttributes(isBold: false)))
    
    return text
}

print(boldText(src: "Congratulations!", toBold: "winner"))
print(boldText(src: "Congratulations! You're a winner!", toBold: "winner"))
print(boldText(src: "Winner, winner, chicken dinner!", toBold: "winner"))