Convert an NSRange in NSString into a Range<String.Index>

NSRangeToRange.swift

//: Convert an NSRange from NSString into a Range<String.Index>

import Foundation

let swiftString: String = "🤖 This is an über-👍 string"
let swiftStringCharCount = swiftString.characters.count

let nsString = (swiftString as NSString).decomposedStringWithCanonicalMapping as NSString
let nsStringLength = nsString.length

let invalidNsRange: NSRange = NSMakeRange(1, 2)
let validNsRange: NSRange = NSMakeRange(20, 2)

let invalidNsSubstring: NSString = nsString.substring(with: invalidNsRange) as NSString
let validNsSubstring: NSString = nsString.substring(with: validNsRange) as NSString

func characterRangeFrom(nsRange: NSRange, nsString: NSString) -> Range<String.Index>? {
    let swiftString: String = nsString.precomposedStringWithCanonicalMapping as String
    var lowerBound: String.Index? = nil
    var upperBound = swiftString.startIndex
    var lowerBoundValid = false
    var upperBoundValid = false
    nsString.enumerateSubstrings(in: NSRange(location: 0, length: nsRange.location + nsRange.length),
                                 options: .byComposedCharacterSequences) { (substring, substringRange, enclosingRange, stop) in
                                    if substringRange.location <= nsRange.location {
                                        lowerBound = lowerBound == nil ? swiftString.startIndex : swiftString.index(after: lowerBound!)
                                        if substringRange.location == nsRange.location {
                                            lowerBoundValid = true
                                        }
                                    }
                                    if substringRange.location + substringRange.length <= nsRange.location + nsRange.length {
                                        upperBound = swiftString.index(after: upperBound)
                                        if substringRange.location + substringRange.length == nsRange.location + nsRange.length {
                                            upperBoundValid = true
                                        }
                                    }
    }
    
    guard lowerBoundValid && upperBoundValid else {
        return nil
    }
    
    return Range(uncheckedBounds: (lowerBound!, upperBound))
}

if let validRange = characterRangeFrom(nsRange: validNsRange, nsString: nsString) {
    let validSubstring = swiftString[validRange]
}

if let invalidRange = characterRangeFrom(nsRange: invalidNsRange, nsString: nsString) {
    let invalidSubstring = swiftString[invalidRange]
}