Working with Unicode code points in Swift

Question

If you are not interested in the details of Mongolian but just want a quick answer about using and converting Unicode values in Swift, then skip down to the first part of the accepted answer.

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Background

I want to render Unicode text for traditional Mongolian to be used in iOS apps. The better and long term solution is to use an AAT smart font that would render this complex script. (Such fonts do exist but their license does not allow modification and non-personal use.) However, since I have never made a font, let alone all of the rendering logic for an AAT font, I just plan to do the rendering myself in Swift for now. Maybe at some later date I can learn to make a smart font.

Externally I will use Unicode text, but internally (for display in a UITextView) I will convert the Unicode to individual glyphs that are stored in a dumb font (coded with Unicode PUA values). So my rendering engine needs to convert Mongolian Unicode values (range: U+1820 to U+1842) to glyph values stored in the PUA (range: U+E360 to U+E5CF). Anyway, this is my plan since it is what I did in Java in the past, but maybe I need to change my whole way of thinking.

Example

The following image shows su written twice in Mongolian using two different forms for the letter u (in red). (Mongolian is written vertically with letters being connected like cursive letters in English.)

In Unicode these two strings would be expressed as

var suForm1: String = "\u{1830}\u{1826}"
var suForm2: String = "\u{1830}\u{1826}\u{180B}"

The Free Variation Selector (U+180B) in suForm2 is recognized (correctly) by Swift String to be a unit with the u (U+1826) that precedes it. It is considered by Swift to be a single character, an extended grapheme cluster. However, for the purposes of doing the rendering myself, I need to differentiate u (U+1826) and FVS1 (U+180B) as two distinct UTF-16 code points.

For internal display purposes, I would convert the above Unicode strings to the following rendered glyph strings:

suForm1 = "\u{E46F}\u{E3BA}" 
suForm2 = "\u{E46F}\u{E3BB}"

Question

I have been playing around with Swift String and Character. There are a lot of convenient things about them, but since in my particular case I deal exclusively with UTF-16 code units, I wonder if I should be using the old NSString rather than Swift's String. I realize that I can use String.utf16 to get UTF-16 code points, but the conversion back to String isn't very nice.

Would it be better to stick with String and Character or should I use NSString and unichar?

What I have read

• Strings and Characters documentation
• Strings in Swift
• NSString and Unicode

Updates to this question have been hidden in order to clean the page up. See the edit history.

Answer 1

Updated for Swift 3

String and Character

For almost everyone in the future who visits this question, String and Character will be the answer for you.

Set Unicode values directly in code:

var str: String = "I want to visit 北京, Москва, मुंबई, القاهرة, and 서울시.                                                                     

                                                        
            

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Answer 2

//Swift 3.0  
// This struct is an array of UInt32 to hold Unicode scalar values
struct ScalarString: Sequence, Hashable, CustomStringConvertible {

    private var scalarArray: [UInt32] = []

    init() {
        // does anything need to go here?
    }

    init(_ character: UInt32) {
        self.scalarArray.append(character)
    }

    init(_ charArray: [UInt32]) {
        for c in charArray {
            self.scalarArray.append(c)
        }
    }

    init(_ string: String) {

        for s in string.unicodeScalars {
            self.scalarArray.append(s.value)
        }
    }

    // Generator in order to conform to SequenceType protocol
    // (to allow users to iterate as in `for myScalarValue in myScalarString` { ... })

    //func generate() -> AnyIterator<UInt32> {
    func makeIterator() -> AnyIterator<UInt32> {

        let nextIndex = 0

        return AnyIterator {
            if (nextIndex > self.scalarArray.count-1) {
                return nil
            }
            return self.scalarArray[nextIndex + 1]
        }
    }

    // append
    mutating func append(scalar: UInt32) {
        self.scalarArray.append(scalar)
    }

    mutating func append(scalarString: ScalarString) {
        for scalar in scalarString {
            self.scalarArray.append(scalar)
        }
    }

    mutating func append(string: String) {
        for s in string.unicodeScalars {
            self.scalarArray.append(s.value)
        }
    }

    // charAt
    func charAt(index: Int) -> UInt32 {
        return self.scalarArray[index]
    }

    // clear
    mutating func clear() {
        self.scalarArray.removeAll(keepingCapacity: true)
    }

    // contains
    func contains(character: UInt32) -> Bool {
        for scalar in self.scalarArray {
            if scalar == character {
                return true
            }
        }
        return false
    }

    // description (to implement Printable protocol)
    var description: String {

        var string: String = ""

        for scalar in scalarArray {
            string.append(String(describing: UnicodeScalar(scalar))) //.append(UnicodeScalar(scalar)!)
        }
        return string
    }

    // endsWith
    func endsWith() -> UInt32? {
        return self.scalarArray.last
    }

    // insert
    mutating func insert(scalar: UInt32, atIndex index: Int) {
        self.scalarArray.insert(scalar, at: index)
    }

    // isEmpty
    var isEmpty: Bool {
        get {
            return self.scalarArray.count == 0
        }
    }

    // hashValue (to implement Hashable protocol)
    var hashValue: Int {
        get {

            // DJB Hash Function
            var hash = 5381

            for i in 0 ..< scalarArray.count {
                hash = ((hash << 5) &+ hash) &+ Int(self.scalarArray[i])
            }
            /*
             for i in 0..< self.scalarArray.count {
             hash = ((hash << 5) &+ hash) &+ Int(self.scalarArray[i])
             }
             */
            return hash
        }
    }

    // length
    var length: Int {
        get {
            return self.scalarArray.count
        }
    }

    // remove character
    mutating func removeCharAt(index: Int) {
        self.scalarArray.remove(at: index)
    }
    func removingAllInstancesOfChar(character: UInt32) -> ScalarString {

        var returnString = ScalarString()

        for scalar in self.scalarArray {
            if scalar != character {
                returnString.append(scalar: scalar) //.append(scalar)
            }
        }

        return returnString
    }

    // replace
    func replace(character: UInt32, withChar replacementChar: UInt32) -> ScalarString {

        var returnString = ScalarString()

        for scalar in self.scalarArray {
            if scalar == character {
                returnString.append(scalar: replacementChar) //.append(replacementChar)
            } else {
                returnString.append(scalar: scalar) //.append(scalar)
            }
        }
        return returnString
    }

    // func replace(character: UInt32, withString replacementString: String) -> ScalarString {
    func replace(character: UInt32, withString replacementString: ScalarString) -> ScalarString {

        var returnString = ScalarString()

        for scalar in self.scalarArray {
            if scalar == character {
                returnString.append(scalarString: replacementString) //.append(replacementString)
            } else {
                returnString.append(scalar: scalar) //.append(scalar)
            }
        }
        return returnString
    }

    // set (an alternative to myScalarString = "some string")
    mutating func set(string: String) {
        self.scalarArray.removeAll(keepingCapacity: false)
        for s in string.unicodeScalars {
            self.scalarArray.append(s.value)
        }
    }

    // split
    func split(atChar splitChar: UInt32) -> [ScalarString] {
        var partsArray: [ScalarString] = []
        var part: ScalarString = ScalarString()
        for scalar in self.scalarArray {
            if scalar == splitChar {
                partsArray.append(part)
                part = ScalarString()
            } else {
                part.append(scalar: scalar) //.append(scalar)
            }
        }
        partsArray.append(part)
        return partsArray
    }

    // startsWith
    func startsWith() -> UInt32? {
        return self.scalarArray.first
    }

    // substring
    func substring(startIndex: Int) -> ScalarString {
        // from startIndex to end of string
        var subArray: ScalarString = ScalarString()
        for i in startIndex ..< self.length {
            subArray.append(scalar: self.scalarArray[i]) //.append(self.scalarArray[i])
        }
        return subArray
    }
    func substring(startIndex: Int, _ endIndex: Int) -> ScalarString {
        // (startIndex is inclusive, endIndex is exclusive)
        var subArray: ScalarString = ScalarString()
        for i in startIndex ..< endIndex {
            subArray.append(scalar: self.scalarArray[i]) //.append(self.scalarArray[i])
        }
        return subArray
    }

    // toString
    func toString() -> String {
        let string: String = ""

        for scalar in self.scalarArray {
            string.appending(String(describing:UnicodeScalar(scalar))) //.append(UnicodeScalar(scalar)!)
        }
        return string
    }

    // values
    func values() -> [UInt32] {
        return self.scalarArray
    }

}

func ==(left: ScalarString, right: ScalarString) -> Bool {

    if left.length != right.length {
        return false
    }

    for i in 0 ..< left.length {
        if left.charAt(index: i) != right.charAt(index: i) {
            return false
        }
    }

    return true
}

func +(left: ScalarString, right: ScalarString) -> ScalarString {
    var returnString = ScalarString()
    for scalar in left.values() {
        returnString.append(scalar: scalar) //.append(scalar)
    }
    for scalar in right.values() {
        returnString.append(scalar: scalar) //.append(scalar)
    }
    return returnString
}