測試Swift 4中CharacterSet是否包含字符的最佳方法是什麼？

Question

我正在尋找一種方法，在Swift 4中測試一個Character是否是任意CharacterSet的成員。我有這個Scanner類將用於一些輕量級的解析。該類中的一個功能是跳過屬於某組可能字符的當前位置的任何字符。

class MyScanner { 
    let str: String 
    var idx: String.Index 
    init(_ string: String) { 
    str = string 
    idx = str.startIndex 
    } 
    var remains: String { return String(str[idx..<str.endIndex])} 

    func skip(charactersIn characters: CharacterSet) { 
    while idx < str.endIndex && characters.contains(str[idx])) { 
     idx = source.index(idx, offsetBy: 1) 
    } 
    } 
} 

let scanner = MyScanner("fizz buzz fizz") 
scanner.skip(charactersIn: CharacterSet.alphanumerics) 
scanner.skip(charactersIn: CharacterSet.whitespaces) 
print("what remains: \"\(scanner.remains)\"") 

我想實現skip(charactersIn:)功能，使上面的代碼將打印buzz fizz。

棘手的部分是characters.contains(str[idx]))在while - .contains()需要Unicode.Scalar，我很茫然，試圖找出下一步。

我知道我可以在String傳遞給skip功能，但我想找到一種方法，使其與一個CharacterSet工作，因爲所有的便利靜態成員（alphanumerics，whitespaces等）。

如何測試CharacterSet如果包含Character？

Answer 1

我知道你想使用CharacterSet而不是String，但CharacterSet不支持（但至少）支持由多個Unicode.Scalar組成的字符。請參閱Apple在WWDC 2017視頻What's New in Swift的字符串討論中演示的「家庭」字符（）或國際標記字符（例如「」或「」）。多膚色表情符號也表現出這種行爲（例如vs）。

因此，我會謹慎使用CharacterSet（這是一組「用於搜索操作的Unicode字符值」）。或者，如果您想爲方便起見而提供此方法，請注意，它將無法正確使用由多個unicode標量表示的字符。

所以，你可能會提供一個掃描儀，提供了skip方法既CharacterSet和String引渡：

class MyScanner { 
    let string: String 
    var index: String.Index 

    init(_ string: String) { 
     self.string = string 
     index = string.startIndex 
    } 

    var remains: String { return String(string[index...]) } 

    /// Skip characters in a string 
    /// 
    /// This rendition is safe to use with strings that have characters 
    /// represented by more than one unicode scalar. 
    /// 
    /// - Parameter skipString: A string with all of the characters to skip. 

    func skip(charactersIn skipString: String) { 
     while index < string.endIndex, skipString.contains(string[index]) { 
      index = string.index(index, offsetBy: 1) 
     } 
    } 

    /// Skip characters in character set 
    /// 
    /// Note, character sets cannot (yet) include characters that are represented by 
    /// more than one unicode scalar (e.g. ‍‍‍ or or). If you want to test 
    /// for these multi-unicode characters, you have to use the `String` rendition of 
    /// this method. 
    /// 
    /// This will simply stop scanning if it encounters a multi-unicode character in 
    /// the string being scanned (because it knows the `CharacterSet` can only represent 
    /// single-unicode characters) and you want to avoid false positives (e.g., mistaking 
    /// the Jamaican flag, , for the Japanese flag,). 
    /// 
    /// - Parameter characterSet: The character set to check for membership. 

    func skip(charactersIn characterSet: CharacterSet) { 
     while index < string.endIndex, 
      string[index].unicodeScalars.count == 1, 
      let character = string[index].unicodeScalars.first, 
      characterSet.contains(character) { 
       index = string.index(index, offsetBy: 1) 
     } 
    } 

} 

因此，您簡單的例子仍然可以工作：

let scanner = MyScanner("fizz buzz fizz") 
scanner.skip(charactersIn: CharacterSet.alphanumerics) 
scanner.skip(charactersIn: CharacterSet.whitespaces) 
print(scanner.remains) // "buzz fizz" 

但使用String如果要跳過的字符可能包含多個Unicode標量：

let family = "\u{200D}\u{200D}\u{200D}" // ‍‍‍ 
let boy = "" 

let charactersToSkip = family + boy 

let string = boy + family + "foobar" // ‍‍‍foobar 

let scanner = MyScanner(string) 
scanner.skip(charactersIn: charactersToSkip) 
print(scanner.remains)    // foobar 

---

正如邁克爾瀑布下面的評論中指出，CharacterSet有缺陷，甚至不正確地處理32位Unicode.Scalar值，這意味着它甚至不正確，如果該值超過0xffff處理單個標字符（包括表情符號等）。然而，上面的String演繹處理正確。

Answer 2

不知道這是否是最有效的方式，但你可以創建一個新的字符集，並檢查它們是否子/超集（集比較是相當快）

let newSet = CharacterSet(charactersIn: "a") 
// let newSet = CharacterSet(charactersIn: "\(character)") 
print(newSet.isSubset(of: CharacterSet.decimalDigits)) // false 
print(newSet.isSubset(of: CharacterSet.alphanumerics)) // true