What is the best way to test if a CharacterSet contains a Character in Swift 4?

Question

I'm looking for a way, in Swift 4, to test if a Character is a member of an arbitrary CharacterSet. I have this Scanner class that will be used for some lightweight parsing. One of the functions in the class is to skip any characters, at the current position, that belong to a certain set of possible characters.

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)\"")

I would like to implement the skip(charactersIn:) function so that the above code would print buzz fizz.

The tricky part is characters.contains(str[idx])) in the while - .contains() requires a Unicode.Scalar, and I'm at a loss trying to figure out the next step.

I know I could pass in a String to the skip function, but I'd like to find a way to make it work with a CharacterSet, because of all the convenient static members (alphanumerics, whitespaces, etc.).

How does one test a CharacterSet if it contains a Character?

Answer 1

Not sure if it's the most efficient way but you can create a new CharSet and check if they are sub/super-sets (Set comparison is rather quick)

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