String Index Explicit Offsets

string_index_explicit_offsets.swift

extension String.Index {
    /// Creates a new index at the specified UTF-16 code unit offset
  ///
  /// - Parameter offset: An offset in UTF-16 code units.
  public init(offset: Int, within utf16: String.UTF16View) {
    let (start, end) = (utf16.startIndex, utf16.endIndex)
    guard offset >= 0,
          let idx = utf16.index(start, offsetBy: offset, limitedBy: end)
    else {
      self = end
      return
    }
    self = idx
  }

  /// Creates a new index at the specified UTF-8 code unit offset
  ///
  /// - Parameter offset: An offset in UTF-8 code units.
  public init(offset: Int, within utf8: String.UTF8View) {
    let (start, end) = (utf8.startIndex, utf8.endIndex)
    guard offset >= 0,
          let idx = utf8.index(start, offsetBy: offset, limitedBy: end)
    else {
      self = end
      return
    }
    self = idx
  }

  /// Creates a new index at the specified Unicode scalar offset
  ///
  /// - Parameter offset: An offset in terms of Unicode scalar values
  public init(offset: Int, within scalars: String.UnicodeScalarView) {
    let (start, end) = (scalars.startIndex, scalars.endIndex)
    guard offset >= 0,
          let idx = scalars.index(start, offsetBy: offset, limitedBy: end)
    else {
      self = end
      return
    }
    self = idx
  }

  /// Creates a new index at the specified Character offset
  ///
  /// - Parameter offset: An offset in terms of Characters
  public init(offset: Int, within str: String) {
    let (start, end) = (str.startIndex, str.endIndex)
    guard offset >= 0,
          let idx = str.index(start, offsetBy: offset, limitedBy: end)
    else {
      self = end
      return
    }
    self = idx
  }

    /// The UTF-16 code unit offset corresponding to this Index
  public func offset(within utf16: String.UTF16View) -> Int {
    return utf16.distance(from: utf16.startIndex, to: self)
  }
  /// The UTF-8 code unit offset corresponding to this Index
  public func offset(within utf8: String.UTF8View) -> Int {
    return utf8.distance(from: utf8.startIndex, to: self)
  }
  /// The Unicode scalar offset corresponding to this Index
  public func offset(within scalars: String.UnicodeScalarView) -> Int {
    return scalars.distance(from: scalars.startIndex, to: self)
  }
  /// The Character offset corresponding to this Index
  public func offset(within str: String) -> Int {
    return str.distance(from: str.startIndex, to: self)
  }

}

let myString = "abcde\u{301}あいう𓀎𓃶"

for index in myString.indices {
  print("Character offset: \(index.offset(within: myString))")
}
for index in myString.utf8.indices {
  print("UTF-8 offset: \(index.offset(within: myString.utf8))")
}
for index in myString.utf16.indices {
  print("UTF-16 offset: \(index.offset(within: myString.utf16))")
}
for index in myString.unicodeScalars.indices {
  print("Scalar offset: \(index.offset(within: myString.unicodeScalars))")
}

print("UTF-8")
for i in 0..<myString.utf8.count {
  print(myString.utf8[String.Index(offset: i, within: myString.utf8)])
}

print("UTF-16")
for i in 0..<myString.utf16.count {
  print(myString.utf16[String.Index(offset: i, within: myString.utf16)])
}

print("Scalars")
for i in 0..<myString.unicodeScalars.count {
  print(myString.unicodeScalars[String.Index(offset: i, within: myString.unicodeScalars)])
}

print("Characters")
for i in 0..<myString.count {
  print(myString[String.Index(offset: i, within: myString)])
}