Swift 使用序列初始化字符串的时间复杂度是多少

What have I done so far?个

1. 我在公共documentation中寻找初始值设定项.

2. 我看了看open source code，见下面的代码.

Detail个

在看了方法定义之后，我认为我们可以进一步澄清这个问题，即Sequence/Array将以连续的字节存储，而UTF8将被使用.

  /// Creates a string from the given Unicode code units in the specified
  /// encoding.
  ///
  /// - Parameters:
  ///   - codeUnits: A collection of code units encoded in the encoding
  ///     specified in `sourceEncoding`.
  ///   - sourceEncoding: The encoding in which `codeUnits` should be
  ///     interpreted.
  @inlinable
  @inline(__always) // Eliminate dynamic type check when possible
  public init<C: Collection, Encoding: Unicode.Encoding>(
    decoding codeUnits: C, as sourceEncoding: Encoding.Type
  ) where C.Iterator.Element == Encoding.CodeUnit {
    guard _fastPath(sourceEncoding == UTF8.self) else {
      self = String._fromCodeUnits(
        codeUnits, encoding: sourceEncoding, repair: true)!.0
      return
    }

    // Fast path for user-defined Collections and typed contiguous collections.
    //
    // Note: this comes first, as the optimizer nearly always has insight into
    // wCSIA, but cannot prove that a type does not have conformance to
    // _HasContiguousBytes.
    if let str = codeUnits.withContiguousStorageIfAvailable({
      (buffer: UnsafeBufferPointer<C.Element>) -> String in
      Builtin.onFastPath() // encourage SIL Optimizer to inline this closure :-(
      let rawBufPtr = UnsafeRawBufferPointer(buffer)
      return String._fromUTF8Repairing(
        UnsafeBufferPointer(
          start: rawBufPtr.baseAddress?.assumingMemoryBound(to: UInt8.self),
          count: rawBufPtr.count)).0
    }) {
      self = str
      return
    }

    // Fast path for untyped raw storage and known stdlib types
    if let contigBytes = codeUnits as? _HasContiguousBytes,
      contigBytes._providesContiguousBytesNoCopy
    {
      self = contigBytes.withUnsafeBytes { rawBufPtr in
        Builtin.onFastPath() // encourage SIL Optimizer to inline this closure
        return String._fromUTF8Repairing(
          UnsafeBufferPointer(
            start: rawBufPtr.baseAddress?.assumingMemoryBound(to: UInt8.self),
            count: rawBufPtr.count)).0
      }
      return
    }

    self = String._fromNonContiguousUnsafeBitcastUTF8Repairing(codeUnits).0
  }