internal fun idnToAscii(host: String): String? {
  val bufferA = Buffer().writeUtf8(host)
  val bufferB = Buffer()

  // 1. Map, from bufferA to bufferB.
  while (!bufferA.exhausted()) {
    val codePoint = bufferA.readUtf8CodePoint()
    if (!IDNA_MAPPING_TABLE.map(codePoint, bufferB)) return null
  }

  // 2. Normalize, from bufferB to bufferA.
  val normalized = normalizeNfc(bufferB.readUtf8())
  bufferA.writeUtf8(normalized)