enum Payload: Codable {
  case plain(String)
  case pair(key: String, value: String)

  private enum CodingKeys: CodingKey {
    case plain
    case pair
  }

  private enum PlainCodingKeys: CodingKey {
    case _0
  }

  private enum PairCodingKeys: CodingKey {
    case key
    case value
  }

  init(from decoder: any Decoder) throws {
    let container = try decoder.container(keyedBy: Payload.CodingKeys.self)

    var allKeys = ArraySlice(container.allKeys)

    guard let onlyKey = allKeys.popFirst(), allKeys.isEmpty else {
      throw DecodingError.typeMismatch(Payload.self, DecodingError.Context.init(codingPath: container.codingPath, debugDescription: "Invalid number of keys found, expected one.", underlyingError: nil))
    }
    switch onlyKey {
    case .plain:
      let nestedContainer = try container.nestedContainer(keyedBy: Payload.PlainCodingKeys.self, forKey: Payload.CodingKeys.plain)

      self = Payload.plain(try nestedContainer.decode(String.self, forKey: Payload.PlainCodingKeys._0))
    case .pair:
      let nestedContainer = try container.nestedContainer(keyedBy: Payload.PairCodingKeys.self, forKey: Payload.CodingKeys.pair)

      self = Payload.pair(key: try nestedContainer.decode(String.self, forKey: Payload.PairCodingKeys.key), value: try nestedContainer.decode(String.self, forKey: Payload.PairCodingKeys.value))
    }
  }

  func encode(to encoder: any Encoder) throws {
    var container = encoder.container(keyedBy: Payload.CodingKeys.self)

    switch self {
    case .plain(let a0):
      var nestedContainer = container.nestedContainer(keyedBy: Payload.PlainCodingKeys.self, forKey: Payload.CodingKeys.plain)

      try nestedContainer.encode(a0, forKey: Payload.PlainCodingKeys._0)
    case .pair(let key, let value):
      var nestedContainer = container.nestedContainer(keyedBy: Payload.PairCodingKeys.self, forKey: Payload.CodingKeys.pair)

      try nestedContainer.encode(key, forKey: Payload.PairCodingKeys.key)
      try nestedContainer.encode(value, forKey: Payload.PairCodingKeys.value)
    }
  }
}

enum Payload_D: Decodable {
  case plain(String)
  case pair(key: String, value: String)
}

enum Payload_E: Encodable {
  case plain(String)
  case pair(key: String, value: String)
}