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post.scala

def checkAndSave(event: PageViewed,
                 hostname: String,
                 clientIp: Option[String]): F[ValidatedNel[PVError, Unit]] = {
  val normalizedPath = normalize(event.path)

  val uuidF: F[UUID] = Sync[F].delay { UUID.randomUUID() }
  val currentTimeF: F[OffsetDateTime] = getCurrentTimeF
  
  val ipValidationF: F[ValidatedNel[PVError, String]] =
    clientIp
      .toValidNel(PVError.noIp)
      .traverse(checkIp(_, hostname, normalizedPath))
      .map(_.andThen(identity)

  val pathValidationF: F[ValidatedNel[PVError, String]] =
    pageExists(hostname, normalizedPath).map(
      Validated.condNel(_, normalizedPath, PVError.pageNotFound))

  (uuidF, currentTimeF, ipValidationF, pathValidationF).traverseN {
    (uuid, currentTime, ipValidation, pathValidation) =>
      val timestampV: ValidatedNel[PVError, OffsetDateTime] = event
        .displayedAt
        .valid
        .ensure(PVError.invalidTimestamp)(_.isBefore(currentTime))
        .toValidatedNel
        
      (
        uuid.valid,
        pathValidation,
        timestampV,
        hostname.valid,
        ipValidation
      ).mapN(PageView).traverse(repository.persist)
  }.flatten
}