CatsCountDownLatch.scala

import cats.Applicative
import cats.effect.Concurrent
import cats.effect.concurrent.{Deferred, Ref}
import cats.syntax.flatMap._
import cats.syntax.functor._

trait CountDownLatch[F[_]] {
  def countDown: F[Unit]
  def await: F[Unit]
}

object CountDownLatch {
  def apply[F[_]: Concurrent](count: Int): F[CountDownLatch[F]] = for {
    ref <- Ref.of[F, Int](count)
    ready <- Deferred[F, Unit]
  } yield new CountDownLatch[F] {
    override def countDown: F[Unit] =
      ref.modify(current => (current - 1, current - 1)).flatMap {
        case 0 => ready.complete(())
        case _ => Applicative[F].unit
      }

    override def await: F[Unit] = ready.get
  }
}