import kotlinx.coroutines.experimental.*
import kotlinx.coroutines.experimental.channels.*

class FixedPool(val workerCount: Int) {

    val channel = Channel<Task>()
    val jobs = mutableListOf<Job>()

    init {
        start() // start immediately
    }

    fun start() {
        repeat(workerCount) { i ->
            jobs.add(launch(CommonPool) { // or use your own coroutine context
                for (task in channel) {
                    println("worker-$i starts ${task.name}")
                    task()
                    println("worker-$i finished ${task.name}")
                }
            })
        }
    }

    fun execute(block: Task) {
        launch(Unconfined) { // seems safe to use Unconfined
            channel.send(block)
        }
    }

    suspend fun join() {
        for (j in jobs) j.join()
    }

    fun close() = channel.close()
}

class Task(val name: String, val time: Long) {
    operator fun invoke() {
        Thread.sleep(time) // block to simulate real operation
    }
}

runBlocking {
    val pool = FixedPool(2)
    pool.execute(Task("A", 1000))
    pool.execute(Task("B", 2500))
    pool.execute(Task("C", 1000))
    pool.execute(Task("D", 1000))
    pool.execute(Task("E", 1000))

    // We must wait; in long running app maybe not needed
    pool.close()
    pool.join() 
}

// Example how to run on desktop:
// kotlinc-jvm -script coroutine-pool.kts -cp kotlinx-coroutines-core-0.22.2.jar -Xcoroutines=enable