class Task {
    protected $taskId;
    protected $coroutine;
    protected $sendValue = null;
    protected $beforeFirstYield = true;

    public function __construct($taskId, Generator $coroutine) {
        $this->taskId = $taskId;
        $this->coroutine = $coroutine;
    }

    public function getTaskId() {
        return $this->taskId;
    }

    public function setSendValue($sendValue) {
        $this->sendValue = $sendValue;
    }

    public function run() {
        if ($this->beforeFirstYield) {
            $this->beforeFirstYield = false;
            return $this->coroutine->current();
        } else {
            $retval = $this->coroutine->send($this->sendValue);
            $this->sendValue = null;
            return $retval;
        }
    }

    public function isFinished() {
        return !$this->coroutine->valid();
    }
}



class Scheduler {
    protected $maxTaskId = 0;
    protected $taskMap = []; // taskId => task
    protected $taskQueue;

    public function __construct() {
        $this->taskQueue = new SplQueue();
    }

    public function newTask(Generator $coroutine) {
        $tid = ++$this->maxTaskId;
        $task = new Task($tid, $coroutine);
        $this->taskMap[$tid] = $task;
        $this->schedule($task);
        return $tid;
    }

    public function schedule(Task $task) {
        $this->taskQueue->enqueue($task);
    }

    public function run() {
        while (!$this->taskQueue->isEmpty()) {
            $task = $this->taskQueue->dequeue();
            $retval = $task->run();

            if ($retval instanceof SystemCall) {
                $retval($task, $this);
                continue;
            }

            if ($task->isFinished()) {
                unset($this->taskMap[$task->getTaskId()]);
            } else {
                $this->schedule($task);
            }
        }
    }
    public function killTask($tid) {
        if (!isset($this->taskMap[$tid])) {
            return false;
        }

        unset($this->taskMap[$tid]);

        foreach ($this->taskQueue as $i => $task) {
            if ($task->getTaskId() === $tid) {
                unset($this->taskQueue[$i]);
                break;
            }
        }

        return true;
    }
}


class SystemCall {
    protected $callback;

    public function __construct(callable $callback) {
        $this->callback = $callback;
    }

    public function __invoke(Task $task, Scheduler $scheduler) {
        $callback = $this->callback;
        return $callback($task, $scheduler);
    }
}


function newTask(Generator $coroutine) {
    return new SystemCall(
        function(Task $task, Scheduler $scheduler) use ($coroutine) {
            $task->setSendValue($scheduler->newTask($coroutine));
            $scheduler->schedule($task);
        }
    );
}

function killTask($tid) {
    return new SystemCall(
        function(Task $task, Scheduler $scheduler) use ($tid) {
            $task->setSendValue($scheduler->killTask($tid));
            $scheduler->schedule($task);
        }
    );
}



function getTaskId() {
    return new SystemCall(function(Task $task, Scheduler $scheduler) {
        $task->setSendValue($task->getTaskId());
        $scheduler->schedule($task);
    });
}



function childTask() {
    $tid = (yield getTaskId());
    while (true) {
        echo "Child task $tid still alive!\n";
        yield;
    }
}

function task() {
    $tid = (yield getTaskId());
    $childTid = (yield newTask(childTask()));

    for ($i = 1; $i <= 6; ++$i) {
        echo "Parent task $tid iteration $i.\n";
        yield;

        if ($i == 3) yield killTask($childTid);
    }
}


$scheduler = new Scheduler;
$scheduler->newTask(task(10));
$scheduler->newTask(task(5));
$scheduler->newTask(task());
$scheduler->run();