<?php

namespace Amp\Parallel\Worker;

use Amp\Coroutine;
use Amp\Deferred;
use Amp\Parallel\ContextException;
use Amp\Parallel\StatusError;
use Amp\Parallel\Strand;
use Amp\Promise;

/**
 * Base class for most common types of task workers.
 */
abstract class AbstractWorker implements Worker {
    /** @var \Amp\Parallel\Strand */
    private $context;

    /** @var bool */
    private $shutdown = false;

    /** @var \Amp\Deferred[] */
    private $jobQueue = [];

    /** @var callable */
    private $onResolve;

    /**
     * @param \Amp\Parallel\Strand $strand
     */
    public function __construct(Strand $strand) {
        $this->context = $strand;

        $this->onResolve = function ($exception, $data) {
            if ($exception) {
                $this->cancel($exception);
                return;
            }

            if (!$data instanceof Internal\TaskResult) {
                $this->cancel(new ContextException("Context did not return a task result"));
                return;
            }

            $id = $data->getId();

            if (!isset($this->jobQueue[$id])) {
                $this->cancel(new ContextException("Job ID returned by context does not exist"));
                return;
            }

            $deferred = $this->jobQueue[$id];
            unset($this->jobQueue[$id]);
            $empty = empty($this->jobQueue);

            $deferred->resolve($data->promise());

            if (!$empty) {
                $this->context->receive()->onResolve($this->onResolve);
            }
        };
    }

    /**
     * {@inheritdoc}
     */
    public function isRunning(): bool {
        return $this->context->isRunning();
    }

    /**
     * {@inheritdoc}
     */
    public function isIdle(): bool {
        return empty($this->jobQueue);
    }

    /**
     * {@inheritdoc}
     */
    public function start() {
        $this->context->start();
    }

    /**
     * {@inheritdoc}
     */
    public function enqueue(Task $task): Promise {
        if (!$this->context->isRunning()) {
            throw new StatusError("The worker has not been started");
        }

        if ($this->shutdown) {
            throw new StatusError("The worker has been shut down");
        }

        return new Coroutine($this->doEnqueue($task));
    }

    /**
     * @coroutine
     *
     * @param \Amp\Parallel\Worker\Task $task
     *
     * @return \Generator
     * @throws \Amp\Parallel\StatusError
     * @throws \Amp\Parallel\Worker\TaskException
     * @throws \Amp\Parallel\Worker\TaskError
     * @throws \Amp\Parallel\Worker\WorkerException
     */
    private function doEnqueue(Task $task): \Generator {
        $empty = empty($this->jobQueue);

        try {
            $job = new Internal\Job($task);
            $this->jobQueue[$job->getId()] = $deferred = new Deferred;

            if ($empty) {
                $this->context->receive()->onResolve($this->onResolve);
            }

            yield $this->context->send($job);
        } catch (\Throwable $exception) {
            $exception = new WorkerException("Sending the task to the worker failed", $exception);
            $this->cancel($exception);
            throw $exception;
        }

        return yield $deferred->promise();
    }

    /**
     * {@inheritdoc}
     */
    public function shutdown(): Promise {
        if (!$this->context->isRunning() || $this->shutdown) {
            throw new StatusError("The worker is not running");
        }

        return new Coroutine($this->doShutdown());
    }

    /**
     * {@inheritdoc}
     */
    private function doShutdown(): \Generator {
        $this->shutdown = true;

        // If a task is currently running, wait for it to finish.
        yield Promise\any(\array_map(function (Deferred $deferred): Promise {
            return $deferred->promise();
        }, $this->jobQueue));

        yield $this->context->send(0);
        return yield $this->context->join();
    }

    /**
     * {@inheritdoc}
     */
    public function kill() {
        $this->cancel();
    }

    /**
     * Cancels all pending tasks and kills the context.
     *
     * @param \Throwable|null $exception Optional exception to be used as the previous exception.
     */
    protected function cancel(\Throwable $exception = null) {
        if (!empty($this->jobQueue)) {
            $exception = new WorkerException('Worker was shut down', $exception);

            foreach ($this->jobQueue as $job) {
                $job->fail($exception);
            }

            $this->jobQueue = [];
        }

        $this->context->kill();
    }
}