<?php

// the very weird way to calculate factorial ^_^
// we create one thread and synching them with buffered channel
// at fact only one thread is executing at the time 

use parallel\{Channel, Future, Runtime};

for ($n = 0; $n <= 10; $n++) {
    echo "!$n = " . factorial($n) . PHP_EOL;
}

/**
 * Creates $n threads.
 */
function factorial(int $n): int
{
    // buffered channel - using for sync threads ^_^
    $channel = new Channel(1);
    $futureList = [];
    for ($i = 2; $i <= $n; $i++) {
        $runtime = new Runtime();
        $futureList[] = $runtime->run(
            static function (Channel $channel, $multiplier): void {
                $f = $channel->recv();
                $channel->send($f * $multiplier);
            },
            [$channel, $i]
        );
    }

    $channel->send(1);

    // waiting until all threads are done
    do {
        $allDone = array_reduce(
            $futureList,
            function (bool $c, Future $future): bool {

                return $c && $future->done();
            },
            true
        );
    } while (false === $allDone);

    return $channel->recv();
}

// output:
// !0 = 1
// !1 = 1
// !2 = 2
// !3 = 6
// !4 = 24
// !5 = 120
// !6 = 720
// !7 = 5040
// !8 = 40320
// !9 = 362880
// !10 = 3628800