<?php

$array = [1, 2, 3, 4, [], [5, 6], 7, 8, [9], [10, [11, 12], 13, [14, 15, 16, [17, [18, [19, 20, 21]]]]], 22, 23, 24, [25], 26, [27, [28], [29, [30, 31], [32], [33, [34, [35, 36], 37, 38, [39, 40]]]]]];

function laravel_flatten($array, $depth = INF)
{
    return array_reduce($array, function ($result, $item) use ($depth) {
        if (! is_array($item)) {
            return array_merge($result, [$item]);
        } elseif ($depth === 1) {
            return array_merge($result, array_values($item));
        } else {
            return array_merge($result, laravel_flatten($item, $depth - 1));
        }
    }, []);
}

function knapsack_flatten($collection, $levelsToFlatten = -1)
{
    $generatorFactory = function () use ($collection, $levelsToFlatten) {
        $flattenNextLevel = $levelsToFlatten < 0 || $levelsToFlatten > 0;
        $childLevelsToFlatten = $levelsToFlatten > 0 ? $levelsToFlatten - 1 : $levelsToFlatten;
        foreach ($collection as $key => $value) {
            if ($flattenNextLevel && (is_array($value) || $value instanceof Traversable)) {
                foreach (knapsack_flatten($value, $childLevelsToFlatten) as $childKey => $childValue) {
                    yield $childKey => $childValue;
                }
            } else {
                yield $key => $value;
            }
        }
    };

    $collection = $generatorFactory();

    $generatorFactory2 = function() use ($collection) {
        foreach ($collection as $value) {
            yield $value;
        }
    };

    return iterator_to_array($generatorFactory2());
}

$tests = [
    'lstrojny/functional-php' => function ($array) {
        $stack = [$array];
        $result = [];
        while (!empty($stack)) {
            $item = array_shift($stack);
            if (is_array($item)) {
                foreach ($item as $element) {
                    array_unshift($stack, $element);
                }
            } else {
                array_unshift($result, $item);
            }
        }
        return $result;
    },
    'laravel collections' => 'laravel_flatten',
    'dusank/knapsack' => 'knapsack_flatten',
    'recursive_iterator' => function ($array) {
        return iterator_to_array(new RecursiveIteratorIterator(new RecursiveArrayIterator($array)), false);
    },
    'array_walk_recursive' => function ($array) {
        $return = array();
        array_walk_recursive($array, function($a) use (&$return) { $return[] = $a; });

        return $return;
    },
];


foreach ($tests as $suite => $closure) {
    $time = microtime(true);
    for ($i=0;$i<100000;$i++) {
        call_user_func($closure, $array);
    }

    echo $suite.' finished in '.(microtime(true) - $time) . 's'.PHP_EOL;
}

/*
lstrojny/functional-php finished in 3.0966401100159s
laravel collections finished in 2.0682859420776s
dusank/knapsack finished in 4.2100491523743s
recursive_iterator finished in 1.9282908439636s
array_walk_recursive finished in 0.74895691871643s
*/