DusanKasan / Knapsack

 * @param callable $function ($value, $key)

 * @return Collection

 */

function groupBy($collection, callable $function)

{

    $result = [];

    foreach ($collection as $key => $value) {

        $newKey = $function($value, $key);

        $group = isset($result[$newKey]) ? $result[$newKey] : new Collection([]);

        $result[$newKey] = $group->append($value);

    }

    return Collection::from($result);

}

/**

 * Returns a non-lazy collection of items grouped by the value at given key.

 * @param mixed $key

 * @return Collection

 */

function groupByKey($collection, $groupKey)

{

    $result = [];

    $function = getCallable($groupKey);

    foreach ($collection as $key => $value) {

        $newKey = $function($value, $key);

        $group = isset($result[$newKey]) ? $result[$newKey] : new Collection([]);

        $result[$newKey] = $group->append($value);

    }

    return Collection::from($result);

}

/**

 * Executes $function for each item in $collection