Reduce a collection of numbers to groups of ranges

numbersToRanges.php

<?php

/**
 * Reduce a collection of numbers to groups of ranges
 *
 * @example dayRange(collect([1,2,3,5,6,10])) = "1 - 3 & 5 - 6 & 10"
 *
 * @param  Collection $days
 * @return string
 */
public function numbersToRanges(Collection $numbers) : string
{
    return $numbers->reduce(function ($groups, $day) {
        if ($groups->last() && $groups->last()->last() === $day - 1) {
            $groups->last()[1] = $day;
        } else {
            $groups->push(collect([$day]));
        }
        return $groups;
    }, collect())->map->implode(' - ')->implode(' & ');
}