Algorithms in PHP: Deques(withinboredom.info)

    return ($this->head === 0 && $this->tail === $this->size - 1) || $this->head === $this->tail + 1;

     $return = $this->next?->resetPrevious();

    $this->tail =
            $this->tail?->setNext(new Node($this->tail, null, $value))->next
            ?? $this->head = new Node(null, null, $value);

    elseif ($this->tail === $this->size - 1) {
            $this->tail = 0;just a tiny
        }