cartitems.php

<?php

error_reporting(E_ALL);

interface CartItemInterface
{
    public function getPrice(float $from = 0.1, float $to = 100.0): float;
}

class CartItem implements CartItemInterface
{
    public function getPrice(float $from = 0.1, float $to = 100.0): float
    {
        return round((($to - $from) * (mt_rand(0, 100) / 100) + $from), 2);
    }
}

class Discount implements CartItemInterface
{
    public function getPrice(float $from = 0.1, float $to = 100.0): float
    {
        return 6.99;
    }
}

class DiscountCalculatorService
{
    private const CART_LIMIT = 1.0;

    public function addDiscount(CartItemCollection $cartItems): CartItemCollection
    {
        if (count($cartItems) === 0) {
            return $cartItems;
        }
        $total = 0.0;
        foreach ($cartItems as $cartItem) {
            $total += $cartItem->getPrice();
        }

        if ($total > self::CART_LIMIT) {
            $cartItems[] = new Discount();
        }
        return $cartItems;
    }
}
interface Collection extends Iterator, Countable, ArrayAccess, Serializable, Stringable{}
trait CollectionTrait{
    private int $position = 0;
    private array $items =[];
    public function serialize()
    {
        return serialize($this->items);
    }

    public function unserialize(string $data)
    {
        $this->items = unserialize($data);
    }

    public function __serialize(): array
    {
        return $this->items;
    }

    public function __unserialize(array $data): void
    {
        $this->items = $data;
    }

    public function __toString(): string
    {
        return serialize($this);
    }


    public function offsetExists(mixed $offset): bool
    {
        return isset($this->items[$offset]);
    }


    public function offsetUnset(mixed $offset): void
    {
        unset($this->items[$offset]);
    }


    public function count(): int
    {
        return count($this->items);
    }

    public function current(): mixed
    {
        return $this->items[$this->position];
    }

    public function next(): void
    {
        ++$this->position;
    }

    public function key(): mixed
    {
        return $this->position;
    }

    public function valid(): bool
    {
        return isset($this->items[$this->position]);
    }

    public function rewind(): void
    {
        $this->position = 0;
    }

}
class CartItemCollection implements Collection
{
    use CollectionTrait;
    public function __construct(CartItemInterface... $items)
    {
        $this->items = $items;
    }

    public function offsetGet(mixed $offset): CartItemInterface
    {
        return $this->items[$offset];
    }

    public function offsetSet(mixed $offset, mixed $value): void
    {
        if(!$value instanceof CartItemInterface){
            throw new InvalidArgumentException(sprintf("Unexpected Value type, expected %s",CartItemInterface::class));
        }
        if($offset === null){
            $offset = $this->position;
        }
        $this->items[$offset] = $value;
    }

}


$cartItems = [
    new CartItem(),
    new CartItem(),
];
$collection = new CartItemCollection(...$cartItems);
$calculator = new DiscountCalculatorService();
$newCartItems = $calculator->addDiscount($collection);

var_dump($newCartItems);