class Create
{

    /**
     * Key of checkout session ID returned by `createCheckoutSession`
     */
    const CHECKOUT_SESSION_ID_KEY = 'checkoutSessionId';

    /**
     * AmazonPay module object
     *
     * @var AmazonPay
     */
    private $module;

    /**
     * Redirect URL for checkout review page
     *
     * @var string
     */
    private $checkoutReviewUrl;

    /**
     * Sets `$module` member var with dependency injection.
     *
     * @author Evan D Shaw <evandanielshaw@gmail.com>
     * @param AmazonPay $module
     * @param string    $checkoutReviewUrl
     * @return void
     */
    public function __construct(AmazonPay $module, $checkoutReviewUrl) {
        $this->module = $module;
        $this->checkoutReviewUrl = $checkoutReviewUrl;
    }

    /**
     * Creates a new checkout session
     *
     * @see https://amazonpaycheckoutintegrationguide.s3.amazonaws.com/amazon-pay-api-v2/checkout-session.html#create-checkout-session
     * @author Evan D Shaw <evandanielshaw@gmail.com>
     * @return array
     * @throws InvalidArgumentException Thrown by `getAmazonClient`.
     */
    public function post() {
        $opts = $this->module->getActingOpts();
        $payload = [
            'webCheckoutDetails' => [
                'checkoutReviewReturnUrl' => $this->checkoutReviewUrl,
            ],
            'platformId' => Constants::PLATFORM_ID,
            'storeId' => $opts['client_id'],
        ];
        $headers = ['x-amz-pay-Idempotency-Key' => uniqid()];

        try {
            $client = $this->module->getAmazonClient();
            // call `generateButtonSignature` so that an error is thrown if the merchant's
            // public/private key pair is not correct
            $client->generateButtonSignature($payload);
            $result = $client->createCheckoutSession($payload, $headers);
            if ($this->module->errors->hasError($result)) {
                return $this->module->errors->getAmzErrorResponse($result);
            }
        } catch (\Exception $e) {
            return $this->module->errors->getErrorResponse(
                GenericErrorStore::AMAZON_PAY_SDK_CLIENT_EXCEPTION,
                [$e->getMessage()]
            );
        }

        return json_decode($result['response'], true)['checkoutSessionId'];
    }
}