/**
 * Checkout service — the only place the frontend talks to Stripe / the payment
 * API. The UI calls these functions; it never handles card data.
 *
 * `startCheckout` follows Stripe's recommended hosted-checkout flow:
 *   1. Ask our backend to create a Checkout Session (amount resolved server-side).
 *   2. Redirect the browser to the Stripe-hosted `session.url`.
 *   3. Stripe handles payment, then redirects back to success_url / cancel_url.
 *
 * A Stripe.js `redirectToCheckout({ sessionId })` fallback (which uses the
 * publishable key) is kept for older API behavior, but `session.url` is the
 * current default and avoids loading Stripe.js for the redirect.
 */

import { loadStripe } from '@stripe/stripe-js';
import { stripeConfig } from '../config/stripeConfig';

let stripePromise;
const getStripe = () => {
    if (!stripePromise) stripePromise = loadStripe(stripeConfig.publishableKey);
    return stripePromise;
};

/**
 * Create a session for an invoice and redirect to Stripe's hosted page.
 * Throws (with a friendly message) if the backend isn't configured/reachable.
 */
export async function startCheckout({ invoiceId, customerEmail, fallback } = {}) {
    if (!stripeConfig.isConfigured) {
        const err = new Error(
            'Stripe is not configured yet. Add your publishable key to enable payments.'
        );
        err.code = 'stripe_not_configured';
        throw err;
    }

    const res = await fetch(`${stripeConfig.apiBase}/create-checkout-session`, {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        // `fallback` carries the invoice's amount/title for demo invoices the
        // owner created in localStorage (the server's static map won't have them).
        body: JSON.stringify({ invoiceId, customerEmail, fallback }),
    });

    const data = await res.json().catch(() => ({}));
    if (!res.ok) {
        throw new Error(data.error || 'Could not start checkout. Please try again.');
    }

    // Preferred: redirect straight to the Stripe-hosted Checkout URL.
    if (data.url) {
        window.location.assign(data.url);
        return;
    }

    // Fallback: Stripe.js redirect using the session id + publishable key.
    const stripe = await getStripe();
    if (!stripe) throw new Error('Unable to initialize Stripe.');
    const { error } = await stripe.redirectToCheckout({ sessionId: data.id });
    if (error) throw new Error(error.message);
}

/**
 * Fetch the verified status of a completed Checkout Session (used by the
 * success return page). Returns the trimmed status object from the backend.
 */
export async function getSessionStatus(sessionId) {
    const res = await fetch(
        `${stripeConfig.apiBase}/session-status?session_id=${encodeURIComponent(sessionId)}`
    );
    const data = await res.json().catch(() => ({}));
    if (!res.ok) {
        throw new Error(data.error || 'Could not verify payment status.');
    }
    return data;
}