/**
 * Reading feature flags.
 *
 * Server-side only. No flag value reaches the browser bundle, and there is no
 * Unleash dependency in package.json — the SDK lives in FastAPI, which already
 * owns every other piece of data this app renders.
 *
 * Flags are declared in backend/flags.py. A purely front-end flag still has to
 * be declared there; it is a flat data edit, and the return is that one list
 * answers "what flags exist" for the whole system.
 */

export type Flags = Record<string, boolean>;

/*
 * Reading flags pins the calling route to this ISR floor: Next uses the LOWEST
 * revalidate among a route's fetches to set the whole route's revalidation
 * frequency. 300s matches what /school/[slug] already sits at, so a page that
 * reads flags is no more dynamic than a school page already is.
 *
 * It is also what makes a flip propagate without a webhook: five minutes on
 * school pages, an hour on place pages, against flags that flip monthly.
 */
export const FLAGS_REVALIDATE = 300;

const API = process.env.FASTAPI_URL || process.env.NEXT_PUBLIC_API_URL
  || 'http://localhost:8000/api';

/**
 * Every flag and its value. Never throws: an unreadable flag is a dark one.
 *
 * `revalidate` is the caller's, because reading flags pins the whole route to
 * the lowest revalidate among its fetches. Every SEO route here declares
 * 604800; gating one at the 300s default would drop it from a weekly cache to
 * a 5-minute one. Pass the route's own floor and gating costs it nothing.
 *
 * The trade is flag-flip latency: a route that revalidates weekly takes up to
 * a week to notice a flip. Pass a smaller number where a flip must land fast.
 */
export async function getFlags(revalidate: number = FLAGS_REVALIDATE): Promise<Flags> {
  try {
    const res = await fetch(`${API}/flags`, {
      next: { revalidate },
    });
    if (!res.ok) return {};
    return await res.json();
  } catch {
    return {};
  }
}