/**
 * Whether search engines may index a deployment. Only the production site belongs
 * in search results; the testing, staging and branch deployments are copies of it.
 */
export type Indexing = "indexable" | "blocked";

export type RobotsFile = {
  readonly rules: {
    readonly userAgent: string;
    readonly allow?: string;
    readonly disallow: string | readonly string[];
  };
  readonly sitemap?: string;
};

export type RobotsMeta = {
  readonly index: boolean;
  readonly follow: boolean;
  readonly googleBot?: {
    readonly index: boolean;
    readonly follow: boolean;
    readonly "max-snippet": number;
    readonly "max-image-preview": "large";
    readonly "max-video-preview": number;
  };
};

const BLOCKED_TARGETS: readonly unknown[] = ["testing", "staging", "branch"];

/** Paths that proxy analytics and error reports; nothing there is a page. */
const PROXY_PATHS: readonly string[] = ["/stats/", "/errors/"];

/**
 * Only the deployments known to be copies are blocked. Anything else stays
 * indexable, so a production build that lost its target does not drop out of
 * the search index.
 */
export function indexingFor(target: unknown): Indexing {
  return BLOCKED_TARGETS.includes(target) ? "blocked" : "indexable";
}

export function robotsFile(indexing: Indexing, siteUrl: string): RobotsFile {
  if (indexing === "blocked") {
    return { rules: { userAgent: "*", disallow: "/" } };
  }
  return {
    rules: { userAgent: "*", allow: "/", disallow: PROXY_PATHS },
    sitemap: `${siteUrl}/sitemap.xml`,
  };
}

/** The robots meta tag. Google gets no limit on snippet length or image preview size. */
export function robotsMeta(indexing: Indexing): RobotsMeta {
  if (indexing === "blocked") return { index: false, follow: false };
  return {
    index: true,
    follow: true,
    googleBot: {
      index: true,
      follow: true,
      "max-snippet": -1,
      "max-image-preview": "large",
      "max-video-preview": -1,
    },
  };
}