chadtmiller.com/pds.js

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/**
 * Candidate authority domains for an NSID, most specific first.
 *
 * Per AT Protocol spec the authority is every segment but the last, reversed
 * into a domain — app.bsky.feed.post is published at feed.bsky.app. Some
 * authorities only publish at the two-segment domain instead (and some serve a
 * wildcard that answers either), so that shorter form follows as a fallback.
 * Both forms are needed in practice: com.atproto.lexicon.schema resolves only
 * at lexicon.atproto.com, while social.grain.gallery answers at grain.social.
 *
 * @param {string} nsid - e.g., "app.bsky.feed.post"
 * @returns {string[]} - e.g., ["feed.bsky.app", "bsky.app"]
 */
export function nsidToDomains(nsid) {
  const parts = nsid.split('.');
  const domains = [
    parts.slice(0, -1).reverse().join('.'),
    parts.slice(0, 2).reverse().join('.'),
  ];
  return [...new Set(domains.filter(Boolean))];
}

/**
 * The spec-form authority domain for an NSID: every segment but the last,
 * reversed. See nsidToDomains for the fallback an authority may also publish at.
 *
 * @param {string} nsid - e.g., "app.bsky.feed.post"
 * @returns {string} - e.g., "feed.bsky.app"
 */
export function nsidToDomain(nsid) {
  return nsidToDomains(nsid)[0];
}

/**
 * Convert domain to AT Protocol lexicon DNS lookup name.
 * Uses _lexicon subdomain per AT Protocol spec for lexicon authority resolution.
 *
 * @param {string} domain - e.g., "bsky.app"
 * @returns {string} - e.g., "_lexicon.bsky.app"
 */
export function domainToLexiconDns(domain) {
  return `_lexicon.${domain}`;
}