/**
 * Photo redundancy helpers.
 *
 * SkateSpots uploads images to Blossom, which is CONTENT-ADDRESSED: a blob's URL
 * is `<host>/<sha256>` (optionally with a file extension). That means a mirror of
 * the same file on another known host is derivable from the URL alone — no extra
 * bookkeeping needed — so a dead photo host never has to leave a blank spot.
 *
 * We only derive mirrors for Blossom-style hash paths; anything else is left
 * untouched (an external image URL has no meaningful alternative location).
 */

/** Hosts the app uploads to, and therefore the hosts we can mirror across. */
export const BLOSSOM_MIRROR_HOSTS = ['blossom.skate-spots.org', 'blossom.primal.net', 'blossom.band'];

/** `<64-hex>` or `<64-hex>.<ext>` */
const HASH_RE = /^([0-9a-f]{64})(\.[a-z0-9]+)?$/i;

/**
 * Ordered candidate URLs for an image: the original first, then any explicitly
 * supplied fallbacks, then same-hash mirrors on the other known Blossom hosts.
 */
export function imageCandidates(
  src: string | undefined | null,
  extra: string[] = [],
): string[] {
  const out: string[] = [];
  const add = (url?: string | null) => {
    if (typeof url === 'string' && url && !out.includes(url)) out.push(url);
  };

  add(src);
  for (const url of extra) add(url);
  if (!src) return out;

  try {
    const parsed = new URL(src);
    const last = parsed.pathname.split('/').filter(Boolean).pop() ?? '';
    const match = HASH_RE.exec(last);
    if (match) {
      for (const host of BLOSSOM_MIRROR_HOSTS) {
        if (host === parsed.hostname) continue;
        add(`${parsed.protocol}//${host}/${match[1]}${match[2] ?? ''}`);
      }
    }
  } catch {
    // Relative or malformed URL — nothing to mirror.
  }

  return out;
}