type RedirectExtractor = {
  host: string;
  exact?: boolean;
  path: string;
  param: string;
  decode?: 'none' | 'uri' | 'form' | 'b64url';
};

const REDIRECT_EXTRACTORS: RedirectExtractor[] = [
  { host: 'l.facebook.com', exact: true, path: '/l.php', param: 'u', decode: 'uri' },
  { host: 'lm.facebook.com', exact: true, path: '/l.php', param: 'u', decode: 'uri' },
  { host: 'linkedin.com', exact: false, path: '/safety/go', param: 'url', decode: 'uri' },
  { host: 'youtube.com', exact: false, path: '/redirect', param: 'q', decode: 'uri' },
  { host: 'go.bsky.app', exact: true, path: '/redirect', param: 'u', decode: 'uri' },
  { host: 'googleadservices.com', exact: false, path: '/pagead/aclk', param: 'adurl', decode: 'uri' },
  { host: 'target.georiot.com', exact: true, path: '/Proxy.ashx', param: 'GR_URL', decode: 'uri' },
  { host: 'click.linksynergy.com', exact: true, path: '/link', param: 'murl', decode: 'uri' },
];

function decodeParam(value: string, mode: 'none' | 'uri' | 'form' | 'b64url'): string | null {
  try {
    switch (mode) {
      case 'none': return value;
      case 'uri': return decodeURIComponent(value);
      case 'form': return decodeURIComponent(value.replace(/\+/g, ' '));
      case 'b64url':
        return new TextDecoder().decode(
          Uint8Array.from(atob(value.replace(/-/g, '+').replace(/_/g, '/')), (c) => c.charCodeAt(0)),
        );
    }
  } catch { return null; }
}

function isHttp(s: string): boolean { return /^https?:\/\//i.test(s); }

export function unwrapRedirect(url: string): string | null {
  try {
    const u = new URL(url);
    if (u.hostname === 'out.reddit.com') {
      const target = u.searchParams.get('url');
      if (target && isHttp(target)) {
        const d = decodeURIComponent(target);
        if (isHttp(d)) return d;
      }
      return null;
    }
    for (const ex of REDIRECT_EXTRACTORS) {
      if (ex.exact ? u.hostname !== ex.host : (u.hostname !== ex.host && !u.hostname.endsWith('.' + ex.host))) continue;
      if (!u.pathname.startsWith(ex.path)) continue;
      const raw = u.searchParams.get(ex.param);
      if (!raw) continue;
      const d = decodeParam(raw, ex.decode ?? 'uri');
      if (d && isHttp(d)) return d;
    }
    return null;
  } catch { return null; }
}

export function fullyUnwrapRedirect(url: string, maxHops: number = 5): { url: string; hops: string[] } {
  const hops: string[] = [];
  let cur = url;
  for (let i = 0; i < maxHops; i++) {
    const next = unwrapRedirect(cur);
    if (!next || next === cur) break;
    try { hops.push(new URL(cur).hostname); } catch {}
    cur = next;
  }
  return { url: cur, hops };
}