// Invite-link fragment codec (see docs/onboarding-and-home-server.md).
// New links carry a versioned fingerprint — /join/join#e1.<T> or #e1.<T>~<name> —
// so clients (web paste-to-join, the iOS and Mac apps) can recognise an Eleven
// invite deterministically and offline, and future format changes can bump the
// version. Old unprefixed #<T> links parse identically. Pure and framework-free
// so it's unit-testable in Node (test/invite.test.mjs).

export const INVITE_PREFIX = "e1.";

// The branding key's shape (docs/no-plaintext.md): lowercase base32, minted
// 10 chars, tolerated 8–32 so the length can be dialed without a format bump.
// Used to disambiguate the third fragment slot from legacy names containing a
// raw "~" (encodeURIComponent leaves "~" unescaped; new builders escape it).
const BRANDING_KEY_RE = /^[a-z2-7]{8,32}$/;

// parseInviteFragment("e1.<T>~<name>~<b>") -> { T, name, b }, name URL-decoded
// ("" if absent or undecodable), b the branding key when the link carries one
// (sealed instances put it in every minted link). Accepts unprefixed legacy
// fragments and the older 1- and 2-slot forms; slot order is fixed (T first)
// so old parsers keep claiming from new links.
export function parseInviteFragment(frag) {
  let f = frag || "";
  if (f.startsWith(INVITE_PREFIX)) f = f.slice(INVITE_PREFIX.length);
  const parts = f.split("~");
  const T = parts[0];
  let name = parts.slice(1).join("~");
  let b = "";
  const last = parts[parts.length - 1];
  if (parts.length >= 3 && BRANDING_KEY_RE.test(last)) {
    b = last;
    name = parts.slice(1, -1).join("~");
  }
  return { T, name: safeDecode(name), b };
}

// buildInviteFragment(T, name?, b?) -> "e1.<T>", "e1.<T>~<encoded name>", or
// "e1.<T>~<encoded name>~<b>" (name slot kept, possibly empty, whenever b is
// present). All minters use this, so every new link carries the fingerprint —
// and, on a sealed instance, the branding key.
export function buildInviteFragment(T, name = "", b = "") {
  let f = INVITE_PREFIX + T;
  if (name || b) f += "~" + encodeURIComponent(name).replace(/~/g, "%7E");
  if (b) f += "~" + b;
  return f;
}

// True when the fragment carries the versioned fingerprint — the deterministic
// "this is an Eleven invite" signal (an arbitrary URL with /join/ in it isn't).
export function hasInviteFingerprint(frag) {
  return (frag || "").startsWith(INVITE_PREFIX);
}

function safeDecode(s) {
  try {
    return decodeURIComponent(s);
  } catch {
    return "";
  }
}