// Seat / host PIN locks for offline pass-and-play (hotseat) games.
//
// A PinLock is a salted SHA-256 digest of a 4-digit PIN. This is a SOCIAL lock for a
// shared device — not cryptographic protection. A 4-digit PIN has only 10^4
// combinations and the whole game runs on the client, so a determined peek reads the
// racks straight from memory regardless. The salt only keeps the PIN itself out of a
// casual devtools glance and defeats precomputed tables; it does not make the PIN
// brute-force resistant.

/** PinLock is a stored, salted SHA-256 digest of a PIN (see the file header caveat). */
export interface PinLock {
  hash: string;
  salt: string;
}

// b64url encodes bytes as base64url without padding (matches the idiom in vkid.ts).
function b64url(bytes: Uint8Array): string {
  let s = '';
  for (const b of bytes) s += String.fromCharCode(b);
  return btoa(s).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
}

/** newSalt returns a fresh random base64url salt for a PinLock. */
export function newSalt(): string {
  const a = new Uint8Array(16);
  crypto.getRandomValues(a);
  return b64url(a);
}

/** hashPin returns the base64url SHA-256 digest binding salt to pin. */
export async function hashPin(pin: string, salt: string): Promise<string> {
  const digest = await crypto.subtle.digest('SHA-256', new TextEncoder().encode(`${salt}:${pin}`));
  return b64url(new Uint8Array(digest));
}

/** newLock builds a PinLock for pin with a fresh salt. */
export async function newLock(pin: string): Promise<PinLock> {
  const salt = newSalt();
  return { salt, hash: await hashPin(pin, salt) };
}

/** verifyPin reports whether pin matches the digest stored in lock. */
export async function verifyPin(pin: string, lock: PinLock): Promise<boolean> {
  return (await hashPin(pin, lock.salt)) === lock.hash;
}