/**
 * Edge-runtime variant of the signed-session verification used by middleware.
 *
 * Next.js middleware runs on the Edge runtime, where Node's `crypto` module is
 * unavailable, so we verify the HMAC signature with Web Crypto (SubtleCrypto).
 * Keep this in sync with `src/lib/session.ts` (same secret, same algorithm).
 */

function base64url(bytes: ArrayBuffer): string {
  const view = new Uint8Array(bytes);
  let bin = '';
  for (let i = 0; i < view.length; i++) {
    bin += String.fromCharCode(view[i]);
  }
  return btoa(bin).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
}

function timingSafeEqual(a: string, b: string): boolean {
  if (a.length !== b.length) return false;
  let diff = 0;
  for (let i = 0; i < a.length; i++) {
    diff |= a.charCodeAt(i) ^ b.charCodeAt(i);
  }
  return diff === 0;
}

/**
 * Verify a signed cookie value in the Edge runtime.
 * Returns the user id when the signature is valid, otherwise null.
 */
export async function verifySignedUserIdEdge(
  value: string | undefined | null
): Promise<string | null> {
  if (!value) return null;

  const secret = process.env.NEXTAUTH_SECRET;
  if (!secret) return null;

  const separator = value.lastIndexOf('.');
  if (separator <= 0 || separator === value.length - 1) {
    return null;
  }

  const userId = value.slice(0, separator);
  const providedSig = value.slice(separator + 1);

  const key = await crypto.subtle.importKey(
    'raw',
    new TextEncoder().encode(secret),
    { name: 'HMAC', hash: 'SHA-256' },
    false,
    ['sign']
  );
  const signature = await crypto.subtle.sign('HMAC', key, new TextEncoder().encode(userId));
  const expectedSig = base64url(signature);

  if (!timingSafeEqual(providedSig, expectedSig)) {
    return null;
  }

  return userId;
}