const toHex = (buffer: ArrayBuffer): string =>
  Array.from(new Uint8Array(buffer))
    .map((b) => b.toString(16).padStart(2, '0'))
    .join('');

export const timingSafeEqualHex = (a: string, b: string): boolean => {
  if (a.length !== b.length) return false;
  let mismatch = 0;
  for (let i = 0; i < a.length; i += 1) {
    mismatch |= a.charCodeAt(i) ^ b.charCodeAt(i);
  }
  return mismatch === 0;
};

export const verifyGithubSignature = async (
  secret: string,
  rawBody: string,
  signatureHeader: string | null,
): Promise<boolean> => {
  if (!signatureHeader || !signatureHeader.startsWith('sha256=')) return false;
  // Web Crypto's importKey throws on a zero-length HMAC key; guard so the
  // verifier never throws on attacker-controlled input (returns false instead).
  if (!secret) return false;
  const provided = signatureHeader.slice('sha256='.length);
  const key = await crypto.subtle.importKey(
    'raw',
    new TextEncoder().encode(secret),
    { name: 'HMAC', hash: 'SHA-256' },
    false,
    ['sign'],
  );
  const mac = await crypto.subtle.sign(
    'HMAC',
    key,
    new TextEncoder().encode(rawBody),
  );
  return timingSafeEqualHex(toHex(mac), provided);
};