import { createHmac } from "crypto";

// Base32 (RFC 4648) decode — TOTP secrets are stored/shared in base32.
function base32Decode(input: string): Buffer {
  const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
  const clean = input.toUpperCase().replace(/[^A-Z2-7]/g, "");
  let bits = "";
  for (const char of clean) {
    const val = alphabet.indexOf(char);
    if (val === -1) continue;
    bits += val.toString(2).padStart(5, "0");
  }
  const bytes: number[] = [];
  for (let i = 0; i + 8 <= bits.length; i += 8) {
    bytes.push(parseInt(bits.slice(i, i + 8), 2));
  }
  return Buffer.from(bytes);
}

function hotp(secret: Buffer, counter: number): string {
  const buf = Buffer.alloc(8);
  buf.writeBigInt64BE(BigInt(counter));
  const hmac = createHmac("sha1", secret).update(buf).digest();
  const offset = hmac[hmac.length - 1] & 0xf;
  const code =
    ((hmac[offset] & 0x7f) << 24) |
    ((hmac[offset + 1] & 0xff) << 16) |
    ((hmac[offset + 2] & 0xff) << 8) |
    (hmac[offset + 3] & 0xff);
  return (code % 1_000_000).toString().padStart(6, "0");
}

/**
 * Verifies a 6-digit TOTP code against a base32 secret, allowing ±1 time
 * step (30s each) of clock drift.
 */
export function verifyTotp(base32Secret: string, code: string, stepSeconds = 30): boolean {
  const secret = base32Decode(base32Secret);
  const counter = Math.floor(Date.now() / 1000 / stepSeconds);
  for (const drift of [-1, 0, 1]) {
    if (hotp(secret, counter + drift) === code) return true;
  }
  return false;
}