import crypto from 'crypto'

// scrypt params: N=16384 gives ~100ms on modern hardware — deliberate slowdown
const SCRYPT_N = 16384
const SCRYPT_R = 8
const SCRYPT_P = 1
const KEY_LEN = 64

/** Hashes a password using scrypt. Returns the hex hash and a random hex salt. */
export async function hashPassword(password: string): Promise<{ hash: string; salt: string }> {
  const salt = crypto.randomBytes(16).toString('hex')
  const hash = await deriveKey(password, salt)
  return { hash, salt }
}

/** Constant-time comparison to prevent timing attacks. */
export async function verifyPassword(
  password: string,
  hash: string,
  salt: string,
): Promise<boolean> {
  const derived = await deriveKey(password, salt)
  const a = Buffer.from(derived, 'hex')
  const b = Buffer.from(hash, 'hex')
  if (a.length !== b.length) return false
  return crypto.timingSafeEqual(a, b)
}

function deriveKey(password: string, salt: string): Promise<string> {
  return new Promise((resolve, reject) => {
    crypto.scrypt(password, salt, KEY_LEN, { N: SCRYPT_N, r: SCRYPT_R, p: SCRYPT_P }, (err, key) => {
      if (err) reject(err)
      else resolve(key.toString('hex'))
    })
  })
}