var createHmac = require('create-hmac')
var MAX_ALLOC = Math.pow(2, 30) - 1 // default in iojs

exports.pbkdf2 = pbkdf2
function pbkdf2 (password, salt, iterations, keylen, digest, callback) {
  if (typeof digest === 'function') {
    callback = digest
    digest = undefined
  }

  if (typeof callback !== 'function') {
    throw new Error('No callback provided to pbkdf2')
  }

  var result = pbkdf2Sync(password, salt, iterations, keylen, digest)
  setTimeout(function () {
    callback(undefined, result)
  })
}

exports.pbkdf2Sync = pbkdf2Sync
function pbkdf2Sync (password, salt, iterations, keylen, digest) {
  if (typeof iterations !== 'number') {
    throw new TypeError('Iterations not a number')
  }

  if (iterations < 0) {
    throw new TypeError('Bad iterations')
  }

  if (typeof keylen !== 'number') {
    throw new TypeError('Key length not a number')
  }

  if (keylen < 0 || keylen > MAX_ALLOC) {
    throw new TypeError('Bad key length')
  }

  digest = digest || 'sha1'

  if (!Buffer.isBuffer(password)) password = new Buffer(password, 'binary')
  if (!Buffer.isBuffer(salt)) salt = new Buffer(salt, 'binary')

  var hLen
  var l = 1
  var DK = new Buffer(keylen)
  var block1 = new Buffer(salt.length + 4)
  salt.copy(block1, 0, 0, salt.length)

  var r
  var T

  for (var i = 1; i <= l; i++) {
    block1.writeUInt32BE(i, salt.length)
    var U = createHmac(digest, password).update(block1).digest()

    if (!hLen) {
      hLen = U.length
      T = new Buffer(hLen)
      l = Math.ceil(keylen / hLen)
      r = keylen - (l - 1) * hLen
    }

    U.copy(T, 0, 0, hLen)

    for (var j = 1; j < iterations; j++) {
      U = createHmac(digest, password).update(U).digest()

      for (var k = 0; k < hLen; k++) {
        T[k] ^= U[k]
      }
    }

    var destPos = (i - 1) * hLen
    var len = (i === l ? r : hLen)
    T.copy(DK, destPos, 0, len)
  }

  return DK
}