var zeros = new Buffer(16)
zeros.fill(0)
module.exports = GHASH
function GHASH (key) {
  this.h = key
  this.state = new Buffer(16)
  this.state.fill(0)
  this.cache = new Buffer('')
}
// from http://bitwiseshiftleft.github.io/sjcl/doc/symbols/src/core_gcm.js.html
// by Juho Vähä-Herttua
GHASH.prototype.ghash = function (block) {
  var i = -1
  while (++i < block.length) {
    this.state[i] ^= block[i]
  }
  this._multiply()
}

GHASH.prototype._multiply = function () {
  var Vi = toArray(this.h)
  var Zi = [0, 0, 0, 0]
  var j, xi, lsb_Vi
  var i = -1
  while (++i < 128) {
    xi = (this.state[~~(i / 8)] & (1 << (7 - i % 8))) !== 0
    if (xi) {
      // Z_i+1 = Z_i ^ V_i
      Zi = xor(Zi, Vi)
    }

    // Store the value of LSB(V_i)
    lsb_Vi = (Vi[3] & 1) !== 0

    // V_i+1 = V_i >> 1
    for (j = 3; j > 0; j--) {
      Vi[j] = (Vi[j] >>> 1) | ((Vi[j - 1] & 1) << 31)
    }
    Vi[0] = Vi[0] >>> 1

    // If LSB(V_i) is 1, V_i+1 = (V_i >> 1) ^ R
    if (lsb_Vi) {
      Vi[0] = Vi[0] ^ (0xe1 << 24)
    }
  }
  this.state = fromArray(Zi)
}
GHASH.prototype.update = function (buf) {
  this.cache = Buffer.concat([this.cache, buf])
  var chunk
  while (this.cache.length >= 16) {
    chunk = this.cache.slice(0, 16)
    this.cache = this.cache.slice(16)
    this.ghash(chunk)
  }
}
GHASH.prototype.final = function (abl, bl) {
  if (this.cache.length) {
    this.ghash(Buffer.concat([this.cache, zeros], 16))
  }
  this.ghash(fromArray([
    0, abl,
    0, bl
  ]))
  return this.state
}

function toArray (buf) {
  return [
    buf.readUInt32BE(0),
    buf.readUInt32BE(4),
    buf.readUInt32BE(8),
    buf.readUInt32BE(12)
  ]
}
function fromArray (out) {
  out = out.map(fixup_uint32)
  var buf = new Buffer(16)
  buf.writeUInt32BE(out[0], 0)
  buf.writeUInt32BE(out[1], 4)
  buf.writeUInt32BE(out[2], 8)
  buf.writeUInt32BE(out[3], 12)
  return buf
}
var uint_max = Math.pow(2, 32)
function fixup_uint32 (x) {
  var ret, x_pos
  ret = x > uint_max || x < 0 ? (x_pos = Math.abs(x) % uint_max, x < 0 ? uint_max - x_pos : x_pos) : x
  return ret
}
function xor (a, b) {
  return [
    a[0] ^ b[0],
    a[1] ^ b[1],
    a[2] ^ b[2],
    a[3] ^ b[3]
  ]
}