"use strict"
var Buffer = require("safer-buffer").Buffer

// Note: UTF16-LE (or UCS2) codec is Node.js native. See encodings/internal.js

// == UTF16-BE codec. ==========================================================

exports.utf16be = Utf16BECodec
function Utf16BECodec () {
}

Utf16BECodec.prototype.encoder = Utf16BEEncoder
Utf16BECodec.prototype.decoder = Utf16BEDecoder
Utf16BECodec.prototype.bomAware = true

// -- Encoding

function Utf16BEEncoder () {
}

Utf16BEEncoder.prototype.write = function (str) {
  var buf = Buffer.from(str, "ucs2")
  for (var i = 0; i < buf.length; i += 2) {
    var tmp = buf[i]; buf[i] = buf[i + 1]; buf[i + 1] = tmp
  }
  return buf
}

Utf16BEEncoder.prototype.end = function () {
}

// -- Decoding

function Utf16BEDecoder () {
  this.overflowByte = -1
}

Utf16BEDecoder.prototype.write = function (buf) {
  if (buf.length == 0) { return "" }

  var buf2 = Buffer.alloc(buf.length + 1)
  var i = 0; var j = 0

  if (this.overflowByte !== -1) {
    buf2[0] = buf[0]
    buf2[1] = this.overflowByte
    i = 1; j = 2
  }

  for (; i < buf.length - 1; i += 2, j += 2) {
    buf2[j] = buf[i + 1]
    buf2[j + 1] = buf[i]
  }

  this.overflowByte = (i == buf.length - 1) ? buf[buf.length - 1] : -1

  return buf2.slice(0, j).toString("ucs2")
}

Utf16BEDecoder.prototype.end = function () {
  this.overflowByte = -1
}

// == UTF-16 codec =============================================================
// Decoder chooses automatically from UTF-16LE and UTF-16BE using BOM and space-based heuristic.
// Defaults to UTF-16LE, as it's prevalent and default in Node.
// http://en.wikipedia.org/wiki/UTF-16 and http://encoding.spec.whatwg.org/#utf-16le
// Decoder default can be changed: iconv.decode(buf, 'utf16', {defaultEncoding: 'utf-16be'});

// Encoder uses UTF-16LE and prepends BOM (which can be overridden with addBOM: false).

exports.utf16 = Utf16Codec
function Utf16Codec (codecOptions, iconv) {
  this.iconv = iconv
}

Utf16Codec.prototype.encoder = Utf16Encoder
Utf16Codec.prototype.decoder = Utf16Decoder

// -- Encoding (pass-through)

function Utf16Encoder (options, codec) {
  options = options || {}
  if (options.addBOM === undefined) { options.addBOM = true }
  this.encoder = codec.iconv.getEncoder("utf-16le", options)
}

Utf16Encoder.prototype.write = function (str) {
  return this.encoder.write(str)
}

Utf16Encoder.prototype.end = function () {
  return this.encoder.end()
}

// -- Decoding

function Utf16Decoder (options, codec) {
  this.decoder = null
  this.initialBufs = []
  this.initialBufsLen = 0

  this.options = options || {}
  this.iconv = codec.iconv
}

Utf16Decoder.prototype.write = function (buf) {
  if (!this.decoder) {
    // Codec is not chosen yet. Accumulate initial bytes.
    this.initialBufs.push(buf)
    this.initialBufsLen += buf.length

    if (this.initialBufsLen < 16) // We need more bytes to use space heuristic (see below)
    { return "" }

    // We have enough bytes -> detect endianness.
    var encoding = detectEncoding(this.initialBufs, this.options.defaultEncoding)
    this.decoder = this.iconv.getDecoder(encoding, this.options)

    var resStr = ""
    for (var i = 0; i < this.initialBufs.length; i++) { resStr += this.decoder.write(this.initialBufs[i]) }

    this.initialBufs.length = this.initialBufsLen = 0
    return resStr
  }

  return this.decoder.write(buf)
}

Utf16Decoder.prototype.end = function () {
  if (!this.decoder) {
    var encoding = detectEncoding(this.initialBufs, this.options.defaultEncoding)
    this.decoder = this.iconv.getDecoder(encoding, this.options)

    var resStr = ""
    for (var i = 0; i < this.initialBufs.length; i++) { resStr += this.decoder.write(this.initialBufs[i]) }

    var trail = this.decoder.end()
    if (trail) { resStr += trail }

    this.initialBufs.length = this.initialBufsLen = 0
    return resStr
  }
  return this.decoder.end()
}

function detectEncoding (bufs, defaultEncoding) {
  var b = []
  var charsProcessed = 0
  // Number of ASCII chars when decoded as LE or BE.
  var asciiCharsLE = 0
  var asciiCharsBE = 0

  outerLoop:
  for (var i = 0; i < bufs.length; i++) {
    var buf = bufs[i]
    for (var j = 0; j < buf.length; j++) {
      b.push(buf[j])
      if (b.length === 2) {
        if (charsProcessed === 0) {
          // Check BOM first.
          if (b[0] === 0xFF && b[1] === 0xFE) return "utf-16le"
          if (b[0] === 0xFE && b[1] === 0xFF) return "utf-16be"
        }

        if (b[0] === 0 && b[1] !== 0) asciiCharsBE++
        if (b[0] !== 0 && b[1] === 0) asciiCharsLE++

        b.length = 0
        charsProcessed++

        if (charsProcessed >= 100) {
          break outerLoop
        }
      }
    }
  }

  // Make decisions.
  // Most of the time, the content has ASCII chars (U+00**), but the opposite (U+**00) is uncommon.
  // So, we count ASCII as if it was LE or BE, and decide from that.
  if (asciiCharsBE > asciiCharsLE) return "utf-16be"
  if (asciiCharsBE < asciiCharsLE) return "utf-16le"

  // Couldn't decide (likely all zeros or not enough data).
  return defaultEncoding || "utf-16le"
}