'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),
    i = 0,
    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;
  var asciiCharsLE = 0,
    asciiCharsBE = 0; // Number of ASCII chars when decoded as LE or BE.

  outer_loop: 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 outer_loop;
        }
      }
    }
  }

  // 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';
}