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Filename : utf16.js
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"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() {
}


// == 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.initialBytes = [];
    this.initialBytesLen = 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.initialBytes.push(buf);
        this.initialBytesLen += buf.length;
        
        if (this.initialBytesLen < 16) // We need more bytes to use space heuristic (see below)
            return '';

        // We have enough bytes -> detect endianness.
        var buf = Buffer.concat(this.initialBytes),
            encoding = detectEncoding(buf, this.options.defaultEncoding);
        this.decoder = this.iconv.getDecoder(encoding, this.options);
        this.initialBytes.length = this.initialBytesLen = 0;
    }

    return this.decoder.write(buf);
}

Utf16Decoder.prototype.end = function() {
    if (!this.decoder) {
        var buf = Buffer.concat(this.initialBytes),
            encoding = detectEncoding(buf, this.options.defaultEncoding);
        this.decoder = this.iconv.getDecoder(encoding, this.options);

        var res = this.decoder.write(buf),
            trail = this.decoder.end();

        return trail ? (res + trail) : res;
    }
    return this.decoder.end();
}

function detectEncoding(buf, defaultEncoding) {
    var enc = defaultEncoding || 'utf-16le';

    if (buf.length >= 2) {
        // Check BOM.
        if (buf[0] == 0xFE && buf[1] == 0xFF) // UTF-16BE BOM
            enc = 'utf-16be';
        else if (buf[0] == 0xFF && buf[1] == 0xFE) // UTF-16LE BOM
            enc = 'utf-16le';
        else {
            // No BOM found. Try to deduce encoding from initial content.
            // 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.
            var asciiCharsLE = 0, asciiCharsBE = 0, // Counts of chars in both positions
                _len = Math.min(buf.length - (buf.length % 2), 64); // Len is always even.

            for (var i = 0; i < _len; i += 2) {
                if (buf[i] === 0 && buf[i+1] !== 0) asciiCharsBE++;
                if (buf[i] !== 0 && buf[i+1] === 0) asciiCharsLE++;
            }

            if (asciiCharsBE > asciiCharsLE)
                enc = 'utf-16be';
            else if (asciiCharsBE < asciiCharsLE)
                enc = 'utf-16le';
        }
    }

    return enc;
}