"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';
}