decoder.js

'use strict';

/*
    Decode functions adapted from:
    Version 1.0 12/25/99 Copyright (C) 1999 Masanao Izumo <iz@onicos.co.jp>
    http://www.onicos.com/staff/iz/amuse/javascript/expert/base64.txt
*/

const Stream = require('stream');


const internals = {
    decodeChars: [
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
        52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1,
        -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
        15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,
        -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
        41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1
    ]
};


exports.decode = function (buffer) {

    const decodeChars = internals.decodeChars;
    const len = buffer.length;
    const allocated = Math.ceil(len / 4) * 3;
    const result = Buffer.alloc(allocated);

    let c1;
    let c2;
    let c3;
    let c4;
    let j = 0;

    for (let i = 0; i < len; ) {
        do {
            c1 = decodeChars[buffer[i++] & 0xff];
        }
        while (i < len && c1 === -1);

        if (c1 === -1) {
            break;
        }

        do {
            c2 = decodeChars[buffer[i++] & 0xff];
        }
        while (i < len && c2 === -1);

        if (c2 === -1) {
            break;
        }

        result[j++] = (c1 << 2) | ((c2 & 0x30) >> 4);

        do {
            c3 = buffer[i++] & 0xff;
            if (c3 === 61) {                        // =
                return result.slice(0, j);
            }

            c3 = decodeChars[c3];
        }
        while (i < len && c3 === -1);

        if (c3 === -1) {
            break;
        }

        result[j++] = ((c2 & 0x0f) << 4) | ((c3 & 0x3c) >> 2);

        do {
            c4 = buffer[i++] & 0xff;
            if (c4 === 61) {                        // =
                return result.slice(0, j);
            }

            c4 = decodeChars[c4];
        }
        while (i < len && c4 === -1);

        if (c4 !== -1) {
            result[j++] = ((c3 & 0x03) << 6) | c4;
        }
    }

    return (j === allocated ? result : result.slice(0, j));
};


exports.Decoder = class Decoder extends Stream.Transform {
    constructor() {

        super();
        this._reminder = null;
    }

    _transform(chunk, encoding, callback) {

        let part = this._reminder ? Buffer.concat([this._reminder, chunk]) : chunk;
        const remaining = part.length % 4;
        if (remaining) {
            this._reminder = part.slice(part.length - remaining);
            part = part.slice(0, part.length - remaining);
        }
        else {
            this._reminder = null;
        }

        this.push(exports.decode(part));
        return callback();
    }

    _flush(callback) {

        if (this._reminder) {
            this.push(exports.decode(this._reminder));
        }

        return callback();
    }
};