Read data directly from base64 without decoding

1Jul/110

Read data directly from base64 without decoding

So here's the catch. You have a binary input you want to mess with in your browser for whatever reason. First problem you encounter is when you load the file. In IE you will have trouble reading a binary string because it uses null terminated strings, and your file will end at the first 0x00 byte found. If you manage to overcome that but trust me it is very CPU intensive. See the source of Read/Load files from ZIP in Javascript post. You will have to create a vbScript that will convert the received AJAX response into an array with decimal values for each byte in the file. Depending on your file size, it will take accordingly (10+ seconds * file KB). For every other browser it works just fine with plain strings.

Okay, one other potential issue may come if you need to receive the data into a JSON string or XML or whatever beside a plain binary file. Then you have to use BASE64 encoding and decode your data into a variable but must keep in mind the first issue with the null characters. My solution to this problem is to create a method to read directly from the BASE64 encoded string at arbitrary locations without decoding the entire thing.

Here's an example of a binary file:

Offset(h)00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
 
00000000 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F     ...............
00000010 19 84 74 68 69 73 20 69 73 20 61 20 74 65 73 74     ..this is a test

You have 32 bytes in this file, with BASE64 encoding you get a 46 chars length string:

   AAECAwQFBgcICQoLDA0ODxmEdGhpcyBpcyBhIHRlc3Q=

A usage example:

    var data = new Base64Native("AAECAwQFBgcICQoLDA0ODxmEdGhpcyBpcyBhIHRlc3Q=");
    console.log(data.length); // outputs 32;
    console.log(data.readBoolean()); // outputs false; - reads a byte and casts it to a boolean value
    console.log(data.readByte()); // outputs 1;
    console.log(data.readWord()); // outputs 515; - reads a 16 bit number
    console.log(data.readInt()); // outputs 67438087; - reads a 32 bit number
    console.log(data.seek(28)); // moves the pointer to byte 28 (0x1C)
    console.log(data.readString(data.bytesAvailable())); // outputs 'test'; reads a string from position 28 to the end of the file

And here's the class

var Base64Native = function(data) {
    var keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=",
        length = data.length - (data.length / 4) - (data.split(/=/g).length - 1),
        ptr = 0;
 
    function keyIdx(idx) {
        return keyStr.indexOf(data.charAt(idx));
    }
 
    function readByte() {
        if (!(length - ptr)) { throw Error('End of stream!'); }
        var blockIndex = Math.floor(ptr / 3) * 4, subIndex = ptr++ % 3, ptrChr = blockIndex + subIndex;
        return !subIndex && ((keyIdx(ptrChr) < < 2) | (keyIdx(ptrChr + 1) >> 4)) || subIndex === 1 &&
               (((keyIdx(ptrChr) & 15) < < 4) | (keyIdx(ptrChr + 1) >> 2)) || ((keyIdx(ptrChr) & 3)  < < 6) |  keyIdx(ptrChr + 1);
    }
 
    return {
        /* Returns the available bytes left in the file to read */
        bytesAvailable : function() {
            return length - ptr;
        },
        /* Reads and returns a 8 bit number and increments the position of the index accordingly */
        readByte : function() {
            return readByte();
        },
        /* Reads and returns a 32 bit number and increments the position of the index accordingly */
        readInt : function() {
            return (readByte() << 24) | (readByte() << 16) | (readByte() << 8) | readByte();
        },
        /* Reads 1 byte and returns the boolean value of it and increments the position of the index accordingly */
        readBoolean : function() {
            return Boolean(readByte());
        },
        /* Reads and returns a 16 bit number and increments the position of the index accordingly */
        readWord : function() {
            return readByte() << 8 | readByte()
        },
        /* Reads and returns a string of length 'len' and increments the position of the index accordingly */
        readString : function(/*String*/len) {
            var data = '';
            for (var i = 0; i < len; i++) {
                data += String.fromCharCode(readByte());
            }
            return data;
        },
        /* Moves the index into the file at location newpos */
        seek : function(newpos) {
            ptr = newpos;
        },
        /* Returns the position of the index in the file */
        position : function() {
            return ptr;
        },
        /* The length of the data */
        length : length
    };
};

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Read data directly from base64 without decoding

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