Langweiliges Zeug. Was sonst.

ArrayBuffer <-> Blob <-> BinaryString conversion

You can speed up your javascript encryption stuff by using ArrayBuffers. However, if you want to convert it back to a binary string for other API calls, it’s really slow. At least if you do it by hand using something like

for(var i=0; i< view.length; ++i)
    str += String.fromCharCode(view[i]);

Of course you can somewhat improve it by using a stringbuilder approach, like

function convertFaster(view)
{
    var array = new Array(view.length);
    for(var i=0; i< view.length; ++i)
        array[i] = String.fromCharCode(view[i]);
    return array.join('');
}

However, there is a much faster way using the FileReader API. Although its name implies only usage for files, it takes a Blob as an argument. Since every File is also a Blob, this makes it possible to read Files. But this also means, we can feed it with custom blobs. To make use of this we can convert any ArrayBuffer to a blob by using the brand new BlobBuilder

var buffer = new ArrayBuffer(3)
var array = new Uint8Array(buffer);
var i=0;
array[i++] = 0x48;
array[i++] = 0x69;
array[i++] = 0x21;

var blobBuilder = new BlobBuilder();
blobBuilder.append(buffer);
buffer = null;
var blob = blobBuilder.getBlob();
var fileReader =new FileReader();
fileReader.onload = function (e) {
	//conversion done
};
fileReader.readAsBinaryString(blob);

To convert a binary string to a blob we can again use the BlobBuilder. Append also accept DOMStrings as argument.

Last but not least a nice performance comparison (ArrayBuffer to BinaryString): click me.

Some results with 5000000 bytes on a Core2Duo 6300:

Firefox 9.0.1: 884ms vs 89ms
Chromium 14.0.835.202: 1886ms vs 101ms

Conclusion: Speedup between 10x and 20x :)

stringToBytes JavaScript Performance

Searching for a simple function to convert javascript strings to byte arrays, I stumbled over this stackoverflow answer.

While it’s implementation is correct, it has a significant performance problem. For large strings, the internal byte array is recreated too often. It’s somewhat similar to using the += operator on strings in java instead of using a StringBuilder. A faster function is below.

function stringToBytesFaster ( str ) { 
    var ch, st, re = [], j=0;
    for (var i = 0; i < str.length; i++ ) { 
        ch = str.charCodeAt(i);
        if(ch < 127)
        {
            re[j++] = ch & 0xFF;
        }
        else
        {
            st = [];    // clear stack
            do {
                st.push( ch & 0xFF );  // push byte to stack
                ch = ch >> 8;          // shift value down by 1 byte
            }
            while ( ch );
            // add stack contents to result
            // done because chars have "wrong" endianness
            st = st.reverse();
            for(var k=0;k<st.length; ++k)
                re[j++] = st[k];
        }
    }   
    // return an array of bytes
    return re; 
}

To round up things, you can start a performance test here: clickme

Example output on Firefox 8, core2duo and strlength = 10000:
Old: 2021ms
New: 4ms

Speedup factor:505.25