We need to calculate the CRC32 value of files to upload to ensure file integrity. We use the standard FileReader which provides an onload method which fires when the data is in memory. In the event handler we calculate the CRC value, then we read the next file and do the same.

In the following test we create a random file, read the file data and create the CRC. But, the engine does not wait until the whole process finished, instead it returns after the random file data has been created.

Like other browsers, we've expected that Chromiom based browsers finish all loops. Online demo: http://www.backload.org/lab/crc32/run.html


Code:

// Lookup table for the polynomial 3988292384 (0xEDB88320)
var crc32Lookup = new Int32Array(256);
(function () {
    var i, j, n;

    for (var i = 0; i < 256; i++) {
        n = i;
        for (var j = 8; j > 0; j--) {
            if ((n & 1) == 1) {
                n = (n >>> 1) ^ 3988292384
            } else {
                n = n >>> 1
            }
        }
        crc32Lookup[i] = n

    }
})();


// Calculate CRC32
function crc32(data) {
    var crc = 0xFFFFFFFF

    for (var i = 0; i < data.length; i++)
        crc = (crc >>> 8) ^ crc32Lookup[data[i] ^ (crc & 0x000000FF)];

    crc = crc = ~crc;
    return crc < 0 ? (0xFFFFFFFF + crc + 1) : crc;
}

// Main test method
function runTest(result) {
    var len = 50000000, // 50MB
        data = new Uint8Array(len),
        data2,
        blob,
        crc;



    // Fill array with random values from 0-255 
    result.timeStart = performance.now();
    for (var i = 0; i < len; i++) {
        data[i] = Math.floor(Math.random() * 256);
    }
    result.timeCreate = performance.now() - result.timeStart;

    // Simulate file data (blob)
    blob = new Blob([data], { type: 'application/octet-binary' });


    // Using a FileReader to read the blob into an ArrayBuffer
    var reader = new FileReader();
    reader.onload = function (e) {
        var data = new Uint8Array(e.target.result, 0, e.target.result.byteLength);

        // Calculate CRC
        result.timeCrc = performance.now();
        result.crc = crc32(data);
        var end = performance.now()


        // Calculate compute time for the current and all iterations
        result.timeCrc = end - result.timeCrc;
        result.timeTotal = end - result.timeStart;
        result.totalCreate += result.timeCreate;
        result.totalCrc += result.timeCrc;
        result.totalTime += result.timeTotal;

        // Print result and next iteration
        result.iter += 1;
        result.printResult();

		reader = null;
        data = null;
        r = null;
        if (result.iter < result.totalIter)
            runTest(result);
    };
    reader.readAsArrayBuffer(blob);
}