Question

I’ve got a function that is taking on average 250ms to complete. I would like to do this in much less time, <20ms if I can <10ms would be best.

function updateDisplay() {
    var start = new Date().getTime();
    var $this = $(this);
    var data = $this.data('ansi');
    var html = '';
    for (var i = 0; i < data.screen.length; i++) {
        for (var j = 0; j < data.screen[i].length; j++) {
            html += data.screen[i][j];
        }
        html += '<br />';
    }
    var create = new Date().getTime();
    console.log('Build html: ' + (create-start));
    $this.html(html);
    var end = new Date().getTime();
    console.log('Update html: ' +(end-create));
}

I’m calling this function in side a setInterval to update my display, building the html string is 0ms to 1ms each frame, but the update html is anywhere from 100ms to 300ms. Is there anyway to get this to be faster?

Bah, having chrome inspector open to watch the console was adding a huge delay
This is my current function (Basically a drop if from CD Sanchez). Without the inspector open It’s running at about 50ms for the update html. This is much better, but would like to get it to <20ms if I can.

function updateDisplay() {
    var start = new Date().getTime();
    var $this = $(this);
    var data = $this.data('ansi');
    var html = Array();
    for (var i = 0, length1 = data.screen.length; i < length1; ++i) {
        var a = data.screen[i]; // cache object
        for (var j = 0, length2 = a.length; j < length2; ++j) {
            html.push(a[j]); // push to array
        }
        html.push('<br />');
    }
    var create = new Date().getTime();
    this.innerHTML = html.join(''); // use innerHTML
    var end = new Date().getTime();
    $('#debug').html('Build html: ' + (create-start) + '<br/>Update html: ' + (end-create));
}

Sample value of html – 1st row, up to the <br>

<span style="background-color:#000000;color:#ffffff;">&nbsp;</span><span style="background-color:#000000;color:#ffffff;">&nbsp;</span><span style="background-color:#000000;color:#ffffff;">&nbsp;</span><span style="background-color:#000000;color:#ffffff;">&nbsp;</span><span style="background-color:#000000;color:#ffffff;">&nbsp;</span><span style="background-color:#000000;color:#ffffff;">&nbsp;</span><span style="background-color:#000000;color:#ffffff;">┌</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">─</span><span style="background-color:#000000;color:#ffffff;">┐</span><span style="background-color:#000000;color:#ffffff;">&nbsp;</span><span style="background-color:#000000;color:#ffffff;">&nbsp;</span><span style="background-color:#000000;color:#ffffff;">&nbsp;</span><span style="background-color:#000000;color:#ffffff;">&nbsp;</span><span style="background-color:#000000;color:#ffffff;">&nbsp;</span><span style="background-color:#000000;color:#ffffff;">&nbsp;</span><span style="background-color:#000000;color:#ffffff;">&nbsp;</span><br>

Example – here, only tested in chrome so far. Known knot to work in IE yet…

Update
I’ve converted my code to use a canvas and draw directly to that. I’m not sure if I’m doing it the best way or not as its my first time using the canvas. As it stands now I’m at around 20ms. Thats the upper end of where I’m happy, 10ms would be much better though.

I’m not sure if I can define a foreground and background color in the style and remove the fillRect call or not, that would speed things up a lot if I could. The other issue is the font doesn’t look as crisp as it did as pure html, not sure if I can fix that or not. The example linked above has been updated.

function updateDisplay() {
    var start = new Date().getTime();
    var $this = $(this);
    var data = $this.data('ansi');
    for (var i = 0, length1 = data.screen.length; i < length1; ++i) {
        var a = data.screen[i]; // cache object
        for (var j = 0, length2 = a.length; j < length2; ++j) {
            data.ctx.fillStyle = a[j][0];
            data.ctx.fillRect (8*j, 14 * i, 8, 14);
            data.ctx.fillStyle = a[j][1];
            data.ctx.fillText(a[j][2], 8*j, 14 * i);
        }
    }
    var end = new Date().getTime();
    $('#debug').html('Frame Time: ' + (end-start));
}

Last Update
ctx.fillText is quite slow and not accurate enough for my purposes. I’ve defined my own font as a 8×16 array and draw each pixel with a ctx.fillRect. This is much much faster and dealing with the font subsystem it seams.

Answer 1

Here are some very small optimizations that I doubt will help much, but here you go anyways:

function updateDisplay() {
    var start = new Date().getTime();
    var $this = $(this);
    var data = $this.data('ansi');
    var html = [];
    for (var i = 0, length1 = data.screen.length; i < length; ++i) {
        var a = data.screen[i]; // cache object
        for (var j = 0, length2 = a.length; j < length2; ++j) {
            html.push(a[j]); // push to array
        }
        html.push('<br />');
    }
    var create = new Date().getTime();
    console.log('Build html: ' + (create-start));
    this.innerHTML = html.join(''); // use innerHTML
    var end = new Date().getTime();
    console.log('Update html: ' +(end-create));
}

Of course, these are just simple JavaScript optimizations (which aren’t really that useful on the newer browsers). It sounds like you need to simplify your HTML and possibly your CSS so that it can be rendered faster by the HTML engine.