favidenticon.user.js

// ==UserScript==
// @name           favidenticon
// @namespace      http://oq.la/
// @include        *
// @require        http://github.com/hgwr/identicon/raw/master/identicon.js
// ==/UserScript==
(function() {
    var icon_1 = document.evaluate('//head/link[@rel="shortcut icon"]', document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
    var icon_2 = document.evaluate('//head/link[@rel="icon"]', document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
    if ( icon_1 || icon_2 ) {
        return;
    }else{
        var req = new XMLHttpRequest();  
        req.open('GET', location.protocol + "//" + location.host + "/favicon.ico", false);   
        req.send(null);
        if(req.status>=400)
            insertFavidenticon();
    }

    function insertFavidenticon(){
        var size = 32;
        var domain = encodeURIComponent(document.domain);
        domain = domain.replace(/[^0-9a-z]/ig,'');
        var domain_id = parseInt(domain, 36);
        while(Math.pow(2,32) > domain_id*10)
            domain_id*=10;
        while(Math.pow(2,32) < domain_id)
            domain_id/=10;
        domain_id = Math.floor(domain_id);
        var can =  document.createElement('canvas');
        can.width = size;
        can.height = size;
        can.id = 'tempcan';
        can.style.display = 'none';
        document.body.appendChild(can);
        var favicon = document.createElement('link');
        favicon.rel = 'shortcut icon';
        favicon.id = 'identifavicon';
        var head = document.getElementsByTagName('head')[0];
        new Identicon(can.id, domain_id, size);
        favicon.href = can.toDataURL();
        head.appendChild(favicon);
    }
})();