var michelf_promo = {
    ads : [
        {
        image : 'http://michelf.com/promo/magic-launch/ml-promo-big.png',
        link : 'http://michelf.com/software/magic-launch/',
        text : 'Magic Launch: Help you files open in the right app.',
        },
        ],
    
    rand : function(min, max) {
        return Math.floor(Math.random() * (max - min + 1)) + min;
    },
    
    makeHTML : function() {
        var index = this.rand(0, this.ads.length - 1);
        var ad = this.ads[index];
    
        if (!navigator.userAgent.search("Mac"))
            return; // Mac-only ad
    
        return '' +
            '<div class="promo">' +
            '<a href="' + ad.link + '">' +
            '<img src="' + ad.image + '" alt="' + ad.text + '" />' +
            '</a>' +
            "</div>";
    },
};

document.write(michelf_promo.makeHTML());