Redirect user to random URL from a list that shrinks upon each visit, then resets

    

// List of URLs (two or more) for the experimental treatment and control groups.
var RedirectURLs = {
    "urls" : [
        { "url" : "http://www.aftonbladet.se/" },
        { "url" : "http://www.dn.se/" },
        { "url" : "http://www.svd.se/" }
    ]
}

// Count the number of URLs in list.
var Groups = RedirectURLs.urls.length;

// Randomize number and get the corresponding index for the URL in list.
var gotoUrl = RedirectURLs.urls[GetRandomNumber(Groups)].url

// Perform redirection.
if (window.location.search == "?debug") {
    // Debug mode: If user supplies a ?debug querystring,
    // then only show the redirect URL, but don't actually redirect.
    document.write("Redirect URL: " + gotoUrl);
} else {
    // Redirect the user to the new URL.
    document.location = gotoUrl;
}

// Perform randomization and save it to a cookie.
function GetRandomNumber(Groups) {
    // Has cookie been set?
    var rndIndex = null;
    if (GetCookie() != "") {
        // Yes, get randomization from cookie.
        rndIndex = GetCookie()
    } else {
        // No, set random value to cookie.
        rndIndex = Math.floor((Math.random() * Groups));
        SetCookie(rndIndex, 30) // Set cookie for 30 days.
    }
    return rndIndex
}

// Set cookie with randomization value.
function SetCookie(CookieValue, DaysUntilExpire) {
    var d = new Date();
    d.setTime(d.getTime() + (DaysUntilExpire * 24 * 60 * 60 * 1000));
    document.cookie = "rnd=" + CookieValue + ";expires=" + d.toUTCString();
}

// Get cookie with randomization value. Return empty string if not set.
function GetCookie() {
    var ca = document.cookie.split(';');
    for(var i = 0; i < ca.length; i++) {
        var c = ca[i];
        while (c.charAt(0) == ' ') c = c.substring(1);
        if (c.indexOf("rnd=") == 0) return c.substring(4, c.length);
    }
    return "";
}