A lot of the Internet uses Google Adsense, and yet very little is known about how it works. I downloaded the show_ads.js script that every adsense page references, and analyzed it to try to see how it works. I found lots of hidden features along the way.

The google show_ads.js script has been optimized for quick loading. Google has removed all whitespace, and renamed variables and funcitons to have single letter names. While this is good for fast page loading, it is not so good for understanding what it does. So I ran this through my Javascript PrettyPrinter, and then renamed the functions and some variables based on what I think they do.

A large amount of this code is for dealing with script ads. That is surprising, considering that it is only possible for premium adsense customers (like about.com or slashdot.org) to use script ads. The rest of us are confined to iframes. It is a strange decision to include all of this additional functionality that can never be used. Why not make two show_ads.js -- one for premium customers and one for regular ones?

Regular iFrame Ads

Web sites that do not yet have 20 million unique visitors have to have iframe ads. The iframe is a separate web page embedded in your web page. For example, here is an iframe containing www.example.com:

1. User script sets variables such as the channels, and ads colours
2. The show_ads.js script translates the variables into a large query string.
3. The script will insert an iframe into your document whose source is the query string.
4. The user's browser retrieves the separate document into the iframe, and the user sees the ad unit.

Script Ads

1. Sides like about.com will call another script to set variables like "google_cust_name" "google_cust_age" etc. if they know this information.
2. The show_ads.js script will build a query string to load the another script from google servers. It will also setup callback functions to call once the other script finishes finding ads.
3. The other script will place the ad content into an array.
4. The other script calls the callback functions. The callbacks will insert div and iframe elements into your web page, containing the ad units.

The end result for script ads is that more customization is possible, and the ads will load more quickly. Instead of making 5 separate queries from your browser to google, your browser will only make 1 query. The disadvantage is that you need a programmer to make the javascript to set all the variables correctly.

It is against Adsense Terms of Service to modify the Adsense code in any way. However, it is interesting to note the extra API available to premium customers. You can see how Slashdot uses the api in this script, which is included just before show_ads.js.

Script Variable	URL Encoding	Meaning
google_ad_output	output	Default: "html". Other values are "js" or "textlink". If a premium customer is using script ads, this is set to "json_html". JSON stands for Javascript Object Notation . Essentially, this output type will be a Javascript array containing the ad content. Only the HTML type will result in an iframe. All other ad types will include a <script> tag into your web page.
google_ad_client	client	This identifies the adsense customer (the web master). For example: "pub-8374822756592383". It is included for non-premium adsense customers too.
google_cpa_choice	cpa_choice	It probably stands for "Code Page" choice, and it controls the unicode character encoding of the ads. If this is set, then the document's character set is appended to the query as "oe". Also, the value of google_cpa_choice is added as well.
google_ad_host	host
Date.getTime()	dt	The current time is added to the request as "dt"
google_language	hl	This variable is added to the request as "hl"
google_country	gl
google_region	gr
google_city	gcs
google_hints	hints
google_safe	adsafe
google_encoding	oe	This must be the charset. Note that if google_cpa_hoice is set, "oe" would be added to the query string twice. They must therefore be mutually exclusive.
google_last_modified_time	lmt
google_alternate_ad_url	alternate_ad_url
google_alternate_color	alt_color
google_skip	skip
google_targeting	targeting
N/A	prev_fmts	Seems to keep track of the number of ad units on your page.
N/A	format	This is derived from the script based on whether the browser is supported.
google_max_num_ads	num_ads
google_ad_output	output	This can be either "json_html" if used by a premium adsense customer, or it's set to whatever google_ad_output is, which usually is null
google_adtest	adtest
google_ad_channel	channel
google_ad_section		This is not encoded in the query string, but read directly by the target script. Slashdot, for example, sets it to 'apple askslashdot backslash books developers'.
N/A	pv_ch
google_page_url	url
google_color_bg	bg
google_color_text	text
google_color_link	link
google_color_url	url
google_color_border	border
google_color_line	line
google_reuse_colors	reuse_colors
google_kw_type	kw_type
google_kw	kw
google_contents	contents
google_num_radlinks	num_radlinks
google_max_radlink_len	max_radlink_len
google_rl_filtering	rl_filtering
google_rl_mode	rl_mode
google_rt	rt
google_ad_type	ad_type
google_image_Size	image_size
google_ad_region	region
google_feedback	feedback_link
google_referrer_url	ref
google_page_location	loc
google_bid	bid
google_cust_age	cust_age
google_cust_gender	cust_gender
google_cust_interests	cust_interests
google_cust_idid	cust_id
google_cust_job	cust_job
google_cust_u_url	cust_l
google_cust_lh	cust_lh
google_cust_ch	cust_ch
google_ed	ed
google_ui_features	ui
	cc	If this is the top frame, this is the ratio of the browser window height divided by the total height of the web page, expressed as a percentage.
google_ad_override	google_ad_override
N/A	flash	This is the version of flash that the user has.
	u_h, u_w	screen height, width
	u_ah, u_aw	available screen height, width
	u_cd	color depth
	u_tz	document.getTimezoneOffset()
	u_his	Number of pages in browser history
	u_java	navigator.javaEnabled()
	u_nplug	Number of plugins installed
	u_nmime	Number of mime types the browser recognizes.

The Dissection

We begin with (function(). This is an odd syntax. However, by defining all of the other functions inside of an unnamed function, Google is giving them their own namespace. That way, you can include the same script numerous times, and include multiple ads per page.

(function()
{

Originally named "n", this function obviously returns its argument with quotes added. It's used for dynamically building HTML.

    function addQuotes(b)
    {
        return b != null ? '"' + b + '"' : '""'
    }

"encodeURIComponent" is a standard javascript function. It turns strings like "http://www.google.com" into "http%3A%2F%2www.google.com". This wrapper checks if it's available in your browser, and if not, it calls escape() instead.

    function encodeUri(b)
    {
        if(typeof encodeURIComponent == "function")
        {
            return encodeURIComponent(b)
        }
        else
        {
            return escape(b)
        }
    }
    
    function appendToQuery(b, a)
    {
        if(a)
        {
            window.google_ad_url += "&" + b + "=" + a
        }
        
    }
    
    function appendToQueryEscaped(b, a)
    {
        if(a)
        {
            appendToQuery(b, encodeUri(a))
        }
        
    }

    function appendColourToQuery(b, a, c)
    {
        if(a && typeof a == "object")
        {
            a = a[c % a.length]
        }
        
        appendToQuery("color_" + b, a)
    }

Here, Google is recording your time zone, screen size, colour depth, and the mime types your browser will accept. Surprisingly, they are also recording how many web pages you have visited in your history (although not which ones). This is a good indication of how long you have been browsing for.

    function appendBrowserCapabilities(b, a)
    {
        var c = b.screen, e = navigator.javaEnabled(),f =- a.getTimezoneOffset();
        if(c)
        {
            appendToQuery("u_h", c.height);
            appendToQuery("u_w", c.width);
            appendToQuery("u_ah", c.availHeight);
            appendToQuery("u_aw", c.availWidth);
            appendToQuery("u_cd", c.colorDepth)
        }
        
        appendToQuery("u_tz", f);
        appendToQuery("u_his", history.length);
        appendToQuery("u_java", e);
        if(navigator.plugins)
        {
            appendToQuery("u_nplug", navigator.plugins.length)
        }
        
        if(navigator.mimeTypes)
        {
            appendToQuery("u_nmime", navigator.mimeTypes.length)
        }
        
    }

This takes a string, makes it lower case, and adds "ca-" in front of it. Maybe it converts it into some kind of name they lookup in a database.

    function convertToCaName(b)
    {
        b = b.toLowerCase();
        if(b.substring(0, 3) !="ca-")
        {
            b = "ca-" + b
        }
        
        return b
    }

This appears to write the start of an add to the document. It writes either javascript, html, or textlint ads. Notice that it does not have the <script> end tag. So the ad still has to be written, and then they have to close off the tag.

    function writeRegularAd(b, a, c)
    {
        c = c.substring(0, 2000);
        c = c.replace( /w ?/, "");
        if(b.google_ad_output == "js" &&(b.google_ad_request_done || b.google_radlink_request_done))
        {
            a.write('<script language="JavaScript1.1" src=' + addQuotes(c) +script>")
        }
        else if(b.google_ad_output == "html")
        {
            if(b.name != "google_ads_frame")
            {
                a.write('<iframe name="google_ads_frame" width=' + addQuotes(b.google_ad_width) +" height=" + addQuotes(b.google_ad_height) +" frameborder=" + addQuotes(b.google_ad_frameborder) +" src=" + addQuotes(c) +' marginwidth="0" marginheight="0" vspace="0" hspace="0" allowtransparency="true" scrolling="no">');
                a.write("</iframe>")
            }
            
        }
        else if(b.google_ad_output == "textlink")
        {
            a.write('<script language="JavaScript1.1" src=' + addQuotes(c) +script>")
        }
        
    }

This function is called to clear all the adsense variables as soon as the script is done with them. That way they don't interfere with the next ad in the page.

    function eraseAllVariables(b)
    {
        var a = null;
        b.google_ad_frameborder = a;
        b.google_ad_format = a;
        b.google_page_url = a;
        b.google_language = a;
        b.google_gl = a;
        b.google_country = a;
        b.google_region = a;
        b.google_city = a;
        b.google_hints = a;
        b.google_safe = a;
        b.google_encoding = a;
        b.google_ad_output = a;
        b.google_max_num_ads = a;
        b.google_ad_channel = a;
        b.google_contents = a;
        b.google_alternate_ad_url = a;
        b.google_alternate_color = a;
        b.google_color_bg = a;
        b.google_color_text = a;
        b.google_color_link = a;
        b.google_color_url = a;
        b.google_color_border = a;
        b.google_color_line = a;
        b.google_reuse_colors = a;
        b.google_adtest = a;
        b.google_kw_type = a;
        b.google_kw = a;
        b.google_num_radlinks = a;
        b.google_max_radlink_len = a;
        b.google_rl_filtering = a;
        b.google_rl_mode = a;
        b.google_rt = a;
        b.google_ad_type = a;
        b.google_image_size = a;
        b.google_feedback = a;
        b.google_skip = a;
        b.google_page_location = a;
        b.google_referrer_url = a;
        b.google_ad_region = a;
        b.google_ad_section = a;
        b.google_bid = a;
        b.google_cpa_choice = a;
        b.google_cust_age = a;
        b.google_cust_gender = a;
        b.google_cust_interests = a;
        b.google_cust_id = a;
        b.google_cust_job = a;
        b.google_cust_u_url = a;
        b.google_cust_l = a;
        b.google_cust_lh = a;
        b.google_cust_ch = a;
        b.google_ed = a;
        b.google_targeting = a;
        b.google_ad_host = a;
        b.google_ad_slotname = a;
        b.google_ad_override = a;
        b.google_ui_features = a
    }

This is the last thing that the script executes, so it must be important. It builds the query URL and begins to write the ad to the document. Notice that google sometimes knows your "cust_age", "cust_gender" and "cust_interest"!

    function buildQueryUrl()
    {
        var b = null, a = window, c = document;
        var e = new Date;
        var curTime = e.getTime();
        var adFormat = a.google_ad_format;
        var scriptAdOkay = false;
        if ( O(a.google_ad_output, a.google_ad_client) && !incompatibleBrowser())
        {
            scriptAdOkay = true
        }
        
        if(a.google_cpa_choice)
        {
            a.google_ad_url = "http://pagead2.googlesyndication.com/cpa/ads?";
            a.google_ad_url += "client=" + escape(E(a.google_ad_client));
            a.google_ad_region = "_google_cpa_region_";
            appendToQuery("cpa_choice", a.google_cpa_choice);
            if(typeof c.characterSet != "undefined")
            {
                appendToQueryEscaped("oe", c.characterSet)
            }
            else if(typeof c.charset != "undefined")
            {
                appendToQueryEscaped("oe", c.charset)
            }
            
        }
        else
        {
            a.google_ad_url = "http://pagead2.googlesyndication.com/pagead/ads?";
            a.google_ad_url += "client=" + escape(E(a.google_ad_client))
        }
        
        appendToQuery("host", a.google_ad_host);
        var j = a.google_num_slots_by_client, C = a.google_num_slots_by_channel, l = a.google_prev_ad_formats_by_region;
        a.onerror = a.google_org_error_handler;
        if(a.google_ad_region == b && a.google_ad_section != b)
        {
            a.google_ad_region = a.google_ad_section
        }
        
        var k = a.google_ad_region == b ? "" : a.google_ad_region, w = false;
        if(adFormat)
        {
            w = i.indexOf("_0ads") >0
        }
        
        if(w)
        {
            if(a.google_num_0ad_slots)
            {
                a.google_num_0ad_slots = a.google_num_0ad_slots + 1
            }
            else
            {
                a.google_num_0ad_slots = 1
            }
            
            if(a.google_num_0ad_slots > 3)
            {
                return
            }
            
        }
        else if( !a.google_cpa_choice)
        {
            if(a.google_num_ad_slots)
            {
                a.google_num_ad_slots = a.google_num_ad_slots + 1
            }
            else
            {
                a.google_num_ad_slots = 1
            }
            
            if(a.google_num_slots_to_rotate)
            {
                l[k] =b;
                if(a.google_num_slot_to_show == b)
                {
                    a.google_num_slot_to_show = curTime % a.google_num_slots_to_rotate + 1
                }
                
                if(a.google_num_slot_to_show != a.google_num_ad_slots)
                {
                    return
                }
                
            }
            else if(a.google_num_ad_slots > 3 && k == "")
            {
                return
            }
            
        }
        
        appendToQuery("dt", e.getTime());
        appendToQuery("hl", a.google_language);
        if(a.google_country)
        {
            appendToQuery("gl", a.google_country)
        }
        else
        {
            appendToQuery("gl", a.google_gl)
        }
        
        appendToQuery("gr", a.google_region);
        appendToQueryEscaped("gcs", a.google_city);
        appendToQueryEscaped("hints", a.google_hints);
        appendToQuery("adsafe", a.google_safe);
        appendToQuery("oe", a.google_encoding);
        appendToQuery("lmt", a.google_last_modified_time);
        appendToQueryEscaped("alternate_ad_url", a.google_alternate_ad_url);
        appendToQuery("alt_color", a.google_alternate_color);
        appendToQuery("skip", a.google_skip);
        appendToQuery("targeting", a.google_targeting);
        var p = a.google_ad_client;
        if( !j[p])
        {
            j[p] =1;
            j.length += 1
        }
        else
        {
            j[p] +=1
        }
        
        if(l[k])
        {
            appendToQueryEscaped("prev_fmts", l[k].toLowerCase());
            if(j.length > 1)
            {
                appendToQuery("slot", j[p])
            }
            
        }
        
        if(adFormat)
        {
            appendToQueryEscaped("format", adFormat.toLowerCase());
            if(l[k])
            {
                l[k] =l[k] +"," + adFormat
            }
            else
            {
                l[k] =adFormat
            }
            
        }
        
        appendToQuery("num_ads", a.google_max_num_ads);
        appendToQuery("output", scriptAdOkay ? "json_html" : a.google_ad_output);
        appendToQuery("adtest", a.google_adtest);
        if(a.google_ad_channel)
        {
            var x = a.google_ad_channel;
            appendToQueryEscaped("channel", x);
            var y = "", z = x.split("+");
            for(var s = 0; s < z.length; s ++) {

                var t = z[s];
                if( !C[t])
                {
                    C[t] =1
                }
                else
                {
                    y += t + "+"
                }
                
            }
            
            appendToQueryEscaped("pv_ch", y)
        }
        
        appendToQueryEscaped("url", a.google_page_url);
        appendColourToQuery("bg", a.google_color_bg, curTime);
        appendColourToQuery("text", a.google_color_text, curTime);
        appendColourToQuery("link", a.google_color_link, curTime);
        appendColourToQuery("url", a.google_color_url, curTime);
        appendColourToQuery("border", a.google_color_border, curTime);
        appendColourToQuery("line", a.google_color_line, curTime);
        appendToQuery("reuse_colors", a.google_reuse_colors);
        appendToQuery("kw_type", a.google_kw_type);
        appendToQueryEscaped("kw", a.google_kw);
        appendToQueryEscaped("contents", a.google_contents);
        appendToQuery("num_radlinks", a.google_num_radlinks);
        appendToQuery("max_radlink_len", a.google_max_radlink_len);
        appendToQuery("rl_filtering", a.google_rl_filtering);
        appendToQuery("rl_mode", a.google_rl_mode);
        appendToQuery("rt", a.google_rt);
        appendToQuery("ad_type", a.google_ad_type);
        appendToQuery("image_size", a.google_image_size);
        appendToQuery("region", a.google_ad_region);
        appendToQuery("feedback_link", a.google_feedback);
        appendToQueryEscaped("ref", a.google_referrer_url);
        appendToQueryEscaped("loc", a.google_page_location);
        appendToQuery("bid", a.google_bid);
        appendToQuery("cust_age", a.google_cust_age);
        appendToQuery("cust_gender", a.google_cust_gender);
        appendToQuery("cust_interests", a.google_cust_interests);
        appendToQuery("cust_id", a.google_cust_id);
        appendToQuery("cust_job", a.google_cust_job);
        appendToQuery("cust_u_url", a.google_cust_u_url);
        appendToQuery("cust_l", a.google_cust_l);
        appendToQuery("cust_lh", a.google_cust_lh);
        appendToQuery("cust_ch", a.google_cust_ch);
        appendToQuery("ed", a.google_ed);
        appendToQueryEscaped("ui", a.google_ui_features);
        if(pageLocationMatches(a, c) &&c.body)
        {
            var A = c.body.scrollHeight, B = c.body.clientHeight;
            if(B && A)
            {
                appendToQueryEscaped("cc", Math.round(B * 100 / A))
            }
            
        }
        
        appendToQuery("google_ad_override", a.google_ad_override);
        appendToQuery("flash", a.google_flash_version);
        appendBrowserCapabilities(a, e);
        if( !scriptAdOkay)
        {
            writeRegularAd(a, c, a.google_ad_url)
        }
        else
        {
            writeScriptAd()
        }
        
        eraseAllVariables(a)
    }
    
    functionbuildQueryUrlAndReturnTrue(b, a, c)
    {
        buildQueryUrl();
        return true
    }
    
    function pageLocationMatches(b, a)
    {
        return b.top.location == a.location
    }

This checks the width and height of the ad. If we don't have at least twice that amount of space available on the page, then it returns false. This is a strange double negative. Someone at google needs to read Code Complete. Also, the page location must be the same as the page that is requesting the ad.

    function addWontFit(b, a)
    {
        var c = a.documentElement;
        if(pageLocationMatches(b, a))return false;
        if(b.google_ad_width && b.google_ad_height)
        {
            var e = 1, f = 1;
            if(b.innerHeight)
            {
                e = b.innerWidth;
                f = b.innerHeight
            }
            else if(c && c.clientHeight)
            {
                e = c.clientWidth;
                f = c.clientHeight
            }
            else if(a.body)
            {
                e = a.body.clientWidth;
                f = a.body.clientHeight
            }
            
            if(f > 2 * b.google_ad_height || e > 2 * b.google_ad_width)
            {
                return false
            }
            
        }
        
        return true
    }
    
    function fillInUnsetVars()
    {
        window.google_org_error_handler = window.onerror;
        window.onerror = buildQueryUrlAndReturnTrue;
        if(window.google_ad_frameborder == null) {
            window.google_ad_frameborder = 0
        }
        
        if(window.google_ad_output == null) {
            window.google_ad_output = "html"
        }
        
        if(window.google_ad_format == null && window.google_ad_output == "html")
        { 
            window.google_ad_format = window.google_ad_width + "x" + window.google_ad_height
        }
        
        if(window.google_page_url == null) {
            window.google_page_url = document.referrer;
            if( !addWontFit(b, document)) {
                window.google_page_url = document.location;
                window.google_last_modified_time = Date.parse(document.lastModified) /1000;
                window.google_referrer_url = document.referrer
            }
            
        } else {
            window.google_page_location = document.referrer;
            if( !adWontFit(b, document)) {
                window.google_page_location = document.location
            }
            
        }
        
        if(window.google_num_slots_by_channel == null) {
            window.google_num_slots_by_channel =[]
        }
        
        if(window.google_num_slots_by_client == null) {
            window.google_num_slots_by_client =[]
        }
        
        if(window.google_prev_ad_formats_by_region == null) {
            window.google_prev_ad_formats_by_region =[]
        }
        
        if(window.google_correlator == null) {
            window.google_correlator =(new Date).getTime()
        }
        
        if(window.google_adslot_loaded == null) {
            window.google_adslot_loaded = { }
            
        }
        
        if(window.google_adContentsBySlot == null) {
            window.google_adContentsBySlot = { }
        }
        
        if(window.google_flash_version == null) {
            window.google_flash_version = getFlashVersion().toString()
        }
        
    }

Here, we are detecting which browser the user has. For example, they might call it with detectBrowser("Firefox"). It caches the result so it can be called many times.

    function detectBrowser(b)
    {
        if(b in q)
        {
            return q[b]
        }
        
        return q[b] =navigator.userAgent.toLowerCase().indexOf(b) !=-1
    }
    
    var q = { } ;

    function isInternetExplorer()
    {
        return detectBrowser("msie") &&!window.opera
    }
    
    function isSarafiBrowser()
    {
        return detectBrowser("safari")
    }
    
    function getInternetExplorerVersion()
    {
        var b = navigator.userAgent, a = b.indexOf("MSIE ");
        if(a ==- 1) {
            return 0
        } else {
            return parseFloat(b.substring(a + 5, b.indexOf(";", a)))
        }
        
    }

This function uses the above to detect if a browser is sufficient to display the ads. It seems we need Intenet Explorer 5, firefox 1.0, Safari 4.12, Operat 9, or Netscape 7.

    function incompatibleBrowser()
    {
        if(isInternetExplorer())
        {
            var b = getInternetExplorerVersion();
            return b <= 5
        }
        else if(detectBrowser("firefox"))
        {
            var a = navigator.userAgent.indexOf("Firefox") +8, c = parseInt(navigator.userAgent.charAt(a)),e = navigator.userAgent.slice(a);
            return c < 1
        }
        else if(isSafariBrowser())
        {
            var a = navigator.userAgent.indexOf("Safari") +7, e = navigator.userAgent.slice(a),f = parseInt(e);
            return f < 412
        }
        else if(window.opera != null)
        {
            var a = navigator.userAgent.indexOf("Opera") +6, e = navigator.userAgent.slice(a),c = parseInt(e);
            return c < 9
        }
        else if( !isInternetExplorer() && !isSafariBrowser() && detectBrowser("mozilla"))
        {
            var a = navigator.userAgent.indexOf("Netscape") +9, e = navigator.userAgent.slice(a),c = parseInt(e);
            return c < 7
        }
        else
        {
            return true
        }
        
    }

I have no idea what this is supposed to be for. O(b, a) returns true only if b is html and a is the string "ca-pub-8954402009499768".

    function O(outputType, clientId)
    {
        if(outputType != "html") {
            return false
        }
        
        var c = { } ;

        c["ca-pub-8954402009499768"] = true;

        return c[clientId] !=null
    }

    function deriveSlotName()
    {
        var b = window;
        b.google_ad_slotname = "_Slot" + b.google_num_ad_slots
    }

This function it an alternative to writeRegularAd() above. It is called instead of writeRegularAd() only under certain circumstances. It can't handle text or html ads, however. Only script ads.

    function writeScriptAd()
    {
        var b = window;
        appendToQueryEscaped("correlator", b.google_correlatorn);
        appendToQuery("impl", "s");
        appendToQuery("callback", "_google_setAdContentsBySlotForSync");

        if( !b.google_ad_slotname) {
            deriveSlotName()
        }
        
        appendToQuery("slotname", b.google_ad_slotname);
        document.write("<script src = '" + b.google_ad_url+"'></script>")
    }

Here, we create a <div> tag with the contents of the ad. This function will be called by the downloaded script.

    window.google_createDivWithContent = function(slot) {
        var id = "google_ads_div_" + slot;
        var div = "<div id=" + id + ">";
        var contents = window.google_adContentsBySlot[slot];

        if(contents == null) {
            return
        }
        
        div += contents._html_;
        div += "n</div>n";
        document.write(div)
    };

This looks for the last script on the web page, and inserts an iframe right after it. The iframe has the given width, height, and contents. Obviously this iframe will contain the ad unit.

    window.google_createDOMIframe = function(id, width, height, contents) {
        var script = document.getElementsByTagName("script");
        var lastScriptOnPage = f[script.length - 1];
        var iframe = document.createElement("iframe");
        iframe.id = "google_ads_iframe_" + id;
        iframe.width = width;
        iframe.height = height;
        iframe.vspace = 0;
        iframe.hspace = 0;
        iframe.allowTransparency = "true";
        iframe.scrolling = "no";
        iframe.marginWidth = 0;
        iframe.marginHeight = 0;
        iframe.frameBorder = 0;
        iframe.style.border = 0;
        lastScriptOnPage.parentNode.appendChild(iframe);
        iframe.contentWindow.document.write(contents);
        iframe.contentWindow.document.close()
    };

This function creates an iframe not using DOM, and writes it directly to the document. You can specify width, height, contents, plus a script function to call when it loads.

    window.google_createiframe = function(id, contents, onLoad, width, height) {
        var divid = "google_ads_div_" + id;
        var frameid = "google_ads_iframe_" + id;
        var div = "<div id=" + i + ">";
        div += "<iframe id=" + frameid + ' width="' +  width+ '" height="' + height + 
            '" vspace="0" hspace="0" allowtransparency="true" scrolling="no" marginwidth="0" marginheight="0" frameborder="0"
            style="border:0px;" src="' + 
            contents.replace( /&/g, "&").replace( /</g, "<").replace( />/g, ">").replace( /"/g, """) +'"';
        div += ' onload="' + onLoad + '"';
        div += "></iframe></div>";
        document.write(div)
    };

This function take an iframe name (which has the format "google_ads_iframe_xxx") and extracts the xxx part, returning the number.

    var iframeNameLength = "google_ads_iframe_".length;
    function getIdFromIFrameName(domTag)
    {
        var attributeValue = domTag.getAttribute("id");
        var c = attibuteValue.substr(iframeNameLength, attributeValue.length - iframeNameLength);
        return c
    }

    
    window._google_syncAdSlotLoaded = function(b)
        {
            var a = getIdFromIFrameName(b);
            window.google_loadiframe(b)
        };

This function looks like it should be called when the ads have finished loading. It takes them from the google_adContentsBySot[] array and creates div and iframe tags for them, with the appropriate content.

    window._google_setAdContentsBySlotForSync = function(slots)
        {
            var a = window;
            for(var slotNum in slots)
            {
                var finishedAd = slots[slotNum];
                a.google_adContentsBySlot[slotNum] = e;
                if(finishedAd == null || finishedAd._empty_) {

                    // nothing to do
                    
                } else if(finishedAd._expandable_) {

                    a.google_createDivWithContent(slotNum)

                } else {

                    if(navigator.userAgent.indexOf("MSIE ") >0)
                    {
                        a.google_adslot_loaded[slotNum] =false;
                        var onLoad = "window._google_syncAdSlotLoaded(this);";
                        a.google_createiframe(slotNum, "about:blank", onLoad, finishedAd._width_, finishedAd._height_)
                    }
                    else
                    {
                        a.google_createDOMIframe(slotNum, finishedAd._width_, finishedAd._height_, finishedAd._html_)
                    }
                }
            }
        };

This is a neat function for splitting a query string into an array. For example, "http://foo.com/?first=steve&last=hanov" would be split into an array with fields "first" and "last", and you can look up the values. Functions like this one exist in all server-side cgi scripts. It is strange to see one on running on the browser, though! It is used for the ad_override functionality.

    function splitQueryString(b) {

        var a = { }

        var c = b.split("?");
        var fields = c[c.length - 1].split("&");

        for(var f = 0; f < fields.length; f++ ) {
            var i = fields[f].split("=");

            if( i[0] ) {

                try {
                    if ( i.length > 1 ) {
                        if ( window.decodeURIComponent ) {
                            a[ i[0].toLowerCase() ] = decodeURIComponent(i[1].replace( /+/g, " "));
                        } else {
                            a[ i[0].toLowerCase() ] = unescape(i[1]));
                        }
                    } else {
                        a[ i[0].toLowerCase() ] = "";
                    }
                } catch(except) {
                    
                }
                
            }
        }
        
        return a
    }

This is function handles something called a "google_ad_override". If, in the document's address, you have the string ?google_ad_override=blah, it will set the variable google_ad_override to "blah" right in the javascript. Hmm, this means that I can inject code into other pages simply by linking to their URL in a special way. Cool!

    function handleAdOverride()
    {
        var b = window, query = splitQueryString(document.URL);
        if(query["google_ad_override"])
        {
            b.google_ad_override = query["google_ad_override"]
        }
    }
    
    function getFlashVersion()
    {
        var b = 0;
        if(navigator.plugins && navigator.mimeTypes.length) {

            var a = navigator.plugins["Shockwave Flash"];
            
            if(a && a.description) {
                b = a.description.replace( /([a - zA - Z] |s) +/,"").split(".")[0]
            }
            
        } else if(navigator.userAgent && navigator.userAgent.indexOf("Windows CE") >=0) {

            b = 3;
            var c = 1;

            while(c) {
                try {
                    c = new ActiveXObject("ShockwaveFlash.ShockwaveFlash." +(b + 1));
                    b ++

                } catch(e) {
                    c = null
                }
            }
            
        } else if(isInternetExplorer()) {
            try
            {
                var c = new ActiveXObject("ShockwaveFlash.ShockwaveFlash.7")
            } catch(e) {
                try {
                    var c = new ActiveXObject("ShockwaveFlash.ShockwaveFlash.6");
                    b = 6;
                    c.AllowScriptAccess = "always"
                } catch(e) {
                    if(b == 6) {
                        return b
                    }
                    
                }
                
                try {
                    c = new ActiveXObject("ShockwaveFlash.ShockwaveFlash")
                } catch(e) {
                    
                }
            }
            
            if(c != null) {
                b = c.GetVariable("$version").split(" ")[1].split(",")[0]
            }
            
        }
        
        return b
    }
    
    handleAdOverride();
    fillInUnsetVars();
    buildQueryUrl();
}

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