Codeprix – Page 2 – Anand Nalya

My first Scheme program

I’ve just started experimenting with Scheme, specifically to follow Structure and Interpretation of Computer Programs. Here is a Scheme procedure that returns the sum of squares of two larger numbers out of the given three.

'protected’ implementation in Actionscript 3 is BROKEN

Consider following code:

public class BaseClass{ public function modifyProperty(property:String, value:Object):void{ this[property] = value; } } public class SubClass extends SubClass{ protected var count:int; } public function test():void{ var testObj:BaseClass = new SubClass(); testObj.modifyProperty('count', 3); }

Theoretically, the call at line no. 13 should not cause any error as count is defined for testObj as a field in SubClass, but it gives a Cannot create property count on SubClass. Interestingly if we move modifyProperty method to SubClass or make count public, it works without any error.

So it can be said that we cannot access protected fields dynamically in a subclass.

How Facebook (probably) implements SMS threads

A lot of web applications provides you the ability to receive or post updates using SMS (text messages). Twitter is the posterboy of such applications.

But the feature that distinguishes Facebook SMS updates from any other service is that you can actually reply to individual messages and your message then becomes a part of the aforementioned thread. You don’t have to specify any special command to do so. Just hit the reply button and you are done.

How do they know which message belongs to which thread? Its simple, each message comes from a different number. For example, say A writes something on your wall, then you will receive a message from +91922305501. Also B sends a message to you, a message comes from +91922305502 and when C comments on your note, you get a message from +91922305503. How many numbers do facebook own? Probably 100 in India, from +91922305500 to +9192230599. A per user mapping of the number from which the message is sent and the object/thread id for that message is kept on the backend. So when the user replies to a particular number, say +91922305502 , it knows that is was a reply to the message sent by B and hence treats as a reply to that message. A rotation policy is used to decide from which number the message is to be sent. So after recieving a message from +9192230599, the next message will be from +91922305500 and now you cannot reply to the thread represented by the earlier message from the same number.

Discalimer: This is just a guess of how this functionality might have been implemented (or at least how I would implement it) and based strictly on SMSes received in India. Actual implementation might (probaly is) be entierly different from what is explained here.

Bing/Google toggle bookmarklet

I’ve been trying out Bing as my default search engine for last few months and the results are definitely at par with that of Google for most of the consumer categories. But sometimes you just need a quick toggle to see what Google would return for your query. So here is a small bookmarklet that I wrote that toggles from Bing to Google and vice versa.

Drag this to you favourites bar: Bing/Google Toogle

Note: Its tested on IE8 only. A few tweaks might be required for Firefox.

Minifying Javascript/css without changing file references in your source

Rule 10 of Steve Souders High Performance Web Sites: Minify Javascript

The most common problem faced while implemnting this is how you handle the full and minified version and how to change there reference in referencing documents. One of the easier ways to do this is to make it part of the deployment process.

Here are the relevent steps involved.

I’m using YUI Compressor.

Now you don’t want to replace all references to x.css or x.js in your development code with references to x-min.css and x-min.js respectively. So what you can do is rewrite all those filenames at the web server level.

For apache the following rewrite rules work fine:

Caution: Remember to delete existing minified css/js file before running the minifying script or you will end up with file names like x-min-min-min.js and so on. One way to do this is to clear the js/css folder before checking out files from your source repository.

Humanizing the time difference ( in django )

django.contrib.humanize is a set of Django template filters that adds human touch to data. It provides naturalday filter that formats date to ‘yesterday’, ‘today’ or ‘tomorrow’ when applicable.

A similar requirement which the humanize pacakge does not address is to display time difference with this human touch. so here is a snippet that does so.

from django import template register = template.Library() MOMENT = 120 # duration in seconds within which the time difference # will be rendered as 'a moment ago' @register.filter def naturalTimeDifference(value): """ Finds the difference between the datetime value given and now() and returns appropriate humanize form """ from datetime import datetime if isinstance(value, datetime): delta = datetime.now() - value if delta.days > 6: return value.strftime("%b %d") # May 15 if delta.days > 1: return value.strftime("%A") # Wednesday elif delta.days == 1: return 'yesterday' # yesterday elif delta.seconds > 3600: return str(delta.seconds / 3600 ) + ' hours ago' # 3 hours ago elif delta.seconds > MOMENT: return str(delta.seconds/60) + ' minutes ago' # 29 minutes ago else: return 'a moment ago' # a moment ago return defaultfilters.date(value) else: return str(value)

Running Glassfish as a service on CentOS

Here is how you run glassfish as a service on CentOS:

Create a user glassfish (you can call it anything you want) under which Glassfish will run.
```
#useradd glassfish
```
Install glassfish in /home/glassfish.
Create the startup script /etc/init.d/glassfish for glassfish.

Install the service

#chmod +x /etc/init.d/glassfish
#chkconfig --add glassfish
#chkconfig --level 3 glassfish on

Start glassfish.
```
#/etc/init.d/glassfish start
```

Embedding flash object in Facebook Apps (FBML)

Yesterday I was trying to include a flash object in a facebook app using FBML fb:swf tag. The flash object needed to change the url of the page it was running on on a particular event. But since facebook prevents direct script access from flash, this could not be done.

Here is a simple workaround:

1. Create a simple html page containing only the flash object that you want to include.

2. Embed this html on the facebook page using the fb:iframe tag.

Now you can make all the Actionscript calls you want to. Only caveat to this approach is that you will no longer be able to make Facebook API calls directly from flash.