Monday, June 29, 2009

A C programmer in Android world : discovering Java allocations

I'm mainly, in my day world, a C / C++ programmer.
So I'm used with this language and sometimes, I feel like I would prefer to use another, more modern language !

So I was quite happy to try Java. I saw Java as a kind of modern, cleaner C++, with some nice features, and a more improved object model.

But for the 'WordProspector' game, in the process of taking the database out, and replacing it with 'something' else, more lighter (see here ), I found a serious limitation of this Java object model...

The issue :
As 'WordProspector' is a word game, it is shipped with a word dictionary.
At first, I stored the dictionary as a SQL database, as it was simpler for me at that time.
but when I found how memory costly it is, I decided to switch to another way to store it.

So I came up with a kind of tree with a letter for each node.
In my effort to compress the dictionary as much as possible, I found a way to store each node as a 8 bits integer plus a 16 bits integer, so it was 3 bytes.
So I had a Node class, with simply a byte and a short.
But I had a big number of node instances in my tree : something like 300 000 nodes.

So when I loaded the tree, I started by allocating all the nodes.
Allocating the nodes with java was something like :

NodeArray = new Node[nbNodes];

for ( int i = 0; i <>
NodeArray[i] = new Node();

When I tried it on the emulator... It took several minutes... Then the program crashed...
Too much allocations of this size !

I was amazed that I just couldn't easily create this really simple array !!

Then I discovered that allocations of simple type arrays didn't demand to allocate each element !

So finally, I get rid of my nice Node class, save the whole thing as a byte array, and interpret, on the fly, the byte array as a byte plus a short.
With that, the allocation is as fast as it could be :

NodeArray = new byte[nbNodes * 3];

With this allocation issue, I had to get rid of my Node class, I just have a big array of bytes.
Acces to member are much more complicated, the code that was once so simple is now much more complex.
Add on this issue the fact that what I really wanted was an unsigned short, and not a short.
My code is now full of bits manipulation to create and interpret some bytes as unsigned short, or as bytes, depending of the situation.

With C/C++, I would never have this problem, and I would have a clean object code !

Saturday, June 20, 2009

Tutorial : How to access a Android database by using a command line.

When your start dealing with a database in your program, it is really important and useful to be able to access it directly, outside your program, to check what the program has just done, and to debug.

And it is important also on Android.

Here is how to do that :

1) Launch the emulator (or connect your real device to your PC ). I usually launch one of my program from Eclipse for this.
2) Launch a command prompt in the android tools directory.
3) type adb shell.
This will launch an unix shell on your emulator / connected device.
4) go to the directory where your database is :
cd data/data
here you have the list of all the applications on your device
Go in your application directory ( beware, Unix is case sensitive !! )
cd com.alocaly.LetterGame
and descend in your databases directory :
cd databases
Here you can find all your databases. In my case, there is only one ( now ) : SCORE_DB
5) Launch sqlite on the database you want to check / change :
sqlite3 SCORE_DB
From here, you can check what tables are present :
6) enter any SQL instruction you want :
select * from Score;

This is quite simple, but every time I need it, I don't know where to find it.

Tuesday, June 9, 2009

Developing with Python on Android : a dream comes true ?

Google still innovates and offers as many tools as they can to us developers !

I'm a big fan of Python, and several times, I wanted to try things on my phone, but without having a development PC around me.
Now they combines both of my implicit wishes, and we can develop in python (and LUA, BeanShell, and other langages to come ) from the mobile !!
I'm really excited about that.

Next question is :
Could I embed my python script that I developed on my phone in a real Java application ?
That would be awesome : it would mean I really could develop parts of my application from my phone, or even have a scripted part of my application that could evolve without a PC !!!


Thank you, Mr google !!!

Google Open Source Blog: Introducing Android Scripting Environment

Edit :
Sadly, this application is for Cupcake only, so I can't use it...
I will have to wait for Cupcake to be avalaible on my phone.
That makes me wonder how many phone are still stuck with the 1.1 OS ?
From what I've read, in the US, the update is finished, so there should be no 1.1 left.
But outside US ???