alternative to "ints" (?)

[Seamus GalIagher]

Hi,

I was reading an article on www.gamedev.net, about memory usage for an application. It stated this

if a particular type of data only requires 3-bits of information try to use a bitfield (or least only 1 8-bit byte or unsigned char) instead of just allocating an "int" (32-bits) to hold it.

I understand what it says, but does this relate to java at all, and if so, is there a way in java to do this? Maybe java only uses the required space and doesnt take into consideration unused space?

thankyou

Seamus

[Hentay Duke]

Look into the other primitive types. A short and a byte each use fewer bits than an int. It's been awhile since I studied this but if you google for it you should find some information.

[Peter Chase]

Two points. Using a smaller data type may not actually use less memory. On many languages and many platforms, it is inefficient to use less than one word (32 bits on a 32-bit platform, 64 on 64 etc.) for a piece of data, so they don't. In Java, you have a guarantee that a "byte" always behaves as having 8 bits and an "int" always behaves as having 32, but I believe there is no guarantee about how much physical memory they will use, and I believe they will use exactly the same amount, on some platforms.

Second, while those working on platforms with very limited resources (e.g. mobile devices), or where performance really is key (e.g games) do want to squeeze as much data as possible into a small space, most other applications don't care and shouldn't bother. It is much more important, for most applications, to write your code clearly, concisely and robustly, than to save a few bytes of memory. Modern machines generally have plenty!

[Jean-Sebastien Abella]

He,

Originally posted by Peter Chase:
It is much more important, for most applications, to write your code clearly, concisely and robustly, than to save a few bytes of memory. Modern machines generally have plenty!

I totally agree with Peter here. I never used them but isnt BitSet usefull for those situation???

[Richard Anderson]

Originally posted by Jean-Sebastien Abella:
He,


I totally agree with Peter here. I never used them but isnt BitSet usefull for those situation???

Okay, normally I would agree with you guys, but I take issue with this because you're assuming that we all build apps on PCs with enough RAM. However, when developing apps on wireless devices, you have to think about memory usage, and saving a few bytes can make all the difference.
[ August 11, 2005: Message edited by: Richard Anderson ]

[Jean-Sebastien Abella]

Originally posted by Richard Anderson:


Okay, normally I would agree with you guys, but I take issue with this because you're assuming that we all build apps on PCs with enough RAM. However, when developing apps on wireless devices, you have to think about memory usage, and saving a few bytes can make all the difference.

What would you suggest then?
The use of bit mask and shift on int or long, or maybe a BitSet?

[Layne Lund]

Originally posted by Jean-Sebastien Abella:


What would you suggest then?
The use of bit mask and shift on int or long, or maybe a BitSet?

I think it all depends on the exact problem you are trying to solve. Different situations with different requirements will require different solutions. Either one may be useful depending on the situation.

One thing you may need to consider is that BitSet is a class. As such, you will need 4 bytes to hold a reference to a BitSet object plus the BitSet internally needs memory to store its value. If you are only using 3 bits, then this doesn't seem to save anything over an int.

Layne

[Layne Lund]

Originally posted by Richard Anderson:


Okay, normally I would agree with you guys, but I take issue with this because you're assuming that we all build apps on PCs with enough RAM. However, when developing apps on wireless devices, you have to think about memory usage, and saving a few bytes can make all the difference.

[ August 11, 2005: Message edited by: Richard Anderson ]

Perhaps you should go back and read Peter's original post again. He did mention that memory usage is an issue on limited devices.

Layne

[Jean-Sebastien Abella]

But peter is not the one asking!

[Seamus GalIagher]

Hi,

it was me asking :-) i was just curious. However, one of the replies did mention memory in smaller devices. In japan especially, mobile phone games have taken off.. Perhaps newer techniques, or more emphasis on how information can be stored as best as possible, is becoming more of a focus.

[Rick O'Shay]

if a particular type of data only requires 3-bits of information try to use a bitfield (or least only 1 8-bit byte or unsigned char) instead of just allocating an "int" (32-bits) to hold it.


Store that data in a byte which uses eight, count 'em, bits. Then write a routine to return tribbles. A tribble is three bits, whereas a nibble is four bits � a dibble is two bits for example. Nibble is an official term, the other two I just made up because they sound neat.

Anyway, three bytes holds eight tribbles so you probably want to round off your tribble arrays to three bytes or anything modulo three.

Mext you will need a routine to address a tribble: set and get from an array where the index is the tribble index. The return and input values are bytes, so you waste five bits but only when passing around individual tribbles.

I will leave the implementation as an exercise for your own personal enjoyment but it goes something like this: locate the byte, isolate the tribble within that byte with a shift, mask off the high bits:

public void getTribble(int index)
{
int slot = index * 3 / 8;
int offset = index * 3 % 8;
return tribbles[ index ] >>> offset & 3;
}

[Stan James]

I like that, Rick. I did similar things in the Carter & Reagan years in 370 asembler and Turbo Pascal. I used another language with 4 bit digits (0-9) and 6 bit letters. Nothing had to be on any boundaries so you could mix & match at will.

[Guy Allard]

public void getTribble(int index)
{
int slot = index * 3 / 8;
int offset = index * 3 % 8;
return tribbles[ index ] >>> offset & 3;
}

That's not going to work. It won't even compile.

Guy

[Rick O'Shay]

>> That's not going to work. It won't even compile.

It works perfectly: it's pseudo code which I warned about it the preface.

I just fixed it up with the casts and the return value, compiled it and ran it. To my surprise it worked! Needs paint however: boundary conditions won't let you use it as is, even with the corrections.

Anyhoo, the idea was not to provide the implementation but the general pattern and a general outline as a special bonus prize.

[Philip Heller]

Rick -

"Dibbles" and "Tribbles" ... love 'em!

Another issue is the arithmetic concersion issue. If two dibbles, represented as bytes, are to be multiplied, Java will convert the bytes to ints. Then after the multiplication, the result has to be cast back to byte. If arithmetic isn't carefully managed, all the conversion and casting could chew up a lot of time.

Thinking out loud here, I would be tempted to use lookup tables for arithmetic. Suppose I want to multiply a pair of dibbles, one in bits 1:0 of its byte and one in bits 3:2 of its (different) byte. Assuming all other bits are zero (that is, the other dibbles or tribbles have been masked out and I'm now dealing with ints), I would add the dibbles and use the result as an index into a byte array. I would have one array for addition results, one for subtraction, etc.

I'm really enjoying typing "dibble" and "tribble" so I'm going to do it once more before signing off. Dibble. Tribble.

-- Phil

[Ilja Preuss]

Originally posted by Philip Heller:
"Dibbles" and "Tribbles" ... love 'em!

Tribbles are cute!