Using bit fields in ActionScript 3
Update: I just confirmed with the player team that a Boolean actually takes up 4 bytes.
I’ve been doing a lot of research lately on different uses for ByteArrays. This is in preparation for my ByteArrays for Beginners session that I will be doing at FITC Toronto. While in Amsterdam recently, Thibault Imbert of bytearray.org mentioned how he uses bit fields in WiiFlash to make boolean values as small as possible. Basically when the WiiFlash server sends over the state of the Wiimote buttons to Flash, it sends over a single byte where each bit represents the state of a particular button.
So why would you want to do this? Well if you have eight boolean values it would take up 32 bytes of space if you use the native Boolean class, as each one takes up 4 bytes. But in reality that is 32 times as big as it needs to be. If you set each bit of a single byte to either 1 or 0 based on your boolean values it will only take up a single byte. Now this is not going to be worth it for most situations, but if you need really high-speed binary communication than it will be more efficient. Below is an example of how you could pack eight booleans into a single byte:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | var ba:ByteArray = new ByteArray(); var bf:uint = 0; bf = 1<<7; //Set bit 8 to true bf |= 1<<6; //Set bit 7 to true bf |= 0<<5; //Set bit 6 to false bf |= 1<<4; //Set bit 5 to true bf |= 1<<3; //Set bit 4 to true bf |= 1<<2; //Set bit 3 to true bf |= 1<<1; //Set bit 2 to true bf |= 1; //Set bit 1 to true ba.writeByte(bf); trace(ba[0].toString(2)); //11011111 |
In the above code all of the boolean values are set to true except for bit number 6. This way of setting individual bits is probably the easiest to read, at least for me. Now when you want to retrieve the value of an individual bit you can simply do an AND on the byte like I do below.
1 | if(ba[0]&32) trace("Bit 6 is true"); //Won't trace anything |
The reason I chose the number 32 is that it is the perfect binary mask to retrieve the 6th bit of the byte. In binary the number 32 is represented as 00100000. I will go over this more during my session and will also do a tutorial on it soon.
Lee
Deep….
I could see this being used in a situation where you were passing data to another app or over the web with a lot of yes/no settings. I suppose the WiiFlash example is proof enough. Thanks Lee.
I first used a Byte key (technically a 32 bit Integer key) in Flash 5 for a calendar application for PricewaterhouseCoopers. What’s great about Byte keys is an entire year of ‘on/off’ data can be stored in only 12 Integers (One Integer per month; no month greater than 31 days, so you’re golden). We were able to shoot out a year of calendar data tracking 4 different categories per day with only 48 Integers then (4×12x32bits). It sped up the results so fast, we actually needed to add a ‘delay timer’ since none of the users in 1999 were expecting the data to be saved that quickly. Byte keys are a GREAT way to get out of a pickle, a righteous hack that’s good to know and I’m stoked you’ll be sharing it with the broader Flash community. Nice work.
Nice, Finally understand what you were talking bout yesterday. Can this only be used to send Booleans or can I send other things such as Sprites or Array even??
Its also easy to toggle the individual bits:
var bits :int = 0;
bits |= 4; // (1 << 2)
bits |= 32; // (1 << 5)
trace(bits.toString(2)); // 00100100
bits ^= 4; // mask off bit 3
trace(bits.toString(2)); // 00100000
bits ^= 32; // mask off bit 6
trace(bits.toString(2)); // 00000000
Also see http://www.vipan.com/htdocs/bitwisehelp.html
I could see how this would be useful in a project where extreme speed is of the essence… like in a socket protocol or something, but for everyday stuff, I don’t know if I would ever actually use this. Maybe the next AVM should condense Boolean values to bits so we don’t even have to do this?
Very nice! Cool thinking that one byte could hold say 8 pieces of personal information such as:
Married / Single
Straight / Gay
Unemployed /Employed
Renting / Buying
Veteran / Not Veteran
Lease / Own
Pet / No Pet
Apple / PC
Or am I way off track with this?
Looking forward to the future videos.
Nice. Looking forward to the video Lee
Thought I might see your name popup on the WebDU speakers list but no such luck
I find byte arrays and bit manipulation in general a very good way to save data out to a database in some cases. I am working on a scheduling app right now and each time-slot has a day of the week, start time, and end time and I can fit it all in a single UINT. Obviously this isn’t good for readability but it’s way more efficient in terms of space, and can speed up load times if the users needs a lot of info.
While it is true that a Boolean value in low-level languages (which may or may not represent it as a boolean-specific data type) would take up one byte (lowest amount of addressable space), am I wrong in remembering having seen somewhere that all variables in Flash take up more memory?
EDIT: Found the link saying 12 bytes, might just be relevant to AVM1. Can you spread some light?
http://www.adobe.com/go/tn_14437
It would make sense, since not only does the AVM need to hold the actual value, but the name of or other reference id for the variable, any metadata, et c.
I use this technique quite often in php/mysql when I need to store various selections for e.g. a profile in a database and don’t want to have lot’s of fields in my table. The speed aspect of it never came to mind.
Thanks!
Next stop: POKE and PEEK!
This kind of thing is used a lot in setting things like sort parameters or dialog button choices. For instance, sorting Arrays in AS. trace out:
trace(Array.CASEINSENSITIVE);
trace(Array.DESCENDING);
trace(Array.UNIQUESORT);
trace(Array.RETURNINDEXEDARRAY);
trace(Array.NUMERIC);
you get
1 or 00001
2 or 00010
4 or 00100
8 or 01000
16 or 10000
This way you can combine multiple options with the bitwise OR operator: |
array.sort(Array.CASEINSENSITIVE | Array.DESCENDING);
This passes 00011, or 3.
I use this sort of thing for page tracking of my e-learning courses. We are limited in using 255 character strings for this. Sometimes a client asks to go beyond the specs and using bit data alleviated some of my concerns.
Looking forward to the video Lee. I will see you in TO for the workshop, albeit I cannot attend your session because I have to get home.
DJ
@Richard Olsson
in the AVM2, according to the tuning guide published by Adobe on their website, a integer without annotations takes 4 bytes, a uint is the exact same. Booleans, however, qualify as Atoms still in AS3. Which would make their size 4 bytes from what I have read. Lee, if I have something incorrect, feel free to correct me.
Cool stuff. I’m a big fan of bit-twiddling though there is little use for that kind of knowledge now-a-days. In any case check out the following link I occassionally use as a resource for bit-hacks:
http://www-graphics.stanford.edu/~seander/bithacks.html
Indeed, this way of handling Booleans is a little cumbersome for everyday use, especially for beginners. But for speed or memory space it’s just excellent.
This means we should pay more attention to bitwise operations since they’re way faster. Also, it could be used when working with Boolean flags and save their states in a single integer value, as it was previously pointed out.
Thank you Lee.
[...] The Flash Blog that a Boolean takes up 4 bytes (confirmed by the Flash player team). Initially, in his article he wrote about one byte being occupied by a Boolean but he updated it later [...]
this is all because the “native” boolean classes as you called them are really what I call a “womenized booleans”. that’s the reason why it takes 4 bytes. the values that can be assigned are as follows: true / false / maybe / next week
[...] > The Flash Blog » Using bit fields in ActionScript 3 [...]
Thanks for the technical overview! I don’t often need to be this stringent on code, but when I do I’ll now be prepared!
great article. great comments.
it’s great when i learn something so empowering. i look forward to more..
thanks lee
thanks community
[...] I was just reading one of my favorite blogs, rightfully named theFlashBlog.com, where Lee posted a great article on ByteArrays. He’s having a session FITC Toronto called ByteArrays for Beginners. I wish I [...]
This changes the way I fell about ByteArray. Great post thanks Lee.
bit 6 is false because it’s never set. i think you meant:
bf &= 0 << 5
[...] Lee Brimelow – ByteArrays for Beginners [...]
Great article.
I’m still searching for your conference notes and/or tutorial on ByteArrays. Have these been posted somewhere?
You’ve helped me finally understand what ByteArrays are all about. Thank you very much!