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Author Topic: Bitwise gems - fast integer math  (Read 2100 times)
gjehle
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« on: June 28, 2007, 03:42:32 AM »
Quote
Fast modulo operation using bitwise AND

If the divisor is a power of 2, the modulo (%) operation can be done with:
modulus = numerator & (divisor - 1);
This is about 600% faster.

[copy or print]
x = 131 % 4;

//equals:
x = 131 & (4 - 1);

Every programmer should know about the simple shift-left, shift-right for base2 multiplication and division (if you don't, you're missing something).
This page, however, lists a couple of real gems (as the title suggests) like the one quoted above.
Enjoy!
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f0dder
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« Reply #1 on: June 28, 2007, 05:32:50 AM »
"This is about 600% faster." - I love people making bold claims without making any reference to which platform they tested it on :] (ah, following the link they mention AS3 - flash actionscript?)

Bitwise math is good to know, though. I generally don't do it for highlevel code, but once there's a bottleneck, it can be worth it (usually ends up in assembly code then). Keep in mind that decent compilers know a bunch of these tricks already, which is good reason you wouldn't do ">> 2" to divide by four in C.
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Eóin
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« Reply #2 on: June 28, 2007, 07:03:35 AM »
I'd highly recommend everyone learning why those gems work. It would teach you a lot about how computers represent numbers and that could lead you to understand when and where those trick are really applicable.
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f0dder
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« Reply #3 on: June 28, 2007, 07:34:06 AM »
Sage advice, Eóin.
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gjehle
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« Reply #4 on: June 28, 2007, 09:39:17 AM »
Quote from: f0dder on June 28, 2007, 05:32:50 AM
"This is about 600% faster." - I love people making bold claims without making any reference to which platform they tested it on :] (ah, following the link they mention AS3 - flash actionscript?)
[...]
Keep in mind that decent compilers know a bunch of these tricks already, which is good reason you wouldn't do ">> 2" to divide by four in C.

hehe yes! exactly
the only reason why some of those bitwise ops are that much faster has to be a bad intermediate compiler / interpreter.
after all you want the script to start running, not compile for 5 minutes.
nevertheless, this list shows how to do some nice operations in bitwise and being who i am (someone who enjoys obfuscated c and perl and fancy/different ways to approach simple tasks) i appreciate websites like this one just for the fact that they list the operations ;-)
it's like a work of art
or maybe i already studied too much comp sci :x


Quote from: Eóin on June 28, 2007, 07:03:35 AM
I'd highly recommend everyone learning why those gems work. It would teach you a lot about how computers represent numbers and that could lead you to understand when and where those trick are really applicable.

true indeed
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