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1. 

   Chad Smith
   Admin

   jQuery Duff's Device

   Summary

   Use Duff's device for faster^* loops.
   
   

   Author

   Chad Smith (email)
   
   

   Requires

   • jQuery

   Download

   jquery-duff.js (842 bytes)
   
   

   License

   Dual licensed under the MIT and GPL licenses.
   
   

   Usage

   Use: $(element).duff(function(){...}); or $.duff(element,function(){...});
   In place of: $(element).each(function(){...}); and $.each(element,function(){...});
   
   ^*Duff's device is faster in many cases, but not every case. Loop speed depends on the browser and number of iterations, and can differ on each run.
   
   

   Version History

   • v0.0.2 - August 28th, 2009
   • v0.0.1 - August 27th, 2009

   Support

   Examples

   Posted 11 months ago #
2. 

   Chad Smith
   Admin

   Duff's device explained

   
   Duff's device makes loops faster by using a method called unrolling. When you have a typical loop, the time it takes to execute that loop equals the time to execute function each time plus the time to see if it should continue each time. So the formula for a standard loop would be:
   
   Number of iterations (N)
   Time to execute (E)
   Time to check to continue (C)
   
   T=NE+NC
   
   Duff says if we iterate our loop and only do test every 8 times (plus a few remainders) it would go faster, and it does. Duff's device "unrolls" the loop, or essentially executes more and checks less to see if it should continue. The function it uses is along the lines of:
   
   Number of iterations (N)
   Time to execute (E)
   Time to check to continue (C)
   
   T=NE+C(N%8+floor(N/8))(plus trivial operation time for Floor, % and / one time)
   
   This means Duff's device is faster than a standard loop when
   
   N%8+floor(N/8)<N

   Posted 11 months ago #

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