JavaScript - Technique - Duff's Device

This example displays an example of Duff's Device implemented in JavaScript.

The process of looping are in it self expensive. Doing a for loop is more expensive than a while loop and so on... Duff's Device is a technique to cut down the number of iterations in a loop which boosts speed on an operation. On large number of iterations Duff's Device performs from 50 - 70% faster than a traditional loop.

Usage:

Duff's Device is build on the theory that this:

    var j = 0;
    j++;
    j++;
    j++;
    j++;
    j++;
    j++;
    j++;
    j++;    // which is 8

is faster than this:

    var j = 0;
    var i = 8;
    while(--i){
        j++;    
    }

So by doing something like this:

    var l = 8;  // will be just one.. just a example
    var j = 0;
    var i = parseInt(l / 8);
    do{
        j++;
        j++;
        j++;
        j++;
        j++;
        j++;
        j++;
        j++;
    }while(--i);

we are doing eight times less iterations than with an old fashion loop. In other words; if we have a list of 64 items we want to iterate over we can cut it down to eight iterations with Duff's Device (if we implement Duff's Device to operate with a division of 8). A traditional loop would use 64 iterations.

Test:

These tests perform the same operations (adding one to a variable) in both a traditional descending while loop and with Duff's Device. The intention of the tests are just to time the looping. NOTE: The largest number of iterations can halt your browser!

Code for this test here.

Acknowledgment:

This page is a result of techniques described by Tom Duff

Page by: Trygve Lie