javascript

Chapter 20: Best Practices
This example assumes that there are only three items in the values array and simply calls process()
directly on each item. Unrolling loops in this way eliminates the overhead of setting up a loop and
processing a terminal condition, making the code run faster.
If the number of iterations through the loop can ’ t be determined ahead of time, you may want to
consider using a technique called Duff ’ s device. The technique is named after its creator, Tom Duff, who
first proposed using it in the C programming language. Jeff Greenberg is credited with implementing
Duff ’ s device in JavaScript. The basic idea of Duff ’ s device is to unroll a loop into a series of statements
by calculating the number of iterations as a multiple of 8. Consider the following code example:
//credit: Jeff Greenberg for JS implementation of Duff’s Device
var iterations = Math.ceil(values.length / 8);
var startAt = values.length % 8;
var i = 0;
do {
switch(startAt){
case 0: process(values[i++]);
case 7: process(values[i++]);
case 6: process(values[i++]);
case 5: process(values[i++]);
case 4: process(values[i++]);
case 3: process(values[i++]);
case 2: process(values[i++]);
case 1: process(values[i++]);
}
startAt = 0;
} while (--iterations > 0);
This implementation of Duff ’ s device starts by calculating how many iterations through the loop need to
take place by dividing the total number of items in the values array by 8. The ceiling function is then
used to ensure that the result is a whole number. The startAt variable holds the number of items that
wouldn ’ t be processed if the iterations were based solely on dividing by 8. When the loop executes for
the first time, the startAt variable is checked to see how many extra calls should be made. For instance,
if there are 10 values in the array, startAt would be equal to 2, so process() would be called only
twice the first time through the loop. At the bottom of the loop, startAt is reset to 0 so that each
subsequent time through the loop results in eight calls to process() . This unrolling speeds up
processing of large datasets.
The book Speed Up Your Site by Andrew B. King (New Riders, 2003) proposed an even faster Duff ’ s
device technique that separated the do - while loop into two separate loops. Here’s an example:
//credit: Speed Up Your Site (New Riders, 2003)
var iterations = Math.floor(values.length / 8);
var leftover = values.length % 8;
var i = 0;
if (leftover > 0){
do {
process(values[i++]);
} while (--leftover > 0);
}
(continued)
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