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executed

iTestValue = 0;

} while (--iLoopCount > 0);

The variable

iIterations

contains the number of times that the loop that should be executed. The

variable

iLoopCount

contains the number of times it is necessary to repeat the

do..while

loop. The

iTestValue

variable represents which

case

in the

switch

statement should be executed the very first

time the loop is entered; every other time, execution starts with

case 0

and runs through all the rest

(notice that no

break

statements stop execution after a

case

statement is executed). If you apply Duff’s

Device to the previous example, this is the result:

var aValues = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20];

var iIterations = aValues.length;

var iLoopCount = Math.ceil(iIterations / 8);

var iTestValue = iIterations % 8;

var iSum = 0;

var i = 0;

do {

switch (iTestValue) {

case 0: iSum+= aValues[i++];

case 7: iSum+= aValues[i++];

case 6: iSum+= aValues[i++];

case 5: iSum+= aValues[i++];

case 4: iSum+= aValues[i++];

case 3: iSum+= aValues[i++];

case 2: iSum+= aValues[i++];

case 1: iSum+= aValues[i++];

}

iTestValue = 0;

} while (--iLoopCount > 0);

Notice that

iIterations

has been set to the number of values in the array, and the

iSum

and

variables

have been added. The

iSum

variable, of course, holds the result of adding all the numbers in the array; the

variable is used as an iterator to move through the array (just as with the

for

and

do..while

loops).

Before the loop is executed,

iLoopCount

is equal to

, meaning that the loop is executed three times. The

value of

iTestValue

is equal to

, so when execution enters the first loop, it skips down to case 4 in the

switch statement. After those four lines are executed,

iTestValue

is set back to

. From that point on,

whenever the loop executes, all cases beginning at

case 0

are executed.

Greenburg further optimized his JavaScript version of Duff’s Device by splitting the single

do..while

loop into two separate loops. The algorithm is as follows:

var iLoopCount = iIterations % 8;

while (iLoopCount--) {

[execute statement]

}

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