Duff的设备在Swift中

概述我们知道 Duff’s device使用隔行开关和循环的结构交错,如： send(to, from, count)register short *to, *from;register count;{ register n = (count + 7) / 8; switch (count % 8) { case 0: do { *to = *from++; ca 我们知道 Duff’s device使用隔行开关和循环的结构交错,如：

send(to,from,count)register short *to,*from;register count;{    register n = (count + 7) / 8;    switch (count % 8) {    case 0: do { *to = *from++;    case 7:      *to = *from++;    case 6:      *to = *from++;    case 5:      *to = *from++;    case 4:      *to = *from++;    case 3:      *to = *from++;    case 2:      *to = *from++;    case 1:      *to = *from++;            } while (--n > 0);    }}

现在,在Swif 2.1中,switch-case control flows并没有像我们在Swift文档中读到的那样隐含起来：

No Implicit Fallthrough

In contrast with switch statements in C and Objective-C,switch
statements in Swift do not fall through the bottom of each case and
into the next one by default. Instead,the entire switch statement
finishes its execution as soon as the first matching switch case is
completed,without requiring an explicit break statement. This makes
the switch statement safer and easIEr to use than in C,and avoIDs
executing more than one switch case by mistake.

现在,鉴于在Swift中明确存在一个突破性副作用,有一个通过条款：

Fallthrough

Switch statements in Swift do not fall through the bottom of each case
and into the next one. Instead,the entire switch statement completes
its execution as soon as the first matching case is completed. By
contrast,C requires you to insert an explicit break statement at the
end of every switch case to prevent fallthrough. AvoIDing default
fallthrough means that Swift switch statements are much more concise
and predictable than their counterparts in C,and thus they avoID
executing multiple switch cases by mistake.

这非常像：

let integerToDescribe = 5var description = "The number \(integerToDescribe) is"switch integerToDescribe {case 2,3,5,7,11,13,17,19:    description += " a prime number,and also"    fallthroughdefault:    description += " an integer."}print(description)// prints "The number 5 is a prime number,and also an integer."

考虑到维基百科提醒我们,这些设备来自这个问题

A straightforward code to copy items from an array to a memory-mapped output register might look like this:do {                          /* count > 0 assumed */    *to = *from++;            /* "to" pointer is NOT incremented,see explanation below */} while(--count > 0);

哪个是Swift中Duff设备的确切实现？

这只是一种语言和编码问题,它不适用于真正的Swift应用程序.

您可以在最高级别的代码中表达您的意图,并信任Swift编译器为您优化它,而不是尝试自己优化它. Swift是一种高级语言.您不需要使用高级语言进行低级循环展开.

特别是在Swift中,您不必担心复制数组(Duff设备的原始应用程序),因为Swift假装在分配数组时使用“copy on write”复制数组.这意味着只要您只是从它们中读取它就会对两个变量使用相同的数组,但只要您修改其中一个变量,它就会在后台创建一个副本.

例如,从https://developer.apple.com/documentation/swift/array开始
修改数组的副本

Each array has an independent value that includes the values of allof its elements. For simple types such as integers and other structures,this means that when you change a value in one array,the value of thatelement does not change in any copIEs of the array. For example:var numbers = [1,2,4,5]var numberscopy = numbersnumbers[0] = 100print(numbers)// Prints "[100,5]"print(numberscopy)// Prints "[1,5]"

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