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<h1 class="menu-title">Go语言圣经</h1>
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<li><em>KusonStack一站式可编程配置技术栈(Go): <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</a></em></li>
<li><em>KCL 配置编程语言(Rust): <a href="https://github.com/KusionStack/KCLVM">https://github.com/KusionStack/KCLVM</a></em></li>
<li><em>凹语言™: <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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<hr>
<h2 id="76-sortinterface接口"><a class="header" href="#76-sortinterface接口">7.6. sort.Interface接口</a></h2>
<p>排序操作和字符串格式化一样是很多程序经常使用的操作。尽管一个最短的快排程序只要15行就可以搞定但是一个健壮的实现需要更多的代码并且我们不希望每次我们需要的时候都重写或者拷贝这些代码。</p>
<p>幸运的是sort包内置的提供了根据一些排序函数来对任何序列排序的功能。它的设计非常独到。在很多语言中排序算法都是和序列数据类型关联同时排序函数和具体类型元素关联。相比之下Go语言的sort.Sort函数不会对具体的序列和它的元素做任何假设。相反它使用了一个接口类型sort.Interface来指定通用的排序算法和可能被排序到的序列类型之间的约定。这个接口的实现由序列的具体表示和它希望排序的元素决定序列的表示经常是一个切片。</p>
<p>一个内置的排序算法需要知道三个东西序列的长度表示两个元素比较的结果一种交换两个元素的方式这就是sort.Interface的三个方法</p>
<pre><code class="language-go">package sort
type Interface interface {
Len() int
Less(i, j int) bool // i, j are indices of sequence elements
Swap(i, j int)
}
</code></pre>
<p>为了对序列进行排序我们需要定义一个实现了这三个方法的类型然后对这个类型的一个实例应用sort.Sort函数。思考对一个字符串切片进行排序这可能是最简单的例子了。下面是这个新的类型StringSlice和它的LenLess和Swap方法</p>
<pre><code class="language-go">type StringSlice []string
func (p StringSlice) Len() int { return len(p) }
func (p StringSlice) Less(i, j int) bool { return p[i] &lt; p[j] }
func (p StringSlice) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
</code></pre>
<p>现在我们可以通过像下面这样将一个切片转换为一个StringSlice类型来进行排序</p>
<pre><code class="language-go">sort.Sort(StringSlice(names))
</code></pre>
<p>这个转换得到一个相同长度容量和基于names数组的切片值并且这个切片值的类型有三个排序需要的方法。</p>
<p>对字符串切片的排序是很常用的需要所以sort包提供了StringSlice类型也提供了Strings函数能让上面这些调用简化成sort.Strings(names)。</p>
<p>这里用到的技术很容易适用到其它排序序列中例如我们可以忽略大小写或者含有的特殊字符。本书使用Go程序对索引词和页码进行排序也用到了这个技术对罗马数字做了额外逻辑处理。对于更复杂的排序我们使用相同的方法但是会用更复杂的数据结构和更复杂地实现sort.Interface的方法。</p>
<p>我们会运行上面的例子来对一个表格中的音乐播放列表进行排序。每个track都是单独的一行每一列都是这个track的属性像艺术家标题和运行时间。想象一个图形用户界面来呈现这个表格并且点击一个属性的顶部会使这个列表按照这个属性进行排序再一次点击相同属性的顶部会进行逆向排序。让我们看下每个点击会发生什么响应。</p>
<p>下面的变量tracks包含了一个播放列表。One of the authors apologizes for the other authors musical tastes.每个元素都不是Track本身而是指向它的指针。尽管我们在下面的代码中直接存储Tracks也可以工作sort函数会交换很多对元素所以如果每个元素都是指针而不是Track类型会更快指针是一个机器字码长度而Track类型可能是八个或更多。</p>
<p><u><i>gopl.io/ch7/sorting</i></u></p>
<pre><code class="language-go">type Track struct {
Title string
Artist string
Album string
Year int
Length time.Duration
}
var tracks = []*Track{
{&quot;Go&quot;, &quot;Delilah&quot;, &quot;From the Roots Up&quot;, 2012, length(&quot;3m38s&quot;)},
{&quot;Go&quot;, &quot;Moby&quot;, &quot;Moby&quot;, 1992, length(&quot;3m37s&quot;)},
{&quot;Go Ahead&quot;, &quot;Alicia Keys&quot;, &quot;As I Am&quot;, 2007, length(&quot;4m36s&quot;)},
{&quot;Ready 2 Go&quot;, &quot;Martin Solveig&quot;, &quot;Smash&quot;, 2011, length(&quot;4m24s&quot;)},
}
func length(s string) time.Duration {
d, err := time.ParseDuration(s)
if err != nil {
panic(s)
}
return d
}
</code></pre>
<p>printTracks函数将播放列表打印成一个表格。一个图形化的展示可能会更好点但是这个小程序使用text/tabwriter包来生成一个列整齐对齐和隔开的表格像下面展示的这样。注意到<code>*tabwriter.Writer</code>是满足io.Writer接口的。它会收集每一片写向它的数据它的Flush方法会格式化整个表格并且将它写向os.Stdout标准输出</p>
<pre><code class="language-go">func printTracks(tracks []*Track) {
const format = &quot;%v\t%v\t%v\t%v\t%v\t\n&quot;
tw := new(tabwriter.Writer).Init(os.Stdout, 0, 8, 2, ' ', 0)
fmt.Fprintf(tw, format, &quot;Title&quot;, &quot;Artist&quot;, &quot;Album&quot;, &quot;Year&quot;, &quot;Length&quot;)
fmt.Fprintf(tw, format, &quot;-----&quot;, &quot;------&quot;, &quot;-----&quot;, &quot;----&quot;, &quot;------&quot;)
for _, t := range tracks {
fmt.Fprintf(tw, format, t.Title, t.Artist, t.Album, t.Year, t.Length)
}
tw.Flush() // calculate column widths and print table
}
</code></pre>
<p>为了能按照Artist字段对播放列表进行排序我们会像对StringSlice那样定义一个新的带有必须的LenLess和Swap方法的切片类型。</p>
<pre><code class="language-go">type byArtist []*Track
func (x byArtist) Len() int { return len(x) }
func (x byArtist) Less(i, j int) bool { return x[i].Artist &lt; x[j].Artist }
func (x byArtist) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
</code></pre>
<p>为了调用通用的排序程序我们必须先将tracks转换为新的byArtist类型它定义了具体的排序</p>
<pre><code class="language-go">sort.Sort(byArtist(tracks))
</code></pre>
<p>在按照artist对这个切片进行排序后printTrack的输出如下</p>
<pre><code>Title Artist Album Year Length
----- ------ ----- ---- ------
Go Ahead Alicia Keys As I Am 2007 4m36s
Go Delilah From the Roots Up 2012 3m38s
Ready 2 Go Martin Solveig Smash 2011 4m24s
Go Moby Moby 1992 3m37s
</code></pre>
<p>如果用户第二次请求“按照artist排序”我们会对tracks进行逆向排序。然而我们不需要定义一个有颠倒Less方法的新类型byReverseArtist因为sort包中提供了Reverse函数将排序顺序转换成逆序。</p>
<pre><code class="language-go">sort.Sort(sort.Reverse(byArtist(tracks)))
</code></pre>
<p>在按照artist对这个切片进行逆向排序后printTrack的输出如下</p>
<pre><code>Title Artist Album Year Length
----- ------ ----- ---- ------
Go Moby Moby 1992 3m37s
Ready 2 Go Martin Solveig Smash 2011 4m24s
Go Delilah From the Roots Up 2012 3m38s
Go Ahead Alicia Keys As I Am 2007 4m36s
</code></pre>
<p>sort.Reverse函数值得进行更近一步的学习因为它使用了§6.3章中的组合这是一个重要的思路。sort包定义了一个不公开的struct类型reverse它嵌入了一个sort.Interface。reverse的Less方法调用了内嵌的sort.Interface值的Less方法但是通过交换索引的方式使排序结果变成逆序。</p>
<pre><code class="language-go">package sort
type reverse struct{ Interface } // that is, sort.Interface
func (r reverse) Less(i, j int) bool { return r.Interface.Less(j, i) }
func Reverse(data Interface) Interface { return reverse{data} }
</code></pre>
<p>reverse的另外两个方法Len和Swap隐式地由原有内嵌的sort.Interface提供。因为reverse是一个不公开的类型所以导出函数Reverse返回一个包含原有sort.Interface值的reverse类型实例。</p>
<p>为了可以按照不同的列进行排序我们必须定义一个新的类型例如byYear</p>
<pre><code class="language-go">type byYear []*Track
func (x byYear) Len() int { return len(x) }
func (x byYear) Less(i, j int) bool { return x[i].Year &lt; x[j].Year }
func (x byYear) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
</code></pre>
<p>在使用sort.Sort(byYear(tracks))按照年对tracks进行排序后printTrack展示了一个按时间先后顺序的列表</p>
<pre><code>Title Artist Album Year Length
----- ------ ----- ---- ------
Go Moby Moby 1992 3m37s
Go Ahead Alicia Keys As I Am 2007 4m36s
Ready 2 Go Martin Solveig Smash 2011 4m24s
Go Delilah From the Roots Up 2012 3m38s
</code></pre>
<p>对于我们需要的每个切片元素类型和每个排序函数我们需要定义一个新的sort.Interface实现。如你所见Len和Swap方法对于所有的切片类型都有相同的定义。下个例子具体的类型customSort会将一个切片和函数结合使我们只需要写比较函数就可以定义一个新的排序。顺便说下实现了sort.Interface的具体类型不一定是切片类型customSort是一个结构体类型。</p>
<pre><code class="language-go">type customSort struct {
t []*Track
less func(x, y *Track) bool
}
func (x customSort) Len() int { return len(x.t) }
func (x customSort) Less(i, j int) bool { return x.less(x.t[i], x.t[j]) }
func (x customSort) Swap(i, j int) { x.t[i], x.t[j] = x.t[j], x.t[i] }
</code></pre>
<p>让我们定义一个多层的排序函数它主要的排序键是标题第二个键是年第三个键是运行时间Length。下面是该排序的调用其中这个排序使用了匿名排序函数</p>
<pre><code class="language-go">sort.Sort(customSort{tracks, func(x, y *Track) bool {
if x.Title != y.Title {
return x.Title &lt; y.Title
}
if x.Year != y.Year {
return x.Year &lt; y.Year
}
if x.Length != y.Length {
return x.Length &lt; y.Length
}
return false
}})
</code></pre>
<p>这下面是排序的结果。注意到两个标题是“Go”的track按照标题排序是相同的顺序但是在按照year排序上更久的那个track优先。</p>
<pre><code>Title Artist Album Year Length
----- ------ ----- ---- ------
Go Moby Moby 1992 3m37s
Go Delilah From the Roots Up 2012 3m38s
Go Ahead Alicia Keys As I Am 2007 4m36s
Ready 2 Go Martin Solveig Smash 2011 4m24s
</code></pre>
<p>尽管对长度为n的序列排序需要 O(n log n)次比较操作检查一个序列是否已经有序至少需要n-1次比较。sort包中的IsSorted函数帮我们做这样的检查。像sort.Sort一样它也使用sort.Interface对这个序列和它的排序函数进行抽象但是它从不会调用Swap方法这段代码示范了IntsAreSorted和Ints函数在IntSlice类型上的使用</p>
<pre><code class="language-go">values := []int{3, 1, 4, 1}
fmt.Println(sort.IntsAreSorted(values)) // &quot;false&quot;
sort.Ints(values)
fmt.Println(values) // &quot;[1 1 3 4]&quot;
fmt.Println(sort.IntsAreSorted(values)) // &quot;true&quot;
sort.Sort(sort.Reverse(sort.IntSlice(values)))
fmt.Println(values) // &quot;[4 3 1 1]&quot;
fmt.Println(sort.IntsAreSorted(values)) // &quot;false&quot;
</code></pre>
<p>为了使用方便sort包为[]int、[]string和[]float64的正常排序提供了特定版本的函数和类型。对于其他类型例如[]int64或者[]uint尽管路径也很简单还是依赖我们自己实现。</p>
<p><strong>练习 7.8</strong> 很多图形界面提供了一个有状态的多重排序表格插件主要的排序键是最近一次点击过列头的列第二个排序键是第二最近点击过列头的列等等。定义一个sort.Interface的实现用在这样的表格中。比较这个实现方式和重复使用sort.Stable来排序的方式。</p>
<p><strong>练习 7.9</strong> 使用html/template包§4.6替代printTracks将tracks展示成一个HTML表格。将这个解决方案用在前一个练习中让每次点击一个列的头部产生一个HTTP请求来排序这个表格。</p>
<p><strong>练习 7.10</strong> sort.Interface类型也可以适用在其它地方。编写一个IsPalindrome(s sort.Interface) bool函数表明序列s是否是回文序列换句话说反向排序不会改变这个序列。假设如果!s.Less(i, j) &amp;&amp; !s.Less(j, i)则索引i和j上的元素相等。</p>
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