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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>
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<h2 id="65-示例-bit数组"><a class="header" href="#65-示例-bit数组">6.5. 示例: Bit数组</a></h2>
<p>Go语言里的集合一般会用map[T]bool这种形式来表示T代表元素类型。集合用map类型来表示虽然非常灵活但我们可以以一种更好的形式来表示它。例如在数据流分析领域集合元素通常是一个非负整数集合会包含很多元素并且集合会经常进行并集、交集操作这种情况下bit数组会比map表现更加理想。译注这里再补充一个例子比如我们执行一个http下载任务把文件按照16kb一块划分为很多块需要有一个全局变量来标识哪些块下载完成了这种时候也需要用到bit数组。</p>
<p>一个bit数组通常会用一个无符号数或者称之为“字”的slice来表示每一个元素的每一位都表示集合里的一个值。当集合的第i位被设置时我们才说这个集合包含元素i。下面的这个程序展示了一个简单的bit数组类型并且实现了三个函数来对这个bit数组来进行操作</p>
<p><u><i>gopl.io/ch6/intset</i></u></p>
<pre><code class="language-go">// An IntSet is a set of small non-negative integers.
// Its zero value represents the empty set.
type IntSet struct {
words []uint64
}
// Has reports whether the set contains the non-negative value x.
func (s *IntSet) Has(x int) bool {
word, bit := x/64, uint(x%64)
return word &lt; len(s.words) &amp;&amp; s.words[word]&amp;(1&lt;&lt;bit) != 0
}
// Add adds the non-negative value x to the set.
func (s *IntSet) Add(x int) {
word, bit := x/64, uint(x%64)
for word &gt;= len(s.words) {
s.words = append(s.words, 0)
}
s.words[word] |= 1 &lt;&lt; bit
}
// UnionWith sets s to the union of s and t.
func (s *IntSet) UnionWith(t *IntSet) {
for i, tword := range t.words {
if i &lt; len(s.words) {
s.words[i] |= tword
} else {
s.words = append(s.words, tword)
}
}
}
</code></pre>
<p>因为每一个字都有64个二进制位所以为了定位x的bit位我们用了x/64的商作为字的下标并且用x%64得到的值作为这个字内的bit的所在位置。UnionWith这个方法里用到了bit位的“或”逻辑操作符号|来一次完成64个元素的或计算。在练习6.5中我们还会有程序用到这个64位字的例子。</p>
<p>当前这个实现还缺少了很多必要的特性我们把其中一些作为练习题列在本小节之后。但是有一个方法如果缺失的话我们的bit数组可能会比较难混将IntSet作为一个字符串来打印。这里我们来实现它让我们来给上面的例子添加一个String方法类似2.5节中做的那样:</p>
<pre><code class="language-go">// String returns the set as a string of the form &quot;{1 2 3}&quot;.
func (s *IntSet) String() string {
var buf bytes.Buffer
buf.WriteByte('{')
for i, word := range s.words {
if word == 0 {
continue
}
for j := 0; j &lt; 64; j++ {
if word&amp;(1&lt;&lt;uint(j)) != 0 {
if buf.Len() &gt; len(&quot;{&quot;) {
buf.WriteByte(' ')
}
fmt.Fprintf(&amp;buf, &quot;%d&quot;, 64*i+j)
}
}
}
buf.WriteByte('}')
return buf.String()
}
</code></pre>
<p>这里留意一下String方法是不是和3.5.4节中的intsToString方法很相似bytes.Buffer在String方法里经常这么用。当你为一个复杂的类型定义了一个String方法时fmt包就会特殊对待这种类型的值这样可以让这些类型在打印的时候看起来更加友好而不是直接打印其原始的值。fmt会直接调用用户定义的String方法。这种机制依赖于接口和类型断言在第7章中我们会详细介绍。</p>
<p>现在我们就可以在实战中直接用上面定义好的IntSet了</p>
<pre><code class="language-go">var x, y IntSet
x.Add(1)
x.Add(144)
x.Add(9)
fmt.Println(x.String()) // &quot;{1 9 144}&quot;
y.Add(9)
y.Add(42)
fmt.Println(y.String()) // &quot;{9 42}&quot;
x.UnionWith(&amp;y)
fmt.Println(x.String()) // &quot;{1 9 42 144}&quot;
fmt.Println(x.Has(9), x.Has(123)) // &quot;true false&quot;
</code></pre>
<p>这里要注意我们声明的String和Has两个方法都是以指针类型<code>*IntSet</code>来作为接收器的但实际上对于这两个类型来说把接收器声明为指针类型也没什么必要。不过另外两个函数就不是这样了因为另外两个函数操作的是s.words对象如果你不把接收器声明为指针对象那么实际操作的是拷贝对象而不是原来的那个对象。因此因为我们的String方法定义在IntSet指针上所以当我们的变量是IntSet类型而不是IntSet指针时可能会有下面这样让人意外的情况</p>
<pre><code class="language-go">fmt.Println(&amp;x) // &quot;{1 9 42 144}&quot;
fmt.Println(x.String()) // &quot;{1 9 42 144}&quot;
fmt.Println(x) // &quot;{[4398046511618 0 65536]}&quot;
</code></pre>
<p>在第一个Println中我们打印一个<code>*IntSet</code>的指针这个类型的指针确实有自定义的String方法。第二Println我们直接调用了x变量的String()方法这种情况下编译器会隐式地在x前插入&amp;操作符这样相当于我们还是调用的IntSet指针的String方法。在第三个Println中因为IntSet类型没有String方法所以Println方法会直接以原始的方式理解并打印。所以在这种情况下&amp;符号是不能忘的。在我们这种场景下你把String方法绑定到IntSet对象上而不是IntSet指针上可能会更合适一些不过这也需要具体问题具体分析。</p>
<p><strong>练习6.1:</strong> 为bit数组实现下面这些方法</p>
<pre><code class="language-go">func (*IntSet) Len() int // return the number of elements
func (*IntSet) Remove(x int) // remove x from the set
func (*IntSet) Clear() // remove all elements from the set
func (*IntSet) Copy() *IntSet // return a copy of the set
</code></pre>
<p><strong>练习 6.2</strong> 定义一个变参方法(*IntSet).AddAll(...int)这个方法可以添加一组IntSet比如s.AddAll(1,2,3)。</p>
<p><strong>练习 6.3</strong> (*IntSet).UnionWith会用<code>|</code>操作符计算两个集合的并集我们再为IntSet实现另外的几个函数IntersectWith交集元素在A集合B集合均出现DifferenceWith差集元素出现在A集合未出现在B集合SymmetricDifference并差集元素出现在A但没有出现在B或者出现在B没有出现在A</p>
<p><strong>练习 6.4</strong> 实现一个Elems方法返回集合中的所有元素用于做一些range之类的遍历操作。</p>
<p><strong>练习 6.5</strong> 我们这章定义的IntSet里的每个字都是用的uint64类型但是64位的数值可能在32位的平台上不高效。修改程序使其使用uint类型这种类型对于32位平台来说更合适。当然了这里我们可以不用简单粗暴地除64可以定义一个常量来决定是用32还是64这里你可能会用到平台的自动判断的一个智能表达式32 &lt;&lt; (^uint(0) &gt;&gt; 63)</p>
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