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3.4.2. &nbsp; GBK 字符集
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<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M10 20H6V4h7v5h5v3.1l2-2V8l-6-6H6c-1.1 0-2 .9-2 2v16c0 1.1.9 2 2 2h4v-2m10.2-7c.1 0 .3.1.4.2l1.3 1.3c.2.2.2.6 0 .8l-1 1-2.1-2.1 1-1c.1-.1.2-.2.4-.2m0 3.9L14.1 23H12v-2.1l6.1-6.1 2.1 2.1Z"/></svg>
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<h1 id="34">3.4. &nbsp; 字符集与编码<a class="headerlink" href="#34" title="Permanent link">&para;</a></h1>
<p>在计算机中,所有数据都是以二进制数的形式存储的,字符 <code>char</code> 也不例外。为了表示字符,我们需要建立一套「字符集」,规定每个字符和二进制数之间的一一对应关系。有了字符集之后,计算机就可以通过查表完成二进制数到字符的转换。</p>
<h2 id="341-ascii">3.4.1. &nbsp; ASCII 字符集<a class="headerlink" href="#341-ascii" title="Permanent link">&para;</a></h2>
<p>「ASCII 码」是最早出现的字符集,全称为“美国标准信息交换代码”。它使用 7 位二进制数(即一个字节的前 7 位)表示一个字符,最多能够表示 128 个不同的字符。这包括英文字母的大小写、数字 0-9 、一些标点符号,以及一些控制字符(如换行符和制表符)。</p>
<p><img alt="ASCII 码" src="../character_encoding.assets/ascii_table.png" /></p>
<p align="center"> Fig. ASCII 码 </p>
<p>然而ASCII 码仅局限于表示英文。随着计算机的全球化一种能够表示更多语言的字符集「EASCII」应运而生。它在 ASCII 的 7 位基础上扩展到 8 位,能够表示 256 个不同的字符。世界陆续诞生了一批适用于不同地区的 EASCII 字符集。这些字符集的前 128 个字符统一为 ASCII 码,后 128 个字符定义了不同语言的字符。</p>
<h2 id="342-gbk">3.4.2. &nbsp; GBK 字符集<a class="headerlink" href="#342-gbk" title="Permanent link">&para;</a></h2>
<p>EASCII 码仍然无法满足许多语言的字符数量要求。例如,汉字大约有近十万个,光日常使用的就有几千个。为此,中国国家标准总局于 1980 年发布了「GB2312」字符集其收录了 6763 个汉字,基本满足了汉字的计算机处理需要。</p>
<p>然而GB2312 无法处理部分的罕见字和繁体字。之后在 GB2312 的基础上扩展得到了「GBK」字符集它共收录了 21886 个汉字。在 GBK 编码方案中ASCII 字符使用一个字节表示,汉字使用两个字节表示。</p>
<h2 id="343-unicode">3.4.3. &nbsp; Unicode 字符集<a class="headerlink" href="#343-unicode" title="Permanent link">&para;</a></h2>
<p>随着计算机的蓬勃发展,在世界范围内诞生了许多字符集与编码标准,而这带来了许多问题。一方面,这些字符集一般只定义了某种特定语言的字符,无法实现跨语言解析;另一方面,同一种语言也存在多种字符集,如果两台电脑安装的是不同的编码标准,则在信息传递时就会出现乱码。</p>
<p>那个时代的人们会想:如果推出一个足够完整的字符集,将世界范围内的所有语言和符号都纳入其中,不就可以解决跨语言环境和乱码问题了吗?在这种思考下,一个庞大的字符集 Unicode 应运而生。</p>
<p>「Unicode」的全称为“统一字符编码”理论上能容纳一百多万个字符。它致力于将全球范围内的字符纳入到统一的字符集之中提供一种通用的字符集来处理和显示各种语言文字减少因为编码标准不同而产生的乱码问题。</p>
<p>自 1991 年发布以来Unicode 不断扩充新的语言与字符。截止 2022 年 9 月Unicode 已经包含 149186 个字符,包括各种语言的字符、符号、甚至是表情符号等。在庞大的 Unicode 字符集中,常用的字符占用 2 字节,有些生僻的字符占 3 字节甚至 4 字节。</p>
<p>Unicode 是一种字符集标准,本质上是给每个字符分配一个编号(称为“码点”),<strong>但它并没有规定在计算机中如何存储这些字符码点</strong>。我们不禁会问:当多种长度的 Unicode 码点同时出现在同一个文本中时,系统如何解析字符?例如,给定一个长度为 2 字节的编码,系统如何确认它是一个 2 字节的字符还是两个 1 字节的字符?</p>
<p><strong>最直接的解决方案是将所有字符存储为等长的编码</strong>。如下图所示“Hello”中的每个字符占用 1 字节,“算法”中的每个字符占用 2 字节。我们可以通过高位填 0 将“Hello 算法”中的所有字符都编码为 2 字节长度。这样系统就可以每隔 2 字节解析一个字符,恢复出这个短语的内容了。</p>
<p><img alt="Unicode 编码示例" src="../character_encoding.assets/unicode_hello_algo.png" /></p>
<p align="center"> Fig. Unicode 编码示例 </p>
<p>然而ASCII 码已经向我们证明,编码英文只需要 1 字节。若采用上述方案,英文文本占用空间的大小将会是 ASCII 编码下大小的 2 倍,非常浪费内存空间。因此,我们需要一种更加高效的 Unicode 编码方法。</p>
<h2 id="344-utf-8">3.4.4. &nbsp; UTF-8 编码<a class="headerlink" href="#344-utf-8" title="Permanent link">&para;</a></h2>
<p>随着互联网的发展UTF-8 成为国际上使用最广泛的 Unicode 编码方法。<strong>它是一种可变长的编码</strong>,使用 1 到 4 个字节来表示一个字符根据字符的复杂性而变。ASCII 字符只需要 1 个字节,拉丁字母和希腊字母需要 2 个字节,常用的中文字符需要 3 个字节,其他的一些生僻字符需要 4 个字节。UTF-8 的编码规则是:</p>
<ul>
<li>对于长度为 1 字节的字符,将最高位设置为 0 、其余 7 位设置为 Unicode 码点。值得注意的是ASCII 字符在 Unicode 字符集中占据了前 128 个码点。也就是说,<strong>UTF-8 编码可以向下兼容 ASCII 码</strong>。这意味着我们可以使用 UTF-8 来解析年代久远的 ASCII 码文本。</li>
<li>对于长度为 <span class="arithmatex">\(n\)</span> 字节的字符(其中 <span class="arithmatex">\(n &gt; 1\)</span>),将首个字节的高 <span class="arithmatex">\(n\)</span> 位都设置为 <span class="arithmatex">\(1\)</span> 、第 <span class="arithmatex">\(n + 1\)</span> 位设置为 <span class="arithmatex">\(0\)</span> ;从第二个字节开始,将每个字节的高 2 位都设置为 <span class="arithmatex">\(10\)</span> ;其余所有位用于填充字符的 Unicode 码点。</li>
</ul>
<p>下图展示了“Hello算法”对应的 UTF-8 编码。将最高 <span class="arithmatex">\(n\)</span> 位设置为 <span class="arithmatex">\(1\)</span> 比较容易理解,可以向系统指出字符的长度为 <span class="arithmatex">\(n\)</span> 。那么,为什么要将其余所有字节的高 2 位都设置为 <span class="arithmatex">\(10\)</span> 呢?实际上,这个 <span class="arithmatex">\(10\)</span> 能够起到校验符的作用,因为在 UTF-8 编码规则下,不可能有字符的最高两位是 <span class="arithmatex">\(10\)</span> 。这是因为长度为 1 字节的字符的最高一位是 <span class="arithmatex">\(0\)</span> 。假设系统从一个错误的字节开始解析文本,字节头部的 <span class="arithmatex">\(10\)</span> 能够帮助系统快速的判断出异常。</p>
<p><img alt="UTF-8 编码示例" src="../character_encoding.assets/utf-8_hello_algo.png" /></p>
<p align="center"> Fig. UTF-8 编码示例 </p>
<p>除了 UTF-8 之外,常见的编码方式还包括 UTF-16 和 UTF-32 。它们为 Unicode 字符集提供了不同的编码方法。</p>
<ul>
<li><strong>UTF-16 编码</strong>:使用 2 或 4 个字节来表示一个字符。所有的 ASCII 字符和很多常用的中文字符,都用 2 个字节表示。少数字符需要用到 4 个字节表示。</li>
<li><strong>UTF-32 编码</strong>:每个字符都使用 4 个字节。这意味着 UTF-32 会比 UTF-8 和 UTF-16 更占用空间,特别是对于主要使用 ASCII 字符的文本。</li>
</ul>
<p>从存储空间的角度看,使用 UTF-8 表示英文字符非常高效,因为它仅需 1 个字节;使用 UTF-16 编码某些非英文字符(例如中文)会更加高效,因为它只需要 2 个字节,而 UTF-8 可能需要 3 个字节。从兼容性的角度看UTF-8 的通用性最佳,许多工具和库都优先支持 UTF-8 。</p>
<p>如下表所示,为各个编程语言的字符串默认编码方式。由于 UTF-16 和 UTF-32 属于等长的编码方法,因此编程语言可以直接计算字符串的长度,也可以快速地访问字符串中的任意字符。而如果使用 UTF-8 这种变长的编码方法,编程语言往往需要额外维护一个字符数组,才能实现高效的随机访问。</p>
<div class="center-table">
<table>
<thead>
<tr>
<th>编码</th>
<th>编程语言</th>
</tr>
</thead>
<tbody>
<tr>
<td>UTF-8</td>
<td>Python, Go, Rust, Swift</td>
</tr>
<tr>
<td>UTF-16</td>
<td>Java, C#, JavaScript, TypeScript</td>
</tr>
<tr>
<td>UTF-32</td>
<td>/</td>
</tr>
</tbody>
</table>
</div>
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