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<main>
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<h1 id="警惕-utf-8-引发的性能隐患"><a class="header" href="#警惕-utf-8-引发的性能隐患">警惕 UTF-8 引发的性能隐患</a></h1>
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<p>大家应该都知道, 虽然 Rust 的字符串 <code>&str</code>、<code>String</code> 在底层是通过 <code>Vec<u8></code> 实现的：字符串数据以字节数组的形式存在堆上，但在使用时，它们都是 UTF-8 编码的，例如:</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
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let s: &str = "中国人";
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for c in s.chars() {
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println!("{}", c) // 依次输出：中 、 国 、 人
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}
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let c = &s[0..3]; // 1. "中" 在 UTF-8 中占用 3 个字节 2. Rust 不支持字符串索引，因此只能通过切片的方式获取 "中"
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assert_eq!(c, "中");
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}</code></pre></pre>
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<p>从上述代码，可以很清晰看出，Rust 的字符串确实是 UTF-8 编码的，这就带来一个隐患：可能在某个转角，你就会遇到来自糟糕性能的示爱。</p>
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<h2 id="问题描述--解决"><a class="header" href="#问题描述--解决">问题描述 & 解决</a></h2>
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<p>例如我们尝试写一个词法解析器，里面用到了以下代码 <code>self.source.chars().nth(self.index).unwrap();</code> 去获取下一个需要处理的字符，大家可能会以为 <code>.nth</code> 的访问应该非常快吧？事实上它确实很快，但是并不妨碍这段代码在循环处理 70000 长度的字符串时，需要消耗 5s 才能完成！</p>
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<p>这么看来，唯一的问题就在于 <code>.chars()</code> 上了。</p>
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<p>其实原因很简单，简单到我们不需要用代码来说明，只需要文字描述即可传达足够的力量：每一次循环时，<code>.chars().nth(index)</code> 都需要对字符串进行一次 UTF-8 解析，这个解析实际上是相当昂贵的，特别是当配合循环时，算法的复杂度就是平方级的。</p>
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<p>既然找到原因，那解决方法也很简单：只要将 <code>self.source.chars()</code> 的迭代器存储起来就行，这样每次 <code>.nth</code> 调用都会复用已经解析好的迭代器，而不是重新去解析一次 UTF-8 字符串。</p>
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<p>当然，我们还可以使用三方库来解决这个问题，例如 <a href="https://crates.io/crates/str_indices">str_indices</a>。</p>
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<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
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<p>最终的优化结果如下：</p>
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<ul>
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<li>保存迭代器后: 耗时 <code>5s</code> -> <code>4ms</code></li>
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<li>进一步使用 <code>u8</code> 字节数组来替换 <code>char</code>，最后使用 <code>String::from_utf8</code> 来构建 UTF-8 字符串: 耗时 <code>4ms</code> -> <code>400us</code></li>
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</ul>
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<p><strong>肉眼可见的巨大提升，12500 倍！</strong></p>
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<p>总之，我们在热点路径中使用字符串做 UTF-8 的相关操作时，就算不提前优化，也要做到心里有数，这样才能在问题发生时，进退自如。</p>
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