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92 lines
3.5 KiB
HTML
92 lines
3.5 KiB
HTML
<article id="wikiArticle">
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<div></div>
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<h2 id="Summary" name="Summary">概述</h2>
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<p>该函数返回两个参数的类C的32位整数乘法运算的运算结果.</p>
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<h2 id="语法">语法</h2>
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<pre class="syntaxbox"><code>Math.imul(a, b)</code></pre>
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<h3 id="Parameters" name="Parameters">参数</h3>
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<dl>
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<dt><code>a</code></dt>
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<dd>被乘数.</dd>
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<dt><code>b</code></dt>
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<dd>乘数.</dd>
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</dl>
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<h2 id="描述">描述</h2>
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<p><code>Math.imul</code>可以进行快速的,类C语义的32位整数乘法.该特性对于一些项目比如<a class="external" href="http://en.wikipedia.org/wiki/Emscripten" rel="noopener" title="http://en.wikipedia.org/wiki/Emscripten">Emscripten</a>很有用.</p>
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<h2 id="例子">例子</h2>
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<pre class="brush: js">Math.imul(2, 4) // 8
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Math.imul(-1, 8) // -8
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Math.imul(-2, -2) // 4
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Math.imul(0xffffffff, 5) //-5
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Math.imul(0xfffffffe, 5) //-10
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</pre>
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<h2 id="Polyfill">Polyfill</h2>
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<p>下面的JavaScript代码可以实现该函数:</p>
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<pre class="brush: js">function imul(a, b)
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{
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var ah = (a >>> 16) & 0xffff;
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var al = a & 0xffff;
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var bh = (b >>> 16) & 0xffff;
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var bl = b & 0xffff;
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// 右移0位可以修复高位的符号位
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return (al * bl) + (((ah*bl+al*bh)<<16)>>>0);
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}
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</pre>
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<h2 id="浏览器兼容性">浏览器兼容性</h2>
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<p></p><div class="blockIndicator warning"><strong><a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">We're converting our compatibility data into a machine-readable JSON format</a></strong>.
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This compatibility table still uses the old format,
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because we haven't yet converted the data it contains.
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<strong><a class="new" href="/zh-CN/docs/MDN/Contribute/Structures/Compatibility_tables" rel="nofollow">Find out how you can help!</a></strong></div>
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<div class="htab">
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<a id="AutoCompatibilityTable" name="AutoCompatibilityTable"></a>
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<ul>
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<li class="selected"><a>Desktop</a></li>
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<li><a>Mobile</a></li>
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</ul>
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</div><p></p>
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<div id="compat-desktop">
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<table class="compat-table">
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<tbody>
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<tr>
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<th>Feature</th>
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<th>Firefox (Gecko)</th>
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<th>Chrome</th>
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<th>Internet Explorer</th>
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<th>Opera</th>
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<th>Safari</th>
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</tr>
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<tr>
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<td>Basic support</td>
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<td><a href="/en-US/Firefox/Releases/20" title="Released on 2013-04-02.">20</a> (20)</td>
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<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
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<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
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<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
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<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
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</tr>
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</tbody>
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</table>
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</div>
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<div id="compat-mobile">
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<table class="compat-table">
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<tbody>
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<tr>
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<th>Feature</th>
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<th>Firefox Mobile (Gecko)</th>
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<th>Android</th>
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<th>IE Mobile</th>
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<th>Opera Mobile</th>
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<th>Safari Mobile</th>
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</tr>
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<tr>
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<td>Basic support</td>
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<td>20.0 (20)</td>
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<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
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<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
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<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
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<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
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</tr>
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</tbody>
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</table>
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</div>
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<p> </p>
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</article> |