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12.15 随机加权的条件 — randcase</div>
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<div class="Element13"><div class="Element12" id="code00471"><pre class="Element12">statement_item ::= <span style="color: #008000">// 引用自附录A.6.4</span>
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| randcase_statement
randcase_statement ::= <span style="color: #008000">// 引用自附录A.6.7</span>
<strong><span style="color: #FF0000">randcase</span></strong> randcase_item {randcase_item} <strong><span style="color: #FF0000">endcase</span></strong>
randcase_item ::= expression <strong><span style="color: #FF0000">:</span></strong> statement_or_null</pre></div></div>
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<strong>语法12-11 — randcase语法（摘录自附录A）</strong></p><p class="Element10">
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<span style="color: #0000FF">randcase</span>关键字引入了一个条件语句，它可以随机地选择它的一条分支。<span style="color: #0000FF">randcase</span>条目表达式是非负的integral值，它们构成了分支权重。一个条目的权重除以所有权重的和指定了该分支被选中的概率。例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00472"><pre class="Element12"><span style="color: #0000FF">randcase</span>
<span style="color: #FF0000">3</span> <span style="color: #800080">:</span> x <span style="color: #800080">=</span> <span style="color: #FF0000">1</span><span style="color: #800080">;</span>
<span style="color: #FF0000">1</span> <span style="color: #800080">:</span> x <span style="color: #800080">=</span> <span style="color: #FF0000">2</span><span style="color: #800080">;</span>
<span style="color: #FF0000">4</span> <span style="color: #800080">:</span> x <span style="color: #800080">=</span> <span style="color: #FF0000">3</span><span style="color: #800080">;</span>
<span style="color: #0000FF">endcase</span></pre></div></div>
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在这个例子中，所有权重的和为8，所以第一条分支被选中的概率为0.375，第二条分支被选中的概率为0.125，第三条语句被选中的概率为0.5。 </p>
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如果一条分支被指定了为0的权重，那么这条分支不会被选中。如果所有的randcase条目的权重都被指定为0，那么没有条目会被选中，并且可以发布一个警告信息。 </p>
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<span style="color: #0000FF">randcase</span>权重可以是任意表达式，而不仅仅是常量。例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00473"><pre class="Element12"><span style="color: #0000FF">byte</span> a<span style="color: #800080">,</span> b<span style="color: #800080">;</span>
<span style="color: #0000FF">randcase</span>
a <span style="color: #800080">+</span> b <span style="color: #800080">:</span> x <span style="color: #800080">=</span> <span style="color: #FF0000">1</span><span style="color: #800080">;</span>
a <span style="color: #800080">-</span> b <span style="color: #800080">:</span> x <span style="color: #800080">=</span> <span style="color: #FF0000">2</span><span style="color: #800080">;</span>
a <span style="color: #800080">^ ~</span>b <span style="color: #800080">:</span> x <span style="color: #800080">=</span> <span style="color: #FF0000">3</span><span style="color: #800080">;</span>
<span style="color: #FF0000">12</span><span style="color: #800080">'</span><span style="color: #0000FF">b</span><span style="color: #FF0000">800</span> <span style="color: #800080">:</span> x <span style="color: #800080">=</span> <span style="color: #FF0000">4</span><span style="color: #800080">;</span>
<span style="color: #0000FF">endcase</span></pre></div></div>
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每一个权重表达式的精度是依据自决的。权重的和使用标准的加法语义计算（所有权重的最大精度），其中被一个被加数都是无符号的。每一个权重表达式以一个未指定的顺序至多计算一次（具体的实现可以缓存相同的表达式）。在上面的例子中，前三个权重表达式使用8位的精度计算，第四个表达式使用12位精度计算；结果权重以12位精度以无符号值相加。权重选择接下来会使用无符号的12位比较。 </p>
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每一次对<span style="color: #0000FF">rancase</span>的调用都会在0到权重之和的范围内重新获得一个随机数。接下来权重以声明的顺序被选择：较小的随机数对应于第一条（最上面的）权重语句。 </p>
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rancase语句展示了线程稳定性。随机数从<span style="color: #0000FF">$urandom_range</span><span style="color: #800080">()</span>获得，因此随机数独立于线程执行的顺序。这能够导致对<span style="color: #0000FF">$urandom_range</span><span style="color: #800080">()</span>的多次调用以便处理大于32位的情况。</p></div>
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