这样做的 主要原因是基础实现和指令在C语言的声明中可以包装为一个大的switch表达式，如果使用手工的方式来完成无疑将会是非常痛苦的。
One of the primary reasons for this is primitives and instructions are wrapped in a huge switch statement in C, which is a pain to maintain by hand.
如果成千上万的计算全用手工来做的话，等做完的时候，其实用价值也就不存在了。
The millions of calculations involved, had they been done by hand, could have lost all practical value by the time they were finished.
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