问题描述
我很感兴趣，这是通过这种方式计算字节中设置的位数的最佳方式
I am interested, which is the optimal way of calculating the number of bits set in byte by this way
template< unsigned char byte > class BITS_SET
{
public:
enum {
B0 = (byte & 0x01) ? 1:0,
B1 = (byte & 0x02) ? 1:0,
B2 = (byte & 0x04) ? 1:0,
B3 = (byte & 0x08) ? 1:0,
B4 = (byte & 0x10) ? 1:0,
B5 = (byte & 0x20) ? 1:0,
B6 = (byte & 0x40) ? 1:0,
B7 = (byte & 0x80) ? 1:0
};
public:
enum{RESULT = B0+B1+B2+B3+B4+B5+B6+B7};
};
也许在运行时知道字节值是最佳的?是否建议在代码中使用它?
Maybe it is optimal when value of byte is known at run-time? Is it recommended use this in code?
推荐答案
对于 8 位值，只需使用 256 个元素的查找表.
For 8-bit values, just use a 256-element lookup table.
对于较大尺寸的输入，它稍微不那么琐碎.Sean Eron Anderson 在他的 Bit Twiddling Hacks 页面上为此提供了几个不同的功能，所有这些功能都具有不同的性能特征.没有一个是最快的版本，因为它取决于您的处理器的性质(流水线深度、分支预测器、缓存大小等)和您使用的数据.
For larger sized inputs, it's slightly less trivial. Sean Eron Anderson has several different functions for this on his Bit Twiddling Hacks page, all with different performance characteristics. There is not one be-all-end-all-fastest version, since it depends on the nature of your processor (pipeline depth, branch predictor, cache size, etc.) and the data you're using.
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