我如何以更优化的方式提取出位？ [英] How do I extract a bit in a more optimal way?

问题描述

今天我有一个采访中，他们让我写两个C功能，一来提取单个位和其他从一个字符提取范围位。我花了一段时间，这些方法出现了。

I had a interview today where they asked me to write two "C" functions, one to to extract a single bit and other to extract a range of bits from a character. I took a while and came up with these methods.

int extractBit(char byte, int pos) {
    assert( (pos >= 0) && (pos < 8) );
    return ( ( byte & (1<<pos) ) >> pos);
}
char extractBitRange(char byte, int startingPos, int offset) {
   assert( ( (startingPos + offset) >= 0) && ( (startingPos + offset) < 8) );
   return ( byte >> startingPos ) & ~(0xff << (offset + 1));
}

但面试官一直问我，如果我能进一步加快code（CPU周期而言），如果有优化，我可以做些什么来实现它的任何范围。
我显然是不舒服的，我很想知道你会怎么做呢？

But the interviewer kept asking me if I could speed up the code further (in terms of cpu cycles) and if there is any scope of optimization that I could do to achieve it. I was clearly out of sorts and I am curious to know how would you do this?

推荐答案

在extractBit，如果你转移首先，你可以用 1 掩盖而不是（1 LT;＆LT; POS）。
考虑到POS是函数的参数，即节省了计算。

In extractBit, if you shift first, you can mask with 1 instead of (1<<pos). Considering that pos is an argument of the function, that saves a computation.

收益率（字节＆GT;＆GT; POS）及1;

在第二个功能，我就断言， startingPos 和偏移都是正的，而不是声称，他们的总和是积极的，更有意义的方式。

In the second function, I would assert that startingPos and offset are both positive instead of asserting that their sum is positive, it makes more sense that way.

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