尾随/前导零计数为一个字节 [英] Trailing/leading zero count for a byte

问题描述

我正在使用Java，并且正在编写国际象棋引擎.

I'm using Java and I'm coding a chess engine.

我正在尝试查找字节中前1位的索引和后1位的索引.

I'm trying to find the index of the first 1 bit and the index of the last 1 bit in a byte.

我目前在Java中使用Long.numberOfTrailingZeros()(或类似的东西)，并且希望模拟该功能，但字节除外.

I'm currently using Long.numberOfTrailingZeros() (or something like that) in Java, and would like to emulate that functionality, except with bytes.

会是这样吗?

byte b = 0b011000101;
int firstOneBit = bitCount ((b & -b) - 1);

如果是这样，我将如何相对有效地实现bitCount.我不介意解释，请不要只给我代码.

If so, how would I implement bitCount relatively efficiently. I don't mind good explainations, please don't just give me code.

推荐答案

使用具有256个条目的查找表. 创建它:

use a lookup tabel with 256 entries. to create it:

unsigned int bitcount ( unsigned int i ) {
unsigned int r = 0;
while ( i ) { r+=i&1; i>>=1; } /* bit shift is >>> in java afair */
return r; 
}

这当然不需要太快，因为您最多可以执行256次来初始化表格.

this of course does not need to be fast as you do it at most 256 times to init your tabel.

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