C code to count the number of '1' bits in an unsigned char

Question

I need C code to return the number of 1\'s in an unsigned char in C. I need an explanation as to why it works if it\'s not obvious. I\'ve found a lot of code for a 32-bit nu

Answer 1

For a integer as small as an unsigned char you get best performance using a small lookup-table.

I know what population-count algorithms you're mentioning. They work by doing arithmetic of multiple words smaller than an integer stored in a register.

This technique is called SWAR (http://en.wikipedia.org/wiki/SWAR).

For more information I suggest you check out the hackers delight website: www.hackersdelight.org. He has example code and written a book that explains these tricks in detail.