Programming Pearl 9 Chapter 7th obtain the number of 1 in a byte sequence

First, for each byte, obtain the number of 1 in this byte. Of course, the unsiged char value cannot be converted to the binary number, and then the number of 1 is counted. This method is very inefficient, this will think of bit operation, a good method of bit operation is to use the method of X & = X-1, how many times the calculation, how many 1, then for a byte, store the number of 1 in each number (0-255) into an array, so that the number of 1 in each byte can be directly found.

Int get_count (unsigned char X) {int COUNT = 0; while (x> 0) {X & = X-1; count ++;} return count ;} static unsigned char hash_table [256]; void pre_process (void) {unsigned char I; for (I = 0; I <256; I ++) hash_table [I] = get_count (I );}