计算位数的3种方法

方法1：

int count_bits(unsigned int data) {
  int cnt=0;
  
  while(data!=0) {
    data=data&(data-1);
    cnt++;
  }
  return cnt;
}

方法2:(为一个字节打表）

static unsigned char byte_bit_count[256];

void initialize_count_bits() {
  int cnt,i,data;
  for(i=0;i<256;i++) {
    cnt=0;
    data=i;
    while(data!=0) {
      data=data&(data-1);
      cnt++;
    }
    byte_bit_count[i]=cnt;
  }
}

int count_bits(unsigned int data)
{
  const unsigned char *byte=(unsigned char*)&data;
  return byte_bit_count[byte[0]]+byte_bit_count[byte[1]]+
           byte_bit_count[byte[2]]+byte_bit_count[byte[3]];
}

方法3:(计算一个字种所有位(0,1)的和)

int count_bits(unsigned int x)
{
  static unsigned int mask[]={0x55555555,
                                           0x33333333,
                                           0x0F0F0F0F,
                                           0x00FF00FF,
                                           0x0000FFFF};
  int i;
  int shift;

  for(i=0,shift=1;i<5;i++,shift*=2)
    x=(x&mask[i])+((x>>shift)&mask[i]);
  return x;
}