package com.interview.bits;
/**
* http://www.geeksforgeeks.org/count-set-bits-in-an-integer/
*/
public class CountBits {
public CountBits(){
preCalculate();
}
public int countBits(int num){
int count=0;
while(num > 0){
num &= num-1;
count++;
}
return count;
}
private int count[] = new int[256];
void preCalculate(){
for(int i=0; i < 256; i++){
count[i] = countBits(i);
}
}
public int countBitsFaster(int num){
//get 8 bits at a time and check count from count array
int total = 0;
int mask = (1<<8) - 1;
for(int i=0 ; i < 4; i++){
total += count[num & mask];
num = num>>>8;
}
return total;
}
//http://bits.stephan-brumme.com/countBits.html
public int countBitsEvenFaster(int x){
// count bits of each 2-bit chunk
x = x - ((x >> 1) & 0x55555555);
// count bits of each 4-bit chunk
x = (x & 0x33333333) + ((x >> 2) & 0x33333333);
// count bits of each 8-bit chunk
x = x + (x >> 4);
// mask out junk
x &= 0xF0F0F0F;
// add all four 8-bit chunks
return (x * 0x01010101) >> 24;
}
public static void main(String args[]){
CountBits cb = new CountBits();
System.out.println(cb.countBits(3636363));
System.out.println(cb.countBitsFaster(3636363));
System.out.println(cb.countBitsEvenFaster(3636363));
}
}