bitwise operations and bitmasks.txt

1- How does numbers are saved?
  Binary Representation.

Boolean algebra.

What's bitwise operations?

    operation          operator
      and                 &
      or                  |
      Xor                 ^
      invert              ~
      left shift          <<
      right shift         >>

- How to get all the possible permutations of k(e.g k = 3) bits?

  for(int msk = 0 ; msk < (1<<k) ; ++msk){
    // Anything
  }


- How to check oddness of a variable x?

  if(x&1)
    printf("It's Odd\n");
  else
    printf("It's Even\n");


- How to check the state of the bit at position k of some mask?

  if(msk&(1<<k))
    printf("It's ON");
  else
    printf("It's OFF");


- How to ON the bit at position k of some mask?

  msk |= (1<<k);


- How to ON the first n bits of some mask?

  msk |= ((1<<n)-1);


- How to toggle the bit at position k of some mask?

  msk ^= (1<<k);


- How to OFF the bit at position k of some mask?

  msk &= ~(1<<k);


- How to get the most right ON bit of some mask?

  rightON = (msk&(-msk));

  
- How to count the number of ON bits in some mask of n bits?

  int num = 0;
  for(int j = 0 ; j < n ; ++j)
    if(msk&(1<<j))
      ++num;