code: function mod(A, B) {
    var C = 1, D = 0
    var _B = B
    if (B > A) return A
  
    //shift B right until it it's just smaller than A.
   while(B < A) {
      B<<=1; C<<=1
    }
    //now, shift B back, while subtracting.
    do {
      B>>=1; C>>=1
  
      //mark the bits where you could subtract.
      //this becomes the quotent!
      if(B < A) {
        D |= C; A = A - B
      }
      console.log('=', D.toString(2))
    } while(B > _B) 
    console.log(D, D.toString(2))
    return A
  }
docs: calculate A % B