Sum of bit differences among all pairs

Given an integer array of n integers, find sum of bit differences in all pairs that can be formed from array elements. Bit difference of a pair (x, y) is count of different bits at same positions in binary representations of x and y. 
For example, bit difference for 2 and 7 is 2. Binary representation of 2 is 010 and 7 is 111 ( first and last bits differ in two numbers).

Examples :  

Input: arr[] = {1, 2}
Output: 4
All pairs in array are (1, 1), (1, 2)
                      (2, 1), (2, 2)
Sum of bit differences = 0 + 2 +
                        2 + 0
                     = 4

Sum of bit differences among all pairs : https://www.geeksforgeeks.org/sum-of-bit-differences-among-all-pairs/