Butterfly 4 inputs, 4 states


                         ctr1                   ctr0
                      ____|____              ____|____
                     |         |   s3       |         |
      in3 -----------| - -1- -  >-----------| - -1- -  >----- out3
                     |   \ /   |            |   \ /   |
                     |    0    |            |    0    |
                     |   / \   |   s2       |   / \   |
                  +--| - -1- -  >--+     +--| - -1- -  >--+
                  |  |_________|   |     |  |_________|   |
      in2 --+     +                +     +                +-- out2
             \   /                  \   /
              \ /                    \ /
               .                      .
              / \                    / \
             /   \       ctr1       /   \       ctr0
      in1 --+     +   ____|____    +     +   ____|____    +-- out1
                  |  |         |   |     |  |         |   |
                  +--| - -1- -  >--+     +--| - -1- -  >--+
                     |   \ /   |   s1       |   \ /   |
                     |    0    |            |    0    |
                     |   / \   |            |   / \   |
      in0 -----------| - -1- -  >-----------| - -1- -  >----- out0
                     |_________|   s0       |_________|



      crt1  ctr0        out3  out2   out1  out0

         0     0    ->   in0   in1    in2   in3
         0     1    ->   in1   in0    in3   in2
         1     0    ->   in2   in3    in0   in1
         1     1    ->   in3   in2    in1   in0




 Butterfly4  as a Dynamic Element Matching Driver ?


 -----------------------------------------------------
|  ctr          |    '00'     '01'     '10'     '11'  |
 -----------------------------------------------------
|  in   'dcba'  |  'abcd'   'badc'   'cdab'   'dcba'  |
 -----------------------------------------------------


 -----------------------------------------------------
|  ctr          |    '00'     '01'     '10'     '11'  |
 -----------------------------------------------------
|  in   '0000'  |  '0000'   '0000'   '0000'   '0000'  |
|  in   '0001'  |  '1000'   '0100'   '0010'   '0001'  |
|  in   '0011'  |  '1100'   '1100'   '0011'   '0011'  |
|  in   '0111'  |  '1110'   '1101'   '1011'   '0111'  |
|  in   '1111'  |  '1111'   '1111'   '1111'   '1111'  |
 -----------------------------------------------------


Center of Gravity is OK.