Formula
----------------------------------

/* This is just the idea how to build the module, likely it contains some
   errors  and some details that have not been worked through */
/* basic idea partition the switches in each level to odd and even ''fleets''
   where:
   all fleets are of size 2**i
   even fleets  X-like inputs comes from odd fleets of the previous level.
   odd fleets  X-like inputs comes from even fleets of the previous level.
*/

module butterfly(IL,IR,OL,OR)
{
n=2**k
input IL[n-1:0];
input IR[n-1:0];
output OL[n-1:0];
output OR[n-1:0];
wire WL[k:-1][n-1:0]
wire WR[k:-1][n-1:0]

assign WL[-1]=IL;
assign WR[-1]=IR;
assign OL=WL[k];
assign OR=WR[k];

/* levels */
genfor(i=0;i<k;i++)
 /* jumps over 2*fleets */
 genfor(j=0;j<n;j+=2**{i+1) )
 {
   /* even fleet */
   genfor(x=j;x<j+2**i;x++)
   {
     switch  #(addressbit=i) B(.LI(WL[i-1][x]),.RI(WR[i-1][x+2**i],.LO(WL[i][x],.RO(WR[i][x]....)
   }
   /* odd fleet */
   genfor(y=j+2**i;y<j+2**(i+1);y++)
   {
     switch  #(addressbit=i) B(.RI(WR[i-1][x]),.LI(WL[i-1][x-2**i],.LO(WL[i][x],.RO(WR[i][x]....)
   }
 }

endmodule