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module
  barrel_shifter
  #(
    parameter W = 32,
    parameter D = W,
    parameter UB = $clog2(D)
  )
  (
    input  direction,       // right = 0, left = 1
    input  op,              // logical = 0, arithmetic = 1; left arithmetic not supported
    input [UB-1:0] amount,  // 0 not allowed
    input [W-1:0] data_in,
    output [W-1:0] data_out,
 
    input reset,
    input clk
  );
 
  // --------------------------------------------------------------------
  wire [W-1:0] out [UB:0];
  wire shift_in = direction & op ? data_in[W-1] : 0;
  genvar j, k;
 
  // --------------------------------------------------------------------
  generate
    for(k = 0; k < D; k = k + 1)
    begin : reversal
      assign out[5][k] = direction ? data_in[k] : data_in[D - 1 - k];
      assign data_out[k] = direction ? out[0][k] : out[0][D - 1 - k];
    end
  endgenerate
 
  // --------------------------------------------------------------------
  generate
    for(j = 0; j < UB; j = j + 1)
      for(k = 0; k < D; k = k + 1)
      begin : shifter
        if(k > D - 1 - (2**j))
          assign out[j][k] = amount[j] ? shift_in : out[j+1][k];
        else
          assign out[j][k] = amount[j] ? out[j+1][k + (2**j)] : out[j+1][k];
      end
  endgenerate
 
// --------------------------------------------------------------------
endmodule