`define ASSERT(expr) begin if (!(expr)) begin $display("FAIL"); $finish; end end

module priority_encoder_v1(input [3:0] in, output [1:0] out, output v);
  assign v = |in;
  assign out = (in[3] == 1) ? 3 :
               (in[2] == 1) ? 2 :
               (in[1] == 1) ? 1 :
                              0;
endmodule

module priority_encoder_v2(input [3:0] in, output reg [1:0] out, output reg v);
  always @* begin
    v = |in;
    out = (in[3] == 1) ? 3 :
          (in[2] == 1) ? 2 :
          (in[1] == 1) ? 1 :
                         0;
  end
endmodule

module testbench #(parameter SIZE=4);
  reg [SIZE:0] in; // 1 bit bigger
  wire [1:0] out;
  wire v;

  priority_encoder_v1 pe(in[SIZE-1:0], out, v);

  initial begin
    for (in = 0; in < (1<<SIZE); in = in + 1) begin
      #1 $display("in=%d, out=%d, v=%d", in, out, v);
      if (in == 0)
        `ASSERT(v === 0)
      else begin
        `ASSERT(v === 1)
        if (in >= 4'b1000)
          `ASSERT(out === 3)
        else if (in >= 4'b0100)
          `ASSERT(out === 2)
        else if (in >= 4'b0010)
          `ASSERT(out === 1)
        else if (in >= 4'b0001)
          `ASSERT(out === 0)
      end
    end
  end
endmodule