// rr_arbiter.sv — N-way round-robin arbiter, combinational grant
module rr_arbiter #(
  parameter int N = 4
) (
  input  logic         clk,
  input  logic         rst,
  input  logic [N-1:0] req,
  output logic [N-1:0] gnt
);
  localparam int PW = (N > 1) ? $clog2(N) : 1;
  logic [PW-1:0] ptr;                    // requester with the highest priority

  // Grant the first requester at or after ptr (wrapping around)
  always_comb begin
    gnt = '0;
    for (int k = N - 1; k >= 0; k--) begin   // lowest k wins: last assignment sticks
      if (req[(int'(ptr) + k) % N]) begin
        gnt = '0;
        gnt[(int'(ptr) + k) % N] = 1'b1;
      end
    end
  end

  // After a grant, the requester just after the winner gets top priority
  always_ff @(posedge clk) begin
    if (rst) begin
      ptr <= '0;
    end else begin
      for (int i = 0; i < N; i++)
        if (gnt[i]) ptr <= PW'((i + 1) % N);
    end
  end
endmodule