// credit_counter.sv — tracks free buffer credits (contains a deliberate bug)
module credit_counter #(
  parameter int MAX = 12
) (
  input  logic       clk,
  input  logic       rst,       // synchronous, active-high
  input  logic       inc,       // a credit is returned
  input  logic       dec,       // a credit is consumed
  output logic [3:0] credits
);
  always_ff @(posedge clk) begin
    if (rst) begin
      credits <= 4'(MAX);           // all slots free after reset
    end else begin
      case ({inc, dec})
        2'b10:   if (credits != 4'(MAX)) credits <= credits + 4'd1;
        2'b01:   credits <= credits - 4'd1;    // BUG: no guard at zero
        default: credits <= credits;           // both or neither: no change
      endcase
    end
  end
endmodule