// SPDX-License-Identifier: MIT
module packet_gate #(parameter DEBUG = 0)(
  input clk, reset, request, release_slot, debug_request,
  input [3:0] length,
  output accepted, debug_accepted, full, empty, impossible,
  output [1:0] occupancy
);
  reg [1:0] used = 0;
  assign full = used == 3;
  assign empty = used == 0;
  assign impossible = full && empty;
  assign occupancy = used;
  assign accepted = request && !full && length != 0;
  assign debug_accepted = DEBUG && debug_request && accepted;
  always @(posedge clk) begin
    if (reset) used <= 0;
    else if (accepted && !release_slot) used <= used + 1'b1;
    else if (release_slot && !empty && !accepted) used <= used - 1'b1;
  end
endmodule