// SPDX-License-Identifier: MIT
`timescale 1ns/1ps
module tb;
  bit clk = 0;
  bit reset = 1;
  bit valid = 0;
  bit ready = 0;
  bit pending = 0;
  bit [1:0] sent_id = 0;
  bit [1:0] received_id = 0;
  bit [7:0] payload = 0;
  bit previous_stall = 0;
  bit [7:0] held_payload = 0;
  int mode = 0;
  int transfers = 0;
  int outstanding = 0;
  int wait_cycles = 0;
  int progress_alarms = 0;
  int id_errors = 0;
  int limit_errors = 0;
  localparam int WAIT_BUDGET = 4;
  localparam int ACCEPT_CAPACITY = 1;
  always #5 clk = ~clk;

  initial begin
    if (!$value$plusargs("MODE=%d", mode)) mode = 0;
    repeat (2) @(negedge clk);
    reset = 0;
    pending = 1;
    payload = 8'h5a;
    sent_id = mode == 2 ? 2'd2 : 2'd0;
    $display("MODE %0d budget=%0d capacity=%0d", mode,
      WAIT_BUDGET, ACCEPT_CAPACITY);
    for (int cycle = 0; cycle < 8; cycle++) begin
      @(negedge clk);
      if (mode == 1) begin
        // Deliberately illegal source waits for READY.
        valid = pending && ready;
        // Receiver is allowed to wait for VALID.
        ready = valid;
      end else begin
        valid = pending;
        ready = cycle >= 2;
      end
      received_id = mode == 2 ? {1'b0, sent_id[0]} : sent_id;
      @(posedge clk);
      $display("TRACE cycle=%0d pending=%0b valid=%0b ready=%0b id=%0d wire_id=%0d data=%02x",
        cycle, pending, valid, ready, sent_id, received_id, payload);
      if (previous_stall && (!valid || payload != held_payload))
        $fatal(1, "Stalled payload or VALID changed");
      previous_stall = valid && !ready;
      held_payload = payload;
      if (valid && ready) begin
        transfers++;
        outstanding++;
        wait_cycles = 0;
        if (received_id != sent_id) begin
          id_errors++;
          $display("ID_MISMATCH sent=%0d received=%0d", sent_id, received_id);
        end
        if (outstanding > ACCEPT_CAPACITY) begin
          limit_errors++;
          $display("OUTSTANDING_LIMIT count=%0d", outstanding);
        end
        if (mode == 3 && transfers == 1) begin
          // A second request arrives before any response.
          pending = 1;
          sent_id = 2'd1;
        end else pending = 0;
      end else if (pending) begin
        wait_cycles++;
        if (wait_cycles == WAIT_BUDGET) begin
          progress_alarms++;
          $display("BOUNDED_WAIT cycle=%0d waited=%0d", cycle, wait_cycles);
        end
      end
      if (cycle == 6) outstanding = 0;
    end
    $display("RESULT transfers=%0d bounded_wait=%0d id_errors=%0d limit_errors=%0d",
      transfers, progress_alarms, id_errors, limit_errors);
    if (mode == 0 && (transfers != 1 || progress_alarms != 0))
      $fatal(1, "Legal backpressure result differs");
    if (mode == 1 && (transfers != 0 || progress_alarms != 1))
      $fatal(1, "Expected deadlock alarm");
    if (mode == 2 && id_errors != 1) $fatal(1, "Expected ID mismatch");
    if (mode == 3 && limit_errors != 1) $fatal(1, "Expected capacity mismatch");
    $finish;
  end
endmodule