Implement a Mealy-type finite state machine that recognizes the sequence
"101" on an input signal named x. Your FSM should have an output signal,
z, that is asserted to logic-1 when the "101" sequence is detected. Your
FSM should also have a negative edge triggered asynchronous reset. You
may only have 3 states in your state machine. Your FSM should recognize
overlapping sequences.

module TopModule (
  input clk,
  input aresetn,
  input x,
  output reg z
);


Reference solution:

module RefModule (
  input clk,
  input aresetn,
  input x,
  output reg z
);

  parameter S=0, S1=1, S10=2;
  reg[1:0] state, next;

  always@(posedge clk, negedge aresetn)
    if (!aresetn)
      state <= S;
    else
      state <= next;

  always_comb begin
    case (state)
      S: next = x ? S1 : S;
      S1: next = x ? S1 : S10;
      S10: next = x ? S1 : S;
      default: next = 'x;
    endcase
  end

  always_comb begin
    case (state)
      S: z = 0;
      S1: z = 0;
      S10: z = x;
      default: z = 'x;
    endcase
  end

endmodule


Testbench:
`timescale 1 ps/1 ps
`define OK 12
`define INCORRECT 13


module stimulus_gen (
	input clk,
	output logic aresetn,
	output logic x,
	output reg[511:0] wavedrom_title,
	output reg wavedrom_enable,
	input tb_match
);
	reg reset;
	assign aresetn = ~reset;


// Add two ports to module stimulus_gen:
//    output [511:0] wavedrom_title
//    output reg wavedrom_enable

	task wavedrom_start(input[511:0] title = "");
	endtask
	
	task wavedrom_stop;
		#1;
	endtask	


	task reset_test(input async=0);
		bit arfail, srfail, datafail;
	
		@(posedge clk);
		@(posedge clk) reset <= 0;
		repeat(3) @(posedge clk);
	
		@(negedge clk) begin datafail = !tb_match ; reset <= 1; end
		@(posedge clk) arfail = !tb_match;
		@(posedge clk) begin
			srfail = !tb_match;
			reset <= 0;
		end
		if (srfail)
			$display("Hint: Your reset doesn't seem to be working.");
		else if (arfail && (async || !datafail))
			$display("Hint: Your reset should be %0s, but doesn't appear to be.", async ? "asynchronous" : "synchronous");
		// Don't warn about synchronous reset if the half-cycle before is already wrong. It's more likely
		// a functionality error than the reset being implemented asynchronously.
	
	endtask


	initial begin
		x <= 0;
		repeat(3) @(posedge clk);
		@(posedge clk) x <= 1;
		@(posedge clk) x <= 0;
		@(posedge clk) x <= 1;
	end
	
	initial begin
		reset <= 1;
		@(posedge clk) reset <= 0;
		reset_test(1);
		
		@(negedge clk) wavedrom_start();
			@(posedge clk) x <= 0;
			@(posedge clk) x <= 0;
			@(posedge clk) x <= 0;
			@(posedge clk) x <= 1;
			@(posedge clk) x <= 0;
			@(posedge clk) x <= 1;
			@(posedge clk) x <= 0;
			@(posedge clk) x <= 1;
			@(posedge clk) x <= 1;
			@(posedge clk) x <= 0;
			@(posedge clk) x <= 1;
			@(posedge clk) x <= 0;
		@(negedge clk) wavedrom_stop();

		repeat(400) @(posedge clk, negedge clk) begin
			x <= $random;
			reset <= ($random&31) == 0;
		end
		
		$finish;
	end
	
endmodule

module tb();

	typedef struct packed {
		int errors;
		int errortime;
		int errors_z;
		int errortime_z;

		int clocks;
	} stats;
	
	stats stats1;
	
	
	wire[511:0] wavedrom_title;
	wire wavedrom_enable;
	int wavedrom_hide_after_time;
	
	reg clk=0;
	initial forever
		#5 clk = ~clk;

	logic aresetn;
	logic x;
	logic z_ref;
	logic z_dut;

	initial begin 
		$dumpfile("wave.vcd");
		$dumpvars(1, stim1.clk, tb_mismatch ,clk,aresetn,x,z_ref,z_dut );
	end


	wire tb_match;		// Verification
	wire tb_mismatch = ~tb_match;
	
	stimulus_gen stim1 (
		.clk,
		.* ,
		.aresetn,
		.x );
	RefModule good1 (
		.clk,
		.aresetn,
		.x,
		.z(z_ref) );
		
	TopModule top_module1 (
		.clk,
		.aresetn,
		.x,
		.z(z_dut) );

	
	bit strobe = 0;
	task wait_for_end_of_timestep;
		repeat(5) begin
			strobe <= !strobe;  // Try to delay until the very end of the time step.
			@(strobe);
		end
	endtask	

	
	final begin
		if (stats1.errors_z) $display("Hint: Output '%s' has %0d mismatches. First mismatch occurred at time %0d.", "z", stats1.errors_z, stats1.errortime_z);
		else $display("Hint: Output '%s' has no mismatches.", "z");

		$display("Hint: Total mismatched samples is %1d out of %1d samples\n", stats1.errors, stats1.clocks);
		$display("Simulation finished at %0d ps", $time);
		$display("Mismatches: %1d in %1d samples", stats1.errors, stats1.clocks);
	end
	
	// Verification: XORs on the right makes any X in good_vector match anything, but X in dut_vector will only match X.
	assign tb_match = ( { z_ref } === ( { z_ref } ^ { z_dut } ^ { z_ref } ) );
	// Use explicit sensitivity list here. @(*) causes NetProc::nex_input() to be called when trying to compute
	// the sensitivity list of the @(strobe) process, which isn't implemented.
	always @(posedge clk, negedge clk) begin

		stats1.clocks++;
		if (!tb_match) begin
			if (stats1.errors == 0) stats1.errortime = $time;
			stats1.errors++;
		end
		if (z_ref !== ( z_ref ^ z_dut ^ z_ref ))
		begin if (stats1.errors_z == 0) stats1.errortime_z = $time;
			stats1.errors_z = stats1.errors_z+1'b1; end

	end

   // add timeout after 100K cycles
   initial begin
     #1000000
     $display("TIMEOUT");
     $finish();
   end

endmodule