module fifo_address_gray (
  input m_axis_aclk,
  input m_axis_aresetn,
  input m_axis_ready,
  output reg m_axis_valid,
  output reg [ADDRESS_WIDTH:0] m_axis_level,

  input s_axis_aclk,
  input s_axis_aresetn,
  output reg s_axis_ready,
  input s_axis_valid,
  output reg s_axis_empty,
  output [ADDRESS_WIDTH-1:0] s_axis_waddr,
  output reg [ADDRESS_WIDTH:0] s_axis_room
);

parameter ADDRESS_WIDTH = 4;

reg [ADDRESS_WIDTH:0] _s_axis_waddr = 'h00;
reg [ADDRESS_WIDTH:0] _s_axis_waddr_next;

reg [ADDRESS_WIDTH:0] _m_axis_raddr = 'h00;
reg [ADDRESS_WIDTH:0] _m_axis_raddr_next;

reg [ADDRESS_WIDTH:0] s_axis_waddr_gray = 'h00;
wire [ADDRESS_WIDTH:0] s_axis_waddr_gray_next;
wire [ADDRESS_WIDTH:0] s_axis_raddr_gray;

reg [ADDRESS_WIDTH:0] m_axis_raddr_gray = 'h00;
wire [ADDRESS_WIDTH:0] m_axis_raddr_gray_next;
wire [ADDRESS_WIDTH:0] m_axis_waddr_gray;

assign s_axis_waddr = _s_axis_waddr[ADDRESS_WIDTH-1:0];

always @(*)
begin
  if (s_axis_ready && s_axis_valid)
    _s_axis_waddr_next <= _s_axis_waddr + 1;
  else
    _s_axis_waddr_next <= _s_axis_waddr;
end

assign s_axis_waddr_gray_next = _s_axis_waddr_next ^ _s_axis_waddr_next[ADDRESS_WIDTH:1];

always @(posedge s_axis_aclk)
begin
  if (s_axis_aresetn == 1'b0) begin
    _s_axis_waddr <= 'h00;
    s_axis_waddr_gray <= 'h00;
  end else begin
    _s_axis_waddr <= _s_axis_waddr_next;
    s_axis_waddr_gray <= s_axis_waddr_gray_next;
  end
end

always @(*)
begin
  if (m_axis_ready && m_axis_valid)
    _m_axis_raddr_next <= _m_axis_raddr + 1;
  else
    _m_axis_raddr_next <= _m_axis_raddr;
end

assign m_axis_raddr_gray_next = _m_axis_raddr_next ^ _m_axis_raddr_next[ADDRESS_WIDTH:1];

always @(posedge m_axis_aclk)
begin
  if (m_axis_aresetn == 1'b0) begin
    _m_axis_raddr <= 'h00;
    m_axis_raddr_gray <= 'h00;
  end else begin
    _m_axis_raddr <= _m_axis_raddr_next;
    m_axis_raddr_gray <= m_axis_raddr_gray_next;
  end
end

sync_bits #(
  .NUM_OF_BITS(ADDRESS_WIDTH + 1)
) i_waddr_sync (
  .out_clk(m_axis_aclk),
  .out_resetn(m_axis_aresetn),
  .in(s_axis_waddr_gray),
  .out(m_axis_waddr_gray)
);

sync_bits #(
  .NUM_OF_BITS(ADDRESS_WIDTH + 1)
) i_raddr_sync (
  .out_clk(s_axis_aclk),
  .out_resetn(s_axis_aresetn),
  .in(m_axis_raddr_gray),
  .out(s_axis_raddr_gray)
);

always @(posedge s_axis_aclk)
begin
  if (s_axis_aresetn == 1'b0) begin
    s_axis_ready <= 1'b1;
    s_axis_empty <= 1'b1;
  end else begin
    s_axis_ready <= (s_axis_raddr_gray[ADDRESS_WIDTH] == s_axis_waddr_gray_next[ADDRESS_WIDTH] ||
      s_axis_raddr_gray[ADDRESS_WIDTH-1] == s_axis_waddr_gray_next[ADDRESS_WIDTH-1] ||
      s_axis_raddr_gray[ADDRESS_WIDTH-2:0] != s_axis_waddr_gray_next[ADDRESS_WIDTH-2:0]);
    s_axis_empty <= s_axis_raddr_gray == s_axis_waddr_gray_next;
  end
end

always @(posedge m_axis_aclk)
begin
  if (s_axis_aresetn == 1'b0)
    m_axis_valid <= 1'b0;
  else begin
    m_axis_valid <= m_axis_waddr_gray != m_axis_raddr_gray_next;
  end
end

endmodule