module axis_eth_fcs_insert #
(
    parameter ENABLE_PADDING = 0,
    parameter MIN_FRAME_LENGTH = 64
)
(
    input  wire        clk,
    input  wire        rst,
    
    /*
     * AXI input
     */
    input  wire [7:0]  s_axis_tdata,
    input  wire        s_axis_tvalid,
    output wire        s_axis_tready,
    input  wire        s_axis_tlast,
    input  wire        s_axis_tuser,
    
    /*
     * AXI output
     */
    output wire [7:0]  m_axis_tdata,
    output wire        m_axis_tvalid,
    input  wire        m_axis_tready,
    output wire        m_axis_tlast,
    output wire        m_axis_tuser,

    /*
     * Status
     */
    output wire        busy
);

localparam [1:0]
    STATE_IDLE = 2'd0,
    STATE_PAYLOAD = 2'd1,
    STATE_PAD = 2'd2,
    STATE_FCS = 2'd3;

reg [1:0] state_reg = STATE_IDLE, state_next;

// datapath control signals
reg reset_crc;
reg update_crc;

reg [15:0] frame_ptr_reg = 16'd0, frame_ptr_next;

reg busy_reg = 1'b0;

reg s_axis_tready_reg = 1'b0, s_axis_tready_next;

reg [31:0] crc_state = 32'hFFFFFFFF;
wire [31:0] crc_next;

// internal datapath
reg [7:0] m_axis_tdata_int;
reg       m_axis_tvalid_int;
reg       m_axis_tready_int_reg = 1'b0;
reg       m_axis_tlast_int;
reg       m_axis_tuser_int;
wire      m_axis_tready_int_early;

assign s_axis_tready = s_axis_tready_reg;

assign busy = busy_reg;

lfsr #(
    .LFSR_WIDTH(32),
    .LFSR_POLY(32'h4c11db7),
    .LFSR_CONFIG("GALOIS"),
    .LFSR_FEED_FORWARD(0),
    .REVERSE(1),
    .DATA_WIDTH(8),
    .STYLE("AUTO")
)
eth_crc_8 (
    .data_in(m_axis_tdata_int),
    .state_in(crc_state),
    .data_out(),
    .state_out(crc_next)
);

always @* begin
    state_next = STATE_IDLE;

    reset_crc = 1'b0;
    update_crc = 1'b0;

    frame_ptr_next = frame_ptr_reg;

    s_axis_tready_next = 1'b0;

    m_axis_tdata_int = 8'd0;
    m_axis_tvalid_int = 1'b0;
    m_axis_tlast_int = 1'b0;
    m_axis_tuser_int = 1'b0;

    case (state_reg)
        STATE_IDLE: begin
            // idle state - wait for data
            s_axis_tready_next = m_axis_tready_int_early;
            frame_ptr_next = 16'd0;
            reset_crc = 1'b1;

            m_axis_tdata_int = s_axis_tdata;
            m_axis_tvalid_int = s_axis_tvalid;
            m_axis_tlast_int = 1'b0;
            m_axis_tuser_int = 1'b0;

            if (s_axis_tready && s_axis_tvalid) begin
                frame_ptr_next = 16'd1;
                reset_crc = 1'b0;
                update_crc = 1'b1;
                if (s_axis_tlast) begin
                    if (s_axis_tuser) begin
                        m_axis_tlast_int = 1'b1;
                        m_axis_tuser_int = 1'b1;
                        reset_crc = 1'b1;
                        frame_ptr_next = 16'd0;
                        state_next = STATE_IDLE;
                    end else begin
                        s_axis_tready_next = 1'b0;
                        if (ENABLE_PADDING && frame_ptr_reg < MIN_FRAME_LENGTH-5) begin
                            state_next = STATE_PAD;
                        end else begin
                            frame_ptr_next = 16'd0;
                            state_next = STATE_FCS;
                        end
                    end
                end else begin
                    state_next = STATE_PAYLOAD;
                end
            end else begin
                state_next = STATE_IDLE;
            end
        end
        STATE_PAYLOAD: begin
            // transfer payload
            s_axis_tready_next = m_axis_tready_int_early;

            m_axis_tdata_int = s_axis_tdata;
            m_axis_tvalid_int = s_axis_tvalid;
            m_axis_tlast_int = 1'b0;
            m_axis_tuser_int = 1'b0;

            if (s_axis_tready && s_axis_tvalid) begin
                frame_ptr_next = frame_ptr_reg + 16'd1;
                update_crc = 1'b1;
                if (s_axis_tlast) begin
                    if (s_axis_tuser) begin
                        m_axis_tlast_int = 1'b1;
                        m_axis_tuser_int = 1'b1;
                        reset_crc = 1'b1;
                        frame_ptr_next = 16'd0;
                        state_next = STATE_IDLE;
                    end else begin
                        s_axis_tready_next = 1'b0;
                        if (ENABLE_PADDING && frame_ptr_reg < MIN_FRAME_LENGTH-5) begin
                            state_next = STATE_PAD;
                        end else begin
                            frame_ptr_next = 16'd0;
                            state_next = STATE_FCS;
                        end
                    end
                end else begin
                    state_next = STATE_PAYLOAD;
                end
            end else begin
                state_next = STATE_PAYLOAD;
            end
        end
        STATE_PAD: begin
            // insert padding
            s_axis_tready_next = 1'b0;

            m_axis_tdata_int = 8'd0;
            m_axis_tvalid_int = 1'b1;
            m_axis_tlast_int = 1'b0;
            m_axis_tuser_int = 1'b0;

            if (m_axis_tready_int_reg) begin
                frame_ptr_next = frame_ptr_reg + 16'd1;
                update_crc = 1'b1;
                if (frame_ptr_reg < MIN_FRAME_LENGTH-5) begin
                    state_next = STATE_PAD;
                end else begin
                    frame_ptr_next = 16'd0;
                    state_next = STATE_FCS;
                end
            end else begin
                state_next = STATE_PAD;
            end
        end
        STATE_FCS: begin
            // send FCS
            s_axis_tready_next = 1'b0;

            case (frame_ptr_reg)
                2'd0: m_axis_tdata_int = ~crc_state[7:0];
                2'd1: m_axis_tdata_int = ~crc_state[15:8];
                2'd2: m_axis_tdata_int = ~crc_state[23:16];
                2'd3: m_axis_tdata_int = ~crc_state[31:24];
            endcase
            m_axis_tvalid_int = 1'b1;
            m_axis_tlast_int = 1'b0;
            m_axis_tuser_int = 1'b0;

            if (m_axis_tready_int_reg) begin
                frame_ptr_next = frame_ptr_reg + 16'd1;

                if (frame_ptr_reg < 16'd3) begin
                    state_next = STATE_FCS;
                end else begin
                    reset_crc = 1'b1;
                    frame_ptr_next = 16'd0;
                    m_axis_tlast_int = 1'b1;
                    s_axis_tready_next = m_axis_tready_int_early;
                    state_next = STATE_IDLE;
                end
            end else begin
                state_next = STATE_FCS;
            end
        end
    endcase
end

always @(posedge clk) begin
    if (rst) begin
        state_reg <= STATE_IDLE;
        
        frame_ptr_reg <= 1'b0;
        
        s_axis_tready_reg <= 1'b0;

        busy_reg <= 1'b0;

        crc_state <= 32'hFFFFFFFF;
    end else begin
        state_reg <= state_next;

        frame_ptr_reg <= frame_ptr_next;

        s_axis_tready_reg <= s_axis_tready_next;

        busy_reg <= state_next != STATE_IDLE;

        // datapath
        if (reset_crc) begin
            crc_state <= 32'hFFFFFFFF;
        end else if (update_crc) begin
            crc_state <= crc_next;
        end
    end
end

// output datapath logic
reg [7:0] m_axis_tdata_reg = 8'd0;
reg       m_axis_tvalid_reg = 1'b0, m_axis_tvalid_next;
reg       m_axis_tlast_reg = 1'b0;
reg       m_axis_tuser_reg = 1'b0;

reg [7:0] temp_m_axis_tdata_reg = 8'd0;
reg       temp_m_axis_tvalid_reg = 1'b0, temp_m_axis_tvalid_next;
reg       temp_m_axis_tlast_reg = 1'b0;
reg       temp_m_axis_tuser_reg = 1'b0;

// datapath control
reg store_axis_int_to_output;
reg store_axis_int_to_temp;
reg store_axis_temp_to_output;

assign m_axis_tdata = m_axis_tdata_reg;
assign m_axis_tvalid = m_axis_tvalid_reg;
assign m_axis_tlast = m_axis_tlast_reg;
assign m_axis_tuser = m_axis_tuser_reg;

// enable ready input next cycle if output is ready or if both output registers are empty
assign m_axis_tready_int_early = m_axis_tready || (!temp_m_axis_tvalid_reg && !m_axis_tvalid_reg);

always @* begin
    // transfer sink ready state to source
    m_axis_tvalid_next = m_axis_tvalid_reg;
    temp_m_axis_tvalid_next = temp_m_axis_tvalid_reg;

    store_axis_int_to_output = 1'b0;
    store_axis_int_to_temp = 1'b0;
    store_axis_temp_to_output = 1'b0;
    
    if (m_axis_tready_int_reg) begin
        // input is ready
        if (m_axis_tready || !m_axis_tvalid_reg) begin
            // output is ready or currently not valid, transfer data to output
            m_axis_tvalid_next = m_axis_tvalid_int;
            store_axis_int_to_output = 1'b1;
        end else begin
            // output is not ready, store input in temp
            temp_m_axis_tvalid_next = m_axis_tvalid_int;
            store_axis_int_to_temp = 1'b1;
        end
    end else if (m_axis_tready) begin
        // input is not ready, but output is ready
        m_axis_tvalid_next = temp_m_axis_tvalid_reg;
        temp_m_axis_tvalid_next = 1'b0;
        store_axis_temp_to_output = 1'b1;
    end
end

always @(posedge clk) begin
    m_axis_tvalid_reg <= m_axis_tvalid_next;
    m_axis_tready_int_reg <= m_axis_tready_int_early;
    temp_m_axis_tvalid_reg <= temp_m_axis_tvalid_next;

    // datapath
    if (store_axis_int_to_output) begin
        m_axis_tdata_reg <= m_axis_tdata_int;
        m_axis_tlast_reg <= m_axis_tlast_int;
        m_axis_tuser_reg <= m_axis_tuser_int;
    end else if (store_axis_temp_to_output) begin
        m_axis_tdata_reg <= temp_m_axis_tdata_reg;
        m_axis_tlast_reg <= temp_m_axis_tlast_reg;
        m_axis_tuser_reg <= temp_m_axis_tuser_reg;
    end

    if (store_axis_int_to_temp) begin
        temp_m_axis_tdata_reg <= m_axis_tdata_int;
        temp_m_axis_tlast_reg <= m_axis_tlast_int;
        temp_m_axis_tuser_reg <= m_axis_tuser_int;
    end

    if (rst) begin
        m_axis_tvalid_reg <= 1'b0;
        m_axis_tready_int_reg <= 1'b0;
        temp_m_axis_tvalid_reg <= 1'b0;
    end
end

endmodule