module udp_checksum_gen #
(
    parameter PAYLOAD_FIFO_DEPTH = 2048,
    parameter HEADER_FIFO_DEPTH = 8
)
(
    input  wire        clk,
    input  wire        rst,
    
    /*
     * UDP frame input
     */
    input  wire        s_udp_hdr_valid,
    output wire        s_udp_hdr_ready,
    input  wire [47:0] s_eth_dest_mac,
    input  wire [47:0] s_eth_src_mac,
    input  wire [15:0] s_eth_type,
    input  wire [3:0]  s_ip_version,
    input  wire [3:0]  s_ip_ihl,
    input  wire [5:0]  s_ip_dscp,
    input  wire [1:0]  s_ip_ecn,
    input  wire [15:0] s_ip_identification,
    input  wire [2:0]  s_ip_flags,
    input  wire [12:0] s_ip_fragment_offset,
    input  wire [7:0]  s_ip_ttl,
    input  wire [15:0] s_ip_header_checksum,
    input  wire [31:0] s_ip_source_ip,
    input  wire [31:0] s_ip_dest_ip,
    input  wire [15:0] s_udp_source_port,
    input  wire [15:0] s_udp_dest_port,
    input  wire [7:0]  s_udp_payload_axis_tdata,
    input  wire        s_udp_payload_axis_tvalid,
    output wire        s_udp_payload_axis_tready,
    input  wire        s_udp_payload_axis_tlast,
    input  wire        s_udp_payload_axis_tuser,
    
    /*
     * UDP frame output
     */
    output wire        m_udp_hdr_valid,
    input  wire        m_udp_hdr_ready,
    output wire [47:0] m_eth_dest_mac,
    output wire [47:0] m_eth_src_mac,
    output wire [15:0] m_eth_type,
    output wire [3:0]  m_ip_version,
    output wire [3:0]  m_ip_ihl,
    output wire [5:0]  m_ip_dscp,
    output wire [1:0]  m_ip_ecn,
    output wire [15:0] m_ip_length,
    output wire [15:0] m_ip_identification,
    output wire [2:0]  m_ip_flags,
    output wire [12:0] m_ip_fragment_offset,
    output wire [7:0]  m_ip_ttl,
    output wire [7:0]  m_ip_protocol,
    output wire [15:0] m_ip_header_checksum,
    output wire [31:0] m_ip_source_ip,
    output wire [31:0] m_ip_dest_ip,
    output wire [15:0] m_udp_source_port,
    output wire [15:0] m_udp_dest_port,
    output wire [15:0] m_udp_length,
    output wire [15:0] m_udp_checksum,
    output wire [7:0]  m_udp_payload_axis_tdata,
    output wire        m_udp_payload_axis_tvalid,
    input  wire        m_udp_payload_axis_tready,
    output wire        m_udp_payload_axis_tlast,
    output wire        m_udp_payload_axis_tuser,
    
    /*
     * Status signals
     */
    output wire        busy
);

/*

UDP Frame

 Field                       Length
 Destination MAC address     6 octets
 Source MAC address          6 octets
 Ethertype (0x0800)          2 octets
 Version (4)                 4 bits
 IHL (5-15)                  4 bits
 DSCP (0)                    6 bits
 ECN (0)                     2 bits
 length                      2 octets
 identification (0?)         2 octets
 flags (010)                 3 bits
 fragment offset (0)         13 bits
 time to live (64?)          1 octet
 protocol                    1 octet
 header checksum             2 octets
 source IP                   4 octets
 destination IP              4 octets
 options                     (IHL-5)*4 octets

 source port                 2 octets
 desination port             2 octets
 length                      2 octets
 checksum                    2 octets

 payload                     length octets

This module receives a UDP frame with header fields in parallel and payload on
an AXI stream interface, calculates the length and checksum, then produces the
header fields in parallel along with the UDP payload in a separate AXI stream.

*/

parameter HEADER_FIFO_ADDR_WIDTH = $clog2(HEADER_FIFO_DEPTH);

localparam [2:0]
    STATE_IDLE = 3'd0,
    STATE_SUM_HEADER_1 = 3'd1,
    STATE_SUM_HEADER_2 = 3'd2,
    STATE_SUM_HEADER_3 = 3'd3,
    STATE_SUM_PAYLOAD = 3'd4,
    STATE_FINISH_SUM = 3'd5;

reg [2:0] state_reg = STATE_IDLE, state_next;

// datapath control signals
reg store_udp_hdr;
reg shift_payload_in;
reg [31:0] checksum_part;

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

reg [31:0] checksum_reg = 32'd0, checksum_next;

reg [47:0] eth_dest_mac_reg = 48'd0;
reg [47:0] eth_src_mac_reg = 48'd0;
reg [15:0] eth_type_reg = 16'd0;
reg [3:0]  ip_version_reg = 4'd0;
reg [3:0]  ip_ihl_reg = 4'd0;
reg [5:0]  ip_dscp_reg = 6'd0;
reg [1:0]  ip_ecn_reg = 2'd0;
reg [15:0] ip_identification_reg = 16'd0;
reg [2:0]  ip_flags_reg = 3'd0;
reg [12:0] ip_fragment_offset_reg = 13'd0;
reg [7:0]  ip_ttl_reg = 8'd0;
reg [15:0] ip_header_checksum_reg = 16'd0;
reg [31:0] ip_source_ip_reg = 32'd0;
reg [31:0] ip_dest_ip_reg = 32'd0;
reg [15:0] udp_source_port_reg = 16'd0;
reg [15:0] udp_dest_port_reg = 16'd0;

reg hdr_valid_reg = 0, hdr_valid_next;

reg s_udp_hdr_ready_reg = 1'b0, s_udp_hdr_ready_next;
reg s_udp_payload_axis_tready_reg = 1'b0, s_udp_payload_axis_tready_next;

reg busy_reg = 1'b0;

/*
 * UDP Payload FIFO
 */
wire [7:0] s_udp_payload_fifo_tdata;
wire s_udp_payload_fifo_tvalid;
wire s_udp_payload_fifo_tready;
wire s_udp_payload_fifo_tlast;
wire s_udp_payload_fifo_tuser;

wire [7:0] m_udp_payload_fifo_tdata;
wire m_udp_payload_fifo_tvalid;
wire m_udp_payload_fifo_tready;
wire m_udp_payload_fifo_tlast;
wire m_udp_payload_fifo_tuser;

axis_fifo #(
    .DEPTH(PAYLOAD_FIFO_DEPTH),
    .DATA_WIDTH(8),
    .KEEP_ENABLE(0),
    .LAST_ENABLE(1),
    .ID_ENABLE(0),
    .DEST_ENABLE(0),
    .USER_ENABLE(1),
    .USER_WIDTH(1),
    .FRAME_FIFO(0)
)
payload_fifo (
    .clk(clk),
    .rst(rst),
    // AXI input
    .s_axis_tdata(s_udp_payload_fifo_tdata),
    .s_axis_tkeep(0),
    .s_axis_tvalid(s_udp_payload_fifo_tvalid),
    .s_axis_tready(s_udp_payload_fifo_tready),
    .s_axis_tlast(s_udp_payload_fifo_tlast),
    .s_axis_tid(0),
    .s_axis_tdest(0),
    .s_axis_tuser(s_udp_payload_fifo_tuser),
    // AXI output
    .m_axis_tdata(m_udp_payload_fifo_tdata),
    .m_axis_tkeep(),
    .m_axis_tvalid(m_udp_payload_fifo_tvalid),
    .m_axis_tready(m_udp_payload_fifo_tready),
    .m_axis_tlast(m_udp_payload_fifo_tlast),
    .m_axis_tid(),
    .m_axis_tdest(),
    .m_axis_tuser(m_udp_payload_fifo_tuser),
    // Status
    .status_overflow(),
    .status_bad_frame(),
    .status_good_frame()
);

assign s_udp_payload_fifo_tdata = s_udp_payload_axis_tdata;
assign s_udp_payload_fifo_tvalid = s_udp_payload_axis_tvalid && shift_payload_in;
assign s_udp_payload_axis_tready = s_udp_payload_fifo_tready && shift_payload_in;
assign s_udp_payload_fifo_tlast = s_udp_payload_axis_tlast;
assign s_udp_payload_fifo_tuser = s_udp_payload_axis_tuser;

assign m_udp_payload_axis_tdata = m_udp_payload_fifo_tdata;
assign m_udp_payload_axis_tvalid = m_udp_payload_fifo_tvalid;
assign m_udp_payload_fifo_tready = m_udp_payload_axis_tready;
assign m_udp_payload_axis_tlast = m_udp_payload_fifo_tlast;
assign m_udp_payload_axis_tuser = m_udp_payload_fifo_tuser;

/*
 * UDP Header FIFO
 */
reg [HEADER_FIFO_ADDR_WIDTH:0] header_fifo_wr_ptr_reg = {HEADER_FIFO_ADDR_WIDTH+1{1'b0}}, header_fifo_wr_ptr_next;
reg [HEADER_FIFO_ADDR_WIDTH:0] header_fifo_rd_ptr_reg = {HEADER_FIFO_ADDR_WIDTH+1{1'b0}}, header_fifo_rd_ptr_next;

reg [47:0] eth_dest_mac_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [47:0] eth_src_mac_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [15:0] eth_type_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [3:0] ip_version_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [3:0] ip_ihl_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [5:0] ip_dscp_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [1:0] ip_ecn_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [15:0] ip_identification_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [2:0] ip_flags_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [12:0] ip_fragment_offset_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [7:0] ip_ttl_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [15:0] ip_header_checksum_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [31:0] ip_source_ip_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [31:0] ip_dest_ip_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [15:0] udp_source_port_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [15:0] udp_dest_port_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [15:0] udp_length_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];
reg [15:0] udp_checksum_mem[(2**HEADER_FIFO_ADDR_WIDTH)-1:0];

reg [47:0] m_eth_dest_mac_reg = 48'd0;
reg [47:0] m_eth_src_mac_reg = 48'd0;
reg [15:0] m_eth_type_reg = 16'd0;
reg [3:0]  m_ip_version_reg = 4'd0;
reg [3:0]  m_ip_ihl_reg = 4'd0;
reg [5:0]  m_ip_dscp_reg = 6'd0;
reg [1:0]  m_ip_ecn_reg = 2'd0;
reg [15:0] m_ip_identification_reg = 16'd0;
reg [2:0]  m_ip_flags_reg = 3'd0;
reg [12:0] m_ip_fragment_offset_reg = 13'd0;
reg [7:0]  m_ip_ttl_reg = 8'd0;
reg [15:0] m_ip_header_checksum_reg = 16'd0;
reg [31:0] m_ip_source_ip_reg = 32'd0;
reg [31:0] m_ip_dest_ip_reg = 32'd0;
reg [15:0] m_udp_source_port_reg = 16'd0;
reg [15:0] m_udp_dest_port_reg = 16'd0;
reg [15:0] m_udp_length_reg = 16'd0;
reg [15:0] m_udp_checksum_reg = 16'd0;

reg m_udp_hdr_valid_reg = 1'b0, m_udp_hdr_valid_next;

// full when first MSB different but rest same
wire header_fifo_full = ((header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH] != header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH]) &&
                         (header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0] == header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]));
// empty when pointers match exactly
wire header_fifo_empty = header_fifo_wr_ptr_reg == header_fifo_rd_ptr_reg;

// control signals
reg header_fifo_write;
reg header_fifo_read;

wire header_fifo_ready = !header_fifo_full;

assign m_udp_hdr_valid = m_udp_hdr_valid_reg;

assign m_eth_dest_mac = m_eth_dest_mac_reg;
assign m_eth_src_mac = m_eth_src_mac_reg;
assign m_eth_type = m_eth_type_reg;
assign m_ip_version = m_ip_version_reg;
assign m_ip_ihl = m_ip_ihl_reg;
assign m_ip_dscp = m_ip_dscp_reg;
assign m_ip_ecn = m_ip_ecn_reg;
assign m_ip_length = m_udp_length_reg + 16'd20;
assign m_ip_identification = m_ip_identification_reg;
assign m_ip_flags = m_ip_flags_reg;
assign m_ip_fragment_offset = m_ip_fragment_offset_reg;
assign m_ip_ttl = m_ip_ttl_reg;
assign m_ip_protocol = 8'h11;
assign m_ip_header_checksum = m_ip_header_checksum_reg;
assign m_ip_source_ip = m_ip_source_ip_reg;
assign m_ip_dest_ip = m_ip_dest_ip_reg;
assign m_udp_source_port = m_udp_source_port_reg;
assign m_udp_dest_port = m_udp_dest_port_reg;
assign m_udp_length = m_udp_length_reg;
assign m_udp_checksum = m_udp_checksum_reg;

// Write logic
always @* begin
    header_fifo_write = 1'b0;

    header_fifo_wr_ptr_next = header_fifo_wr_ptr_reg;

    if (hdr_valid_reg) begin
        // input data valid
        if (~header_fifo_full) begin
            // not full, perform write
            header_fifo_write = 1'b1;
            header_fifo_wr_ptr_next = header_fifo_wr_ptr_reg + 1;
        end
    end
end

always @(posedge clk) begin
    if (rst) begin
        header_fifo_wr_ptr_reg <= {HEADER_FIFO_ADDR_WIDTH+1{1'b0}};
    end else begin
        header_fifo_wr_ptr_reg <= header_fifo_wr_ptr_next;
    end

    if (header_fifo_write) begin
        eth_dest_mac_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= eth_dest_mac_reg;
        eth_src_mac_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= eth_src_mac_reg;
        eth_type_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= eth_type_reg;
        ip_version_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= ip_version_reg;
        ip_ihl_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= ip_ihl_reg;
        ip_dscp_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= ip_dscp_reg;
        ip_ecn_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= ip_ecn_reg;
        ip_identification_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= ip_identification_reg;
        ip_flags_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= ip_flags_reg;
        ip_fragment_offset_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= ip_fragment_offset_reg;
        ip_ttl_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= ip_ttl_reg;
        ip_header_checksum_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= ip_header_checksum_reg;
        ip_source_ip_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= ip_source_ip_reg;
        ip_dest_ip_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= ip_dest_ip_reg;
        udp_source_port_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= udp_source_port_reg;
        udp_dest_port_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= udp_dest_port_reg;
        udp_length_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= frame_ptr_reg;
        udp_checksum_mem[header_fifo_wr_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]] <= checksum_reg[15:0];
    end
end

// Read logic
always @* begin
    header_fifo_read = 1'b0;

    header_fifo_rd_ptr_next = header_fifo_rd_ptr_reg;

    m_udp_hdr_valid_next = m_udp_hdr_valid_reg;

    if (m_udp_hdr_ready || !m_udp_hdr_valid) begin
        // output data not valid OR currently being transferred
        if (!header_fifo_empty) begin
            // not empty, perform read
            header_fifo_read = 1'b1;
            m_udp_hdr_valid_next = 1'b1;
            header_fifo_rd_ptr_next = header_fifo_rd_ptr_reg + 1;
        end else begin
            // empty, invalidate
            m_udp_hdr_valid_next = 1'b0;
        end
    end
end

always @(posedge clk) begin
    if (rst) begin
        header_fifo_rd_ptr_reg <= {HEADER_FIFO_ADDR_WIDTH+1{1'b0}};
        m_udp_hdr_valid_reg <= 1'b0;
    end else begin
        header_fifo_rd_ptr_reg <= header_fifo_rd_ptr_next;
        m_udp_hdr_valid_reg <= m_udp_hdr_valid_next;
    end

    if (header_fifo_read) begin
        m_eth_dest_mac_reg <= eth_dest_mac_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_eth_src_mac_reg <= eth_src_mac_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_eth_type_reg <= eth_type_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_ip_version_reg <= ip_version_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_ip_ihl_reg <= ip_ihl_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_ip_dscp_reg <= ip_dscp_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_ip_ecn_reg <= ip_ecn_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_ip_identification_reg <= ip_identification_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_ip_flags_reg <= ip_flags_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_ip_fragment_offset_reg <= ip_fragment_offset_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_ip_ttl_reg <= ip_ttl_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_ip_header_checksum_reg <= ip_header_checksum_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_ip_source_ip_reg <= ip_source_ip_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_ip_dest_ip_reg <= ip_dest_ip_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_udp_source_port_reg <= udp_source_port_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_udp_dest_port_reg <= udp_dest_port_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_udp_length_reg <= udp_length_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
        m_udp_checksum_reg <= udp_checksum_mem[header_fifo_rd_ptr_reg[HEADER_FIFO_ADDR_WIDTH-1:0]];
    end
end

assign s_udp_hdr_ready = s_udp_hdr_ready_reg;

assign busy = busy_reg;

always @* begin
    state_next = STATE_IDLE;

    s_udp_hdr_ready_next = 1'b0;
    s_udp_payload_axis_tready_next = 1'b0;

    store_udp_hdr = 1'b0;
    shift_payload_in = 1'b0;

    frame_ptr_next = frame_ptr_reg;
    checksum_next = checksum_reg;

    hdr_valid_next = 1'b0;

    case (state_reg)
        STATE_IDLE: begin
            // idle state
            s_udp_hdr_ready_next = header_fifo_ready;

            if (s_udp_hdr_ready && s_udp_hdr_valid) begin
                store_udp_hdr = 1'b1;
                frame_ptr_next = 0;
                // 16'h0011 = zero padded type field
                // 16'h0010 = header length times two
                checksum_next = 16'h0011 + 16'h0010;
                s_udp_hdr_ready_next = 1'b0;
                state_next = STATE_SUM_HEADER_1;
            end else begin
                state_next = STATE_IDLE;
            end
        end
        STATE_SUM_HEADER_1: begin
            // sum pseudo header and header
            checksum_next = checksum_reg + ip_source_ip_reg[31:16] + ip_source_ip_reg[15:0];
            state_next = STATE_SUM_HEADER_2;
        end
        STATE_SUM_HEADER_2: begin
            // sum pseudo header and header
            checksum_next = checksum_reg + ip_dest_ip_reg[31:16] + ip_dest_ip_reg[15:0];
            state_next = STATE_SUM_HEADER_3;
        end
        STATE_SUM_HEADER_3: begin
            // sum pseudo header and header
            checksum_next = checksum_reg + udp_source_port_reg + udp_dest_port_reg;
            frame_ptr_next = 8;
            state_next = STATE_SUM_PAYLOAD;
        end
        STATE_SUM_PAYLOAD: begin
            // sum payload
            shift_payload_in = 1'b1;

            if (s_udp_payload_axis_tready && s_udp_payload_axis_tvalid) begin
                // checksum computation for payload - alternately store and accumulate
                // add 2 for length calculation (two length fields in pseudo header)
                if (frame_ptr_reg[0]) begin
                    checksum_next = checksum_reg + {8'h00, s_udp_payload_axis_tdata} + 2;
                end else begin
                    checksum_next = checksum_reg + {s_udp_payload_axis_tdata, 8'h00} + 2;
                end

                frame_ptr_next = frame_ptr_reg + 1;

                if (s_udp_payload_axis_tlast) begin
                    state_next = STATE_FINISH_SUM;
                end else begin
                    state_next = STATE_SUM_PAYLOAD;
                end
            end else begin
                state_next = STATE_SUM_PAYLOAD;
            end
        end
        STATE_FINISH_SUM: begin
            // add MSW (twice!) for proper ones complement sum
            checksum_part = checksum_reg[15:0] + checksum_reg[31:16];
            checksum_next = ~(checksum_part[15:0] + checksum_part[16]);
            hdr_valid_next = 1;
            state_next = STATE_IDLE;
        end
    endcase
end

always @(posedge clk) begin
    if (rst) begin
        state_reg <= STATE_IDLE;
        s_udp_hdr_ready_reg <= 1'b0;
        s_udp_payload_axis_tready_reg <= 1'b0;
        hdr_valid_reg <= 1'b0;
        busy_reg <= 1'b0;
    end else begin
        state_reg <= state_next;

        s_udp_hdr_ready_reg <= s_udp_hdr_ready_next;
        s_udp_payload_axis_tready_reg <= s_udp_payload_axis_tready_next;

        hdr_valid_reg <= hdr_valid_next;

        busy_reg <= state_next != STATE_IDLE;
    end

    frame_ptr_reg <= frame_ptr_next;
    checksum_reg <= checksum_next;

    // datapath
    if (store_udp_hdr) begin
        eth_dest_mac_reg <= s_eth_dest_mac;
        eth_src_mac_reg <= s_eth_src_mac;
        eth_type_reg <= s_eth_type;
        ip_version_reg <= s_ip_version;
        ip_ihl_reg <= s_ip_ihl;
        ip_dscp_reg <= s_ip_dscp;
        ip_ecn_reg <= s_ip_ecn;
        ip_identification_reg <= s_ip_identification;
        ip_flags_reg <= s_ip_flags;
        ip_fragment_offset_reg <= s_ip_fragment_offset;
        ip_ttl_reg <= s_ip_ttl;
        ip_header_checksum_reg <= s_ip_header_checksum;
        ip_source_ip_reg <= s_ip_source_ip;
        ip_dest_ip_reg <= s_ip_dest_ip;
        udp_source_port_reg <= s_udp_source_port;
        udp_dest_port_reg <= s_udp_dest_port;
    end
end

endmodule