module axis_tap #
(
    // Width of AXI stream interfaces in bits
    parameter DATA_WIDTH = 8,
    // Propagate tkeep signal
    parameter KEEP_ENABLE = (DATA_WIDTH>8),
    // tkeep signal width (words per cycle)
    parameter KEEP_WIDTH = (DATA_WIDTH/8),
    // Propagate tid signal
    parameter ID_ENABLE = 0,
    // tid signal width
    parameter ID_WIDTH = 8,
    // Propagate tdest signal
    parameter DEST_ENABLE = 0,
    // tdest signal width
    parameter DEST_WIDTH = 8,
    // Propagate tuser signal
    parameter USER_ENABLE = 1,
    // tuser signal width
    parameter USER_WIDTH = 1,
    // tuser value for bad frame marker
    parameter USER_BAD_FRAME_VALUE = 1'b1,
    // tuser mask for bad frame marker
    parameter USER_BAD_FRAME_MASK = 1'b1
)
(
    input  wire                   clk,
    input  wire                   rst,

    /*
     * AXI tap
     */
    input  wire [DATA_WIDTH-1:0]  tap_axis_tdata,
    input  wire [KEEP_WIDTH-1:0]  tap_axis_tkeep,
    input  wire                   tap_axis_tvalid,
    input  wire                   tap_axis_tready,
    input  wire                   tap_axis_tlast,
    input  wire [ID_WIDTH-1:0]    tap_axis_tid,
    input  wire [DEST_WIDTH-1:0]  tap_axis_tdest,
    input  wire [USER_WIDTH-1:0]  tap_axis_tuser,

    /*
     * AXI output
     */
    output wire [DATA_WIDTH-1:0]  m_axis_tdata,
    output wire [KEEP_WIDTH-1:0]  m_axis_tkeep,
    output wire                   m_axis_tvalid,
    input  wire                   m_axis_tready,
    output wire                   m_axis_tlast,
    output wire [ID_WIDTH-1:0]    m_axis_tid,
    output wire [DEST_WIDTH-1:0]  m_axis_tdest,
    output wire [USER_WIDTH-1:0]  m_axis_tuser
);

// datapath control signals
reg store_last_word;

reg [ID_WIDTH-1:0]   last_word_id_reg   = {ID_WIDTH{1'b0}};
reg [DEST_WIDTH-1:0] last_word_dest_reg = {DEST_WIDTH{1'b0}};
reg [USER_WIDTH-1:0] last_word_user_reg = {USER_WIDTH{1'b0}};

// internal datapath
reg  [DATA_WIDTH-1:0] m_axis_tdata_int;
reg  [KEEP_WIDTH-1:0] m_axis_tkeep_int;
reg                   m_axis_tvalid_int;
reg                   m_axis_tready_int_reg = 1'b0;
reg                   m_axis_tlast_int;
reg  [ID_WIDTH-1:0]   m_axis_tid_int;
reg  [DEST_WIDTH-1:0] m_axis_tdest_int;
reg  [USER_WIDTH-1:0] m_axis_tuser_int;
wire                  m_axis_tready_int_early;

localparam [1:0]
    STATE_IDLE = 2'd0,
    STATE_TRANSFER = 2'd1,
    STATE_TRUNCATE = 2'd2,
    STATE_WAIT = 2'd3;

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

reg frame_reg = 1'b0, frame_next;

always @* begin
    state_next = STATE_IDLE;

    store_last_word = 1'b0;

    frame_next = frame_reg;

    m_axis_tdata_int  = {DATA_WIDTH{1'b0}};
    m_axis_tkeep_int  = {KEEP_WIDTH{1'b0}};
    m_axis_tvalid_int = 1'b0;
    m_axis_tlast_int  = 1'b0;
    m_axis_tid_int    = {ID_WIDTH{1'b0}};
    m_axis_tdest_int  = {DEST_WIDTH{1'b0}};
    m_axis_tuser_int  = {USER_WIDTH{1'b0}};

    if (tap_axis_tready && tap_axis_tvalid) begin
        frame_next = !tap_axis_tlast;
    end

    case (state_reg)
        STATE_IDLE: begin
            if (tap_axis_tready && tap_axis_tvalid) begin
                // start of frame
                if (m_axis_tready_int_reg) begin
                    m_axis_tdata_int  = tap_axis_tdata;
                    m_axis_tkeep_int  = tap_axis_tkeep;
                    m_axis_tvalid_int = tap_axis_tvalid && tap_axis_tready;
                    m_axis_tlast_int  = tap_axis_tlast;
                    m_axis_tid_int    = tap_axis_tid;
                    m_axis_tdest_int  = tap_axis_tdest;
                    m_axis_tuser_int  = tap_axis_tuser;
                    if (tap_axis_tlast) begin
                        state_next = STATE_IDLE;
                    end else begin
                        state_next = STATE_TRANSFER;
                    end
                end else begin
                    state_next = STATE_WAIT;
                end
            end else begin
                state_next = STATE_IDLE;
            end
        end
        STATE_TRANSFER: begin
            if (tap_axis_tready && tap_axis_tvalid) begin
                // transfer data
                if (m_axis_tready_int_reg) begin
                    m_axis_tdata_int  = tap_axis_tdata;
                    m_axis_tkeep_int  = tap_axis_tkeep;
                    m_axis_tvalid_int = tap_axis_tvalid && tap_axis_tready;
                    m_axis_tlast_int  = tap_axis_tlast;
                    m_axis_tid_int    = tap_axis_tid;
                    m_axis_tdest_int  = tap_axis_tdest;
                    m_axis_tuser_int  = tap_axis_tuser;
                    if (tap_axis_tlast) begin
                        state_next = STATE_IDLE;
                    end else begin
                        state_next = STATE_TRANSFER;
                    end
                end else begin
                    store_last_word = 1'b1;
                    state_next = STATE_TRUNCATE;
                end
            end else begin
                state_next = STATE_TRANSFER;
            end
        end
        STATE_TRUNCATE: begin
            if (m_axis_tready_int_reg) begin
                m_axis_tdata_int  = {DATA_WIDTH{1'b0}};
                m_axis_tkeep_int  = {{KEEP_WIDTH-1{1'b0}}, 1'b1};
                m_axis_tvalid_int = 1'b1;
                m_axis_tlast_int  = 1'b1;
                m_axis_tid_int    = last_word_id_reg;
                m_axis_tdest_int  = last_word_dest_reg;
                m_axis_tuser_int  = (last_word_user_reg & ~USER_BAD_FRAME_MASK) | (USER_BAD_FRAME_VALUE & USER_BAD_FRAME_MASK);
                if (frame_next) begin
                    state_next = STATE_WAIT;
                end else begin
                    state_next = STATE_IDLE;
                end
            end else begin
                state_next = STATE_TRUNCATE;
            end
        end
        STATE_WAIT: begin
            if (tap_axis_tready && tap_axis_tvalid) begin
                if (tap_axis_tlast) begin
                    state_next = STATE_IDLE;
                end else begin
                    state_next = STATE_WAIT;
                end
            end else begin
                state_next = STATE_WAIT;
            end
        end
    endcase
end

always @(posedge clk) begin
    if (rst) begin
        state_reg <= STATE_IDLE;
        frame_reg <= 1'b0;
    end else begin
        state_reg <= state_next;
        frame_reg <= frame_next;
    end

    if (store_last_word) begin
        last_word_id_reg   <= tap_axis_tid;
        last_word_dest_reg <= tap_axis_tdest;
        last_word_user_reg <= tap_axis_tuser;
    end
end

// output datapath logic
reg [DATA_WIDTH-1:0] m_axis_tdata_reg  = {DATA_WIDTH{1'b0}};
reg [KEEP_WIDTH-1:0] m_axis_tkeep_reg  = {KEEP_WIDTH{1'b0}};
reg                  m_axis_tvalid_reg = 1'b0, m_axis_tvalid_next;
reg                  m_axis_tlast_reg  = 1'b0;
reg [ID_WIDTH-1:0]   m_axis_tid_reg    = {ID_WIDTH{1'b0}};
reg [DEST_WIDTH-1:0] m_axis_tdest_reg  = {DEST_WIDTH{1'b0}};
reg [USER_WIDTH-1:0] m_axis_tuser_reg  = {USER_WIDTH{1'b0}};

reg [DATA_WIDTH-1:0] temp_m_axis_tdata_reg  = {DATA_WIDTH{1'b0}};
reg [KEEP_WIDTH-1:0] temp_m_axis_tkeep_reg  = {KEEP_WIDTH{1'b0}};
reg                  temp_m_axis_tvalid_reg = 1'b0, temp_m_axis_tvalid_next;
reg                  temp_m_axis_tlast_reg  = 1'b0;
reg [ID_WIDTH-1:0]   temp_m_axis_tid_reg    = {ID_WIDTH{1'b0}};
reg [DEST_WIDTH-1:0] temp_m_axis_tdest_reg  = {DEST_WIDTH{1'b0}};
reg [USER_WIDTH-1:0] temp_m_axis_tuser_reg  = {USER_WIDTH{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_tkeep  = KEEP_ENABLE ? m_axis_tkeep_reg : {KEEP_WIDTH{1'b1}};
assign m_axis_tvalid = m_axis_tvalid_reg;
assign m_axis_tlast  = m_axis_tlast_reg;
assign m_axis_tid    = ID_ENABLE   ? m_axis_tid_reg   : {ID_WIDTH{1'b0}};
assign m_axis_tdest  = DEST_ENABLE ? m_axis_tdest_reg : {DEST_WIDTH{1'b0}};
assign m_axis_tuser  = USER_ENABLE ? m_axis_tuser_reg : {USER_WIDTH{1'b0}};

// enable ready input next cycle if output is ready or the temp reg will not be filled on the next cycle (output reg empty or no input)
assign m_axis_tready_int_early = m_axis_tready || (!temp_m_axis_tvalid_reg && (!m_axis_tvalid_reg || !m_axis_tvalid_int));

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

    // datapath
    if (store_axis_int_to_output) begin
        m_axis_tdata_reg <= m_axis_tdata_int;
        m_axis_tkeep_reg <= m_axis_tkeep_int;
        m_axis_tlast_reg <= m_axis_tlast_int;
        m_axis_tid_reg   <= m_axis_tid_int;
        m_axis_tdest_reg <= m_axis_tdest_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_tkeep_reg <= temp_m_axis_tkeep_reg;
        m_axis_tlast_reg <= temp_m_axis_tlast_reg;
        m_axis_tid_reg   <= temp_m_axis_tid_reg;
        m_axis_tdest_reg <= temp_m_axis_tdest_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_tkeep_reg <= m_axis_tkeep_int;
        temp_m_axis_tlast_reg <= m_axis_tlast_int;
        temp_m_axis_tid_reg   <= m_axis_tid_int;
        temp_m_axis_tdest_reg <= m_axis_tdest_int;
        temp_m_axis_tuser_reg <= m_axis_tuser_int;
    end
end

endmodule