`timescale 1ns/1ps

// ================================================================
// CRC32_BYTE TESTBENCH
//
// Oracle: tools/flash_catalog/oracle.py's crc32() (Python stdlib
// zlib.crc32) -- an independent implementation, not derived from
// rtl/crc32.v. Expected values below were generated by running that
// script (see its own __main__ block and WORKLOG.md's F4 entry for
// the exact invocation), not by reading rtl/crc32.v's algorithm and
// re-deriving them by hand.
// ================================================================

module tb;

    reg  [31:0] crc_in;
    reg  [7:0]  data;
    wire [31:0] crc_out;

    crc32_byte dut (.crc_in(crc_in), .data(data), .crc_out(crc_out));

    integer errors;
    integer i;
    reg [31:0] crc;

    task automatic check(input [31:0] got, input [31:0] exp, input [255:0] label);
        begin
            if (got !== exp) begin
                $display("FAIL: %0s got=%08h exp=%08h", label, got, exp);
                errors = errors + 1;
            end else begin
                $display("PASS: %0s = %08h", label, got);
            end
        end
    endtask

    initial begin
        errors = 0;

        // TEST 1: CRC32 of the empty message is, by definition,
        // 0x00000000 (init 0xFFFFFFFF XOR final 0xFFFFFFFF, no bytes
        // processed) -- a fixed, well-known property of this exact
        // algorithm, not something derived from either implementation.
        check(32'hFFFFFFFF ^ 32'hFFFFFFFF, 32'h00000000, "TEST1 empty-message identity");

        // TEST 2: single byte 0x00. Oracle (Python):
        //   >>> zlib.crc32(bytes([0x00])) -> 0xd202ef8d
        crc = 32'hFFFFFFFF;
        #1 crc_in = crc; data = 8'h00; #1;
        crc = crc_out;
        check(crc ^ 32'hFFFFFFFF, 32'hd202ef8d, "TEST2 single byte 0x00");

        // TEST 3: ASCII "123456789" (the standard CRC32 check-value
        // vector quoted by every CRC32 reference table, e.g.
        // Rocksoft's "check value" for CRC-32/ISO-HDLC = 0xCBF43926
        // -- an independent, textbook-published constant, not
        // computed by either this RTL or the Python oracle).
        begin : t3
            reg [7:0] msg [0:8];
            msg[0]=8'h31; msg[1]=8'h32; msg[2]=8'h33; msg[3]=8'h34; msg[4]=8'h35;
            msg[5]=8'h36; msg[6]=8'h37; msg[7]=8'h38; msg[8]=8'h39;
            crc = 32'hFFFFFFFF;
            for (i = 0; i < 9; i = i + 1) begin
                crc_in = crc; data = msg[i]; #1;
                crc = crc_out;
                #1;
            end
            check(crc ^ 32'hFFFFFFFF, 32'hCBF43926, "TEST3 \"123456789\" textbook check-value");
        end

        // TEST 4: 32-byte payload 0x10..0x2F, matching
        // tools/flash_catalog/oracle.py's __main__ self-check output
        // exactly (run: `python3 tools/flash_catalog/oracle.py`) --
        // oracle printed CRC32 = 0x3b8cfe40 for this exact payload.
        begin : t4
            crc = 32'hFFFFFFFF;
            for (i = 0; i < 32; i = i + 1) begin
                crc_in = crc; data = 8'h10 + i[7:0]; #1;
                crc = crc_out;
                #1;
            end
            check(crc ^ 32'hFFFFFFFF, 32'h3b8cfe40, "TEST4 32B payload vs Python oracle.py");
        end

        if (errors == 0)
            $display("ALL TESTS PASSED");
        else
            $display("FAILED: %0d error(s)", errors);

        $finish;
    end

endmodule