local void make_crc_table()

in zlib/crc32.c [310:469]

• 128 lines of code
• 16 McCabe index (conditional complexity)

local void make_crc_table(void) {
    unsigned i, j, n;
    z_crc_t p;

    /* initialize the CRC of bytes tables */
    for (i = 0; i < 256; i++) {
        p = i;
        for (j = 0; j < 8; j++)
            p = p & 1 ? (p >> 1) ^ POLY : p >> 1;
        crc_table[i] = p;
#ifdef W
        crc_big_table[i] = byte_swap(p);
#endif
    }

    /* initialize the x^2^n mod p(x) table */
    p = (z_crc_t)1 << 30;         /* x^1 */
    x2n_table[0] = p;
    for (n = 1; n < 32; n++)
        x2n_table[n] = p = multmodp(p, p);

#ifdef W
    /* initialize the braiding tables -- needs x2n_table[] */
    braid(crc_braid_table, crc_braid_big_table, N, W);
#endif

#ifdef MAKECRCH
    {
        /*
          The crc32.h header file contains tables for both 32-bit and 64-bit
          z_word_t's, and so requires a 64-bit type be available. In that case,
          z_word_t must be defined to be 64-bits. This code then also generates
          and writes out the tables for the case that z_word_t is 32 bits.
         */
#if !defined(W) || W != 8
#  error Need a 64-bit integer type in order to generate crc32.h.
#endif
        FILE *out;
        int k, n;
        z_crc_t ltl[8][256];
        z_word_t big[8][256];

        out = fopen("crc32.h", "w");
        if (out == NULL) return;

        /* write out little-endian CRC table to crc32.h */
        fprintf(out,
            "/* crc32.h -- tables for rapid CRC calculation\n"
            " * Generated automatically by crc32.c\n */\n"
            "\n"
            "local const z_crc_t FAR crc_table[] = {\n"
            "    ");
        write_table(out, crc_table, 256);
        fprintf(out,
            "};\n");

        /* write out big-endian CRC table for 64-bit z_word_t to crc32.h */
        fprintf(out,
            "\n"
            "#ifdef W\n"
            "\n"
            "#if W == 8\n"
            "\n"
            "local const z_word_t FAR crc_big_table[] = {\n"
            "    ");
        write_table64(out, crc_big_table, 256);
        fprintf(out,
            "};\n");

        /* write out big-endian CRC table for 32-bit z_word_t to crc32.h */
        fprintf(out,
            "\n"
            "#else /* W == 4 */\n"
            "\n"
            "local const z_word_t FAR crc_big_table[] = {\n"
            "    ");
        write_table32hi(out, crc_big_table, 256);
        fprintf(out,
            "};\n"
            "\n"
            "#endif\n");

        /* write out braid tables for each value of N */
        for (n = 1; n <= 6; n++) {
            fprintf(out,
            "\n"
            "#if N == %d\n", n);

            /* compute braid tables for this N and 64-bit word_t */
            braid(ltl, big, n, 8);

            /* write out braid tables for 64-bit z_word_t to crc32.h */
            fprintf(out,
            "\n"
            "#if W == 8\n"
            "\n"
            "local const z_crc_t FAR crc_braid_table[][256] = {\n");
            for (k = 0; k < 8; k++) {
                fprintf(out, "   {");
                write_table(out, ltl[k], 256);
                fprintf(out, "}%s", k < 7 ? ",\n" : "");
            }
            fprintf(out,
            "};\n"
            "\n"
            "local const z_word_t FAR crc_braid_big_table[][256] = {\n");
            for (k = 0; k < 8; k++) {
                fprintf(out, "   {");
                write_table64(out, big[k], 256);
                fprintf(out, "}%s", k < 7 ? ",\n" : "");
            }
            fprintf(out,
            "};\n");

            /* compute braid tables for this N and 32-bit word_t */
            braid(ltl, big, n, 4);

            /* write out braid tables for 32-bit z_word_t to crc32.h */
            fprintf(out,
            "\n"
            "#else /* W == 4 */\n"
            "\n"
            "local const z_crc_t FAR crc_braid_table[][256] = {\n");
            for (k = 0; k < 4; k++) {
                fprintf(out, "   {");
                write_table(out, ltl[k], 256);
                fprintf(out, "}%s", k < 3 ? ",\n" : "");
            }
            fprintf(out,
            "};\n"
            "\n"
            "local const z_word_t FAR crc_braid_big_table[][256] = {\n");
            for (k = 0; k < 4; k++) {
                fprintf(out, "   {");
                write_table32hi(out, big[k], 256);
                fprintf(out, "}%s", k < 3 ? ",\n" : "");
            }
            fprintf(out,
            "};\n"
            "\n"
            "#endif\n"
            "\n"
            "#endif\n");
        }
        fprintf(out,
            "\n"
            "#endif\n");

        /* write out zeros operator table to crc32.h */
        fprintf(out,
            "\n"
            "local const z_crc_t FAR x2n_table[] = {\n"
            "    ");
        write_table(out, x2n_table, 32);
        fprintf(out,
            "};\n");
        fclose(out);
    }
#endif /* MAKECRCH */
}