[Bug] UB in s21_add with negative numbers about s21_bigint

[posidoni]

This causes stack overflow. Probably this is also due to incorrect comparison of negative numbers. Andrey was writing & testing his functions with old buggy Stepan's comparisons. Probably we need to write ADD, SUB, MUL, DIV from scratch 🍭 This is not that hard actually. We brainstormed this with Stepa this evening.

Moreover, we will write our own functions & rewrite completely unreadable Andrey's code.

START_TEST(int64_t_negatives) {
    int64_t long_a = get_random_ll();
    int64_t long_b = get_random_ll();

    if (rand() % 2)
        long_a *= -1;

    if (rand() % 2)
        long_b *= -1;

    int64_t a = long_a;
    int64_t b = long_b;

    int64_t sum = a + b;

    s21_decimal res128 = {0};

    for (long long int i = 0, k = 0; i < 2; i++)
        for (long long int j = 0; j < 32; j++, k++)
            if (IS_SET(sum, k))
                ADD_BIT(res128.bits[i], j);

    s21_decimal dec_a = ll_to_decimal(long_a);
    s21_decimal dec_b = ll_to_decimal(long_b);
    s21_decimal dec_sum = {0};

    s21_add(dec_a, dec_b, &dec_sum);
    /* print_bits_r(dec_sum); */

    ck_assert_int_eq(s21_is_equal(res128, dec_sum), TRUE);
}

Suite *suite_s21_add(void) {
    Suite *s = suite_create(PRETTY_PRINT("s21_add"));
    TCase *tc = tcase_create("s21_add_tc");

    /* tcase_add_loop_test(tc, add_test1, 0, 1); */
    tcase_add_loop_test(tc, gcc_128_bits, 0, 10000);
    tcase_add_loop_test(tc, int64_t_negatives, 0, 10000);

    suite_add_tcase(s, tc);
    return s;
}

[posidoni]

It turns out there were two bugs with s21_sub. Now everything works fine. #57 closes this issue.