@Test
    public void test_bigInt_1() throws Exception {
        String str = "-12345678901234567890123456789012345678";
        char[] chars = str.toCharArray();
        BigInteger bigInt1 = new BigInteger(str);

        // isneg ? -1 : 1
        bigInt(chars, -1, 0, 1, chars.length);

        assertEquals(-1, signum);
        assertEquals(mag0, 155824374);
        assertEquals(mag1, -268291309);
        assertEquals(mag2, -1001811888);
        assertEquals(mag3, -566693042);
    }
This mask is used to obtain the value of an int as if it were unsigned.
core/src/test/java/com/alibaba/fastjson2/read/BigIntTest.java