private int firstNonzeroIntNum(int[] mag) {
        for (int i = mag.length - 1, j = 0; i >= 0; i--, j++) {
            if (mag[i] != 0) {
                return j;
            }
        }
        return 0;
    }
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