private static long expand(long i) {
        long t = i << 16;
        i = (i & ~0xFFFF00000000L) | (t & 0xFFFF00000000L);
        t = i << 8;
        i = (i & ~0xFF000000FF0000L) | (t & 0xFF000000FF0000L);
        t = i << 4;
        i = (i & ~0xF000F000F000F00L) | (t & 0xF000F000F000F00L);
        return i & 0x0F0F_0F0F_0F0F_0F0FL;
    }
A faster alternative that is functionally equivalent to Long.expand(i, 0x0F0F_0F0F_0F0F_0F0FL)
core/src/main/java/com/alibaba/fastjson2/JSONWriterUTF8.java