public static int mask(int crc) {
        // Rotate right by 15 bits and add a constant.
        return ((crc >>> 15) | (crc << 17)) + MASK_DELTA;
    }
Return a masked representation of crc.

Motivation: it is problematic to compute the CRC of a string that
contains embedded CRCs.  Therefore we recommend that CRCs stored
somewhere (e.g., in files) should be masked before being stored.
codec/codec-api/src/main/java/com/alipay/sofa/rpc/codec/snappy/Crc32C.java