This crate specifies the CdrEncodingSize trait. It is a helper to the RustDDS library.

The trait is in a separate crate, because it has a derive-macro.

The RTPS specification version 2.5 Section "9.6.4.8 KeyHash (PID_KEY_HASH)" defines an algorithm for computing a 16-byte hash of a DDS Key of a data sample.

The algorithm summary:

• Define a "holder" type corresponding to the data sample type.
• The holder type consists of all the fields that form the Key of the data sample type.
• The holder type is serialized according to PLAIN_CDR2 Big Endian.
• If the holder type has maximum serialized size of at most 16 bytes, the serialized representation is the key hash, padded with zero bytes to 16 bytes.
• If the holder type has maximum serialized size of more then 16 bytes, key hash is the MD5 hash of the serialized representation.
• This always results in a key hash of exactly 16 bytes.

This trait is a means of deciding if the 16-byte limit is exceede, i.e. if MD5 shold be used or not.