What is xxHash64? It has two separate uses in Merceq

xxHash64 is an algorithm that creates a quick fingerprint from file contents. It helps detect accidental corruption and incomplete writes in a copy. It’s not encryption, and it can’t be claimed to verify intentionally modified files at a cryptographic level.

Merceq uses it in two separate ways:

• Transfer verification: The entire source and destination are read, and their xxHash64 checksums are compared. The full-file xxh64 value is also written to ASC MHL records.
• Content identity in the library: A shorter fingerprint is created from the file size and the first and last 1 MB to recognise a file again, associate it with a clip moved to another location, or find possible duplicates. This isn’t the same task as full-file verification.

For this reason, Merceq recognising two library files as similar or having the same content isn’t a substitute for fully verifying a copy written to disk. To assess full-file integrity, check the transfer’s verification result and its manifest, if available.