Function checksum64 

pub fn checksum64(parts: &[&[u8]]) -> u64

64-bit checksum of parts in order: XXH64 of each part, seeded with the checksum of the parts before it, so every bit of the result depends on every part. Used for hash chains and dedup keys, where 32 bits would make accidental matches too likely over billions of entries.

(Two CRC-32C passes with different seeds do not make 64 bits: a CRC is linear, so for inputs of one length the two halves differ by a constant and two inputs that collide in one half collide in both.)