## About XXH64 hash

XXH64 (xxHash64) is a variant of the xxHash family of non-cryptographic hash functions, developed by Yann Collet. It is designed to produce a 64-bit hash output, offering a larger hash size compared to its counterpart, XXH32 (xxHash32). XXH64 maintains the same principles of xxHash, focusing on speed and good distribution of hash values.

### Characteristics of XXH64

**Output Size**:- XXH64 produces a fixed-length output of 64 bits (8 bytes).

**Initialization Value**:- Similar to other variants of xxHash, XXH64 uses an initial seed value (
`seed`

) for hash calculation. This seed can be any arbitrary value chosen by the user to control the starting point of the hash computation.

- Similar to other variants of xxHash, XXH64 uses an initial seed value (
**Hash Calculation**:- XXH64 processes the input data in blocks of 8 bytes (64 bits) at a time.
- It applies a mix of bitwise rotations, XOR operations, and multiplications with prime constants to each block of data.
- The hash state is updated iteratively for each processed block until all data is processed.

**Properties**:- XXH64 is optimized for speed and efficiency, making it suitable for applications requiring rapid computation of hash values.
- It is deterministic and produces the same hash value for identical input data and seed.
- XXH64 is not designed for cryptographic purposes due to its predictable nature and vulnerability to collision attacks.

### Usage

**Applications**: XXH64 is commonly used in applications where a larger hash output size (64 bits) is required for data integrity checks, hash tables, and non-cryptographic hash-based operations.**Implementation**: Implementing XXH64 is straightforward due to its simple algorithm and efficient use of computational resources.

### Example Calculation

Given a sequence of bytes `data`

and an initial seed `seed`

, the XXH64 hash `XXH64(data, seed)`

is calculated as follows:

- Initialize the hash value
`hash`

with the seed value. - Process each 8-byte block of
`data`

using specific mixing operations. - Finalize the hash value to ensure good dispersion and avalanche effect.
- The final XXH64 hash value after processing all data blocks is returned.

### Security Considerations

- XXH64, like other xxHash variants, is not suitable for cryptographic purposes due to its vulnerability to collision attacks and its predictable nature.
- It should only be used in scenarios where speed and good distribution of hash values are more critical than cryptographic security.

### Summary

XXH64 (xxHash64) is a fast and efficient non-cryptographic hash function designed for rapid computation of 64-bit hash values. It is widely adopted in various software applications and systems where quick hash computation with larger output size and good dispersion properties is required. However, it should not be used for cryptographic applications where strong security guarantees are needed.