datasketches 0.5.0

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// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
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// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License.

use crate::error::Error;
use crate::error::ErrorKind;
use crate::hash::MurmurHash3X64128;

/// Computes and checks the 16-bit seed hash from the given long seed.
///
/// The computed seed hash must not be zero in order to maintain compatibility with older
/// serialized versions that did not have this concept.
///
/// # Errors
///
/// Returns an error of `error_kind` if the computed seed hash is zero.
pub fn compute_seed_hash(seed: u64, error_kind: ErrorKind) -> Result<u16, Error> {
    use std::hash::Hasher;

    let mut hasher = MurmurHash3X64128::with_seed(0);
    hasher.write(&seed.to_le_bytes());
    let (h1, _) = hasher.finish128();
    let seed_hash = (h1 & 0xffff) as u16;
    if seed_hash == 0 {
        return Err(Error::new(
            error_kind,
            format!("seed {seed} produces a seed hash of zero; choose a different seed"),
        ));
    }
    Ok(seed_hash)
}

/// Checks that an actual seed hash matches the expected seed hash.
pub fn check_seed_hash(
    expected: u16,
    actual: u16,
    name: &'static str,
    kind: ErrorKind,
) -> Result<(), Error> {
    if actual != expected {
        return Err(Error::new(
            kind,
            format!("incompatible seed hash of {name}: expected {expected}, got {actual}"),
        ));
    }
    Ok(())
}