raindb 1.0.0

A persistent key-value store based on an LSM tree implemented in Rust
Documentation 
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// Copyright (c) 2021 Google LLC
//
// Use of this source code is governed by an MIT-style
// license that can be found in the LICENSE file or at
// https://opensource.org/licenses/MIT.

//! Utilities for checksums.

/// A constant delta for masking and unmasking checksums.
const CRC_MASKING_DELTA: u32 = 0xa282ead8;

/**
Return a masked representation of the checksum.

# Motivation

Per LevelDB, computing the CRC of a string that contains embedded CRC's can be problematic. So they
recommend masking CRC's that will be stored somewhere (e.g. in a file).
*/
pub(crate) fn mask_checksum(checksum: u32) -> u32 {
    // Rotate right by 15 bits and add a constant.
    ((checksum.wrapping_shr(15)) | (checksum.wrapping_shl(17))).wrapping_add(CRC_MASKING_DELTA)
}

/**
Return the unmasked checksum.

The checksum must have been masked with [`crate::utils::crc::mask_checksum`].
*/
pub(crate) fn unmask_checksum(masked_checksum: u32) -> u32 {
    let rotated = masked_checksum.wrapping_sub(CRC_MASKING_DELTA);
    (rotated.wrapping_shr(17)) | (rotated.wrapping_shl(15))
}

#[cfg(test)]
mod tests {
    use crc::{Crc, CRC_32_ISCSI};

    use super::*;
    use pretty_assertions::{assert_eq, assert_ne};

    const CRC_CALCULATOR: Crc<u32> = Crc::<u32>::new(&CRC_32_ISCSI);

    #[test]
    fn can_mask_and_unmask_checksums_correctly() {
        let checksum = CRC_CALCULATOR.checksum(b"foo");

        assert_ne!(checksum, mask_checksum(checksum));
        assert_ne!(checksum, mask_checksum(mask_checksum(checksum)));
        assert_eq!(checksum, unmask_checksum(mask_checksum(checksum)));
        assert_eq!(
            checksum,
            unmask_checksum(unmask_checksum(mask_checksum(mask_checksum(checksum))))
        );
    }
}