GitLab

Created by: asomers

SeaHash does not produce the same result when hashing a whole buffer as when hashing its constituent parts. For example, calling SeaHasher::write on the whole buffer provides a different result than calling SeaHasher::write on subslices, or calling SeaHasher::write_u8 on each byte. Rust's DefaultHasher as well as MetroHash64 and MetroHash128 do have this property. I haven't tested any others. This test case demonstrates the problem. Just replace the type MyHasher line to test different Hasher implementations:

#[test]
fn test_hash_slice() {
    use metrohash::{MetroHash64, MetroHash128};
    use seahash::SeaHasher;
    use std::collections::hash_map::DefaultHasher;
    use std::hash::Hasher;

    type MyHasher = DefaultHasher;

    let sl: Vec<u8> = vec![0, 1, 2, 3, 4, 5, 6, 7];
    let mut slice_hasher = MyHasher::new();
    let mut halfslice_hasher = MyHasher::new();
    let mut u8_hasher = MyHasher::new();
    let mut u16_hasher = MyHasher::new();
    let mut u32_hasher = MyHasher::new();
    let mut u64_hasher = MyHasher::new();
    slice_hasher.write(&sl[..]);
    halfslice_hasher.write(&sl[0..4]);
    halfslice_hasher.write(&sl[4..8]);
    u8_hasher.write_u8(0);
    u8_hasher.write_u8(1);
    u8_hasher.write_u8(2);
    u8_hasher.write_u8(3);
    u8_hasher.write_u8(4);
    u8_hasher.write_u8(5);
    u8_hasher.write_u8(6);
    u8_hasher.write_u8(7);
    u16_hasher.write_u16(u16::from_be(0x0001));
    u16_hasher.write_u16(u16::from_be(0x0203));
    u16_hasher.write_u16(u16::from_be(0x0405));
    u16_hasher.write_u16(u16::from_be(0x0607));
    u32_hasher.write_u32(u32::from_be(0x00010203));
    u32_hasher.write_u32(u32::from_be(0x04050607));
    u64_hasher.write_u64(u64::from_be(0x0001020304050607));
    let slice_hash = slice_hasher.finish();
    assert_eq!(slice_hash, halfslice_hasher.finish());
    assert_eq!(slice_hash, u64_hasher.finish());
    assert_eq!(slice_hash, u32_hasher.finish());
    assert_eq!(slice_hash, u16_hasher.finish());
    assert_eq!(slice_hash, u8_hasher.finish());
}