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Support for alternative HashMap hashing algorithms #913

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alexheretic opened this issue Sep 11, 2023 · 0 comments · May be fixed by #914
Open

Support for alternative HashMap hashing algorithms #913

alexheretic opened this issue Sep 11, 2023 · 0 comments · May be fixed by #914

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@alexheretic
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It is currently possible to derive Message for HashMaps. However it is not possible to use an alternative hashing algorithm like rustc-hash/ahash which may be more optimal in certain situations.

Example

Take a simple Msg message with a int map field. This can be implemented using the default hasher.

use std::collections::HashMap;

#[derive(Clone, PartialEq, prost::Message)]
pub struct Msg {
    #[prost(map = "uint32, uint32", tag = "1")]
    pub map: HashMap<u32, u32>,
}

#[test]
fn decode() {
    use prost::Message;
    const FOO_BYTES: &[u8] = &[10, 4, 8, 3, 16, 4, 10, 4, 8, 1, 16, 2];

    let msg = Msg::decode(FOO_BYTES).unwrap();

    assert_eq!(msg.map.len(), 2);
    assert_eq!(msg.map[&1], 2);
    assert_eq!(msg.map[&3], 4);
}

However, were I to try to use instead pub map: rustc_hash::FxHashMap<u32, u32> as a more optimal hashing algo for u32 Message derive fails to build as it only implements for the default hasher.

$ cargo build
   Compiling prost-alt-hash v0.1.0 (/home/alex/project/prost-alt-hash)
error[E0308]: mismatched types
    --> src/lib.rs:3:28
     |
3    | #[derive(Clone, PartialEq, prost::Message)]
     |                            ^^^^^^^^^^^^^^
     |                            |
     |                            expected `&HashMap<u32, u32>`, found `&HashMap<u32, u32, ...>`
     |                            arguments to this function are incorrect
     |
     = note: expected reference `&HashMap<u32, u32>`
                found reference `&HashMap<u32, u32, BuildHasherDefault<FxHasher>>`
@alexheretic alexheretic linked a pull request Sep 11, 2023 that will close this issue
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Support alternative HashMap hasher alexheretic/prost
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@alexheretic