由于 SipHasher 对于我的用例来说太慢,因此我尝试实现自定义哈希函数。我发现一个例子 https://github.com/rust-lang/rust/blob/master/src/librustc/util/nodemap.rs我用它作为基础来编译所有内容。
我当前的代码如下所示:
use std::collections::hash_state::{DefaultState};
use std::collections::{HashMap, HashSet};
use std::default::Default;
use std::hash::{Hash, Hasher, SipHasher};
use std::marker;
pub struct FnvHasher(u64);
impl Default for FnvHasher {
fn default() -> FnvHasher { FnvHasher(0xcbf29ce484222325) }
}
impl Hasher for FnvHasher {
fn write(&mut self, bytes: &[u8]) {
let FnvHasher(mut hash) = *self;
for byte in bytes {
hash = hash ^ (*byte as u64);
hash = hash * 0x100000001b3;
}
*self = FnvHasher(hash);
}
fn finish(&self) -> u64 { self.0 }
}
fn main() {
let mut set:HashSet<i64, DefaultState<FnvHasher>> = HashSet::with_hash_state(DefaultState::<FnvHasher>);
}
当我编译时,我收到以下错误消息:
$ rustc -V
rustc 1.0.0-nightly (522d09dfe 2015-02-19) (built 2015-02-19)
$ rustc hash.rs
hash.rs:26:86: 26:111 error: mismatched types:
expected `std::collections::hash::state::DefaultState<FnvHasher>`,
found `fn(core::marker::PhantomData<FnvHasher>) -> std::collections::hash::state::DefaultState<FnvHasher> {std::collections::hash::state::DefaultState}`
(expected struct `std::collections::hash::state::DefaultState`,
found fn item) [E0308]
hash.rs:26 let mut set:HashSet<i64, DefaultState<FnvHasher>> = HashSet::with_hash_state(DefaultState::<FnvHasher>);
^~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
如何让上面的示例在 HashSet/HashMap 中编译或使用自定义哈希函数?
Rust 1.7 http://blog.rust-lang.org/2016/03/02/Rust-1.7.html稳定了自定义哈希器的使用。例如,您可以使用FnvHasher
来自fnv https://crates.io/crates/fnv crate
extern crate fnv;
use std::collections::HashMap;
use std::hash::BuildHasherDefault;
use fnv::FnvHasher;
type HashMapFnv<K, V> = HashMap<K, V, BuildHasherDefault<FnvHasher>>;
fn main() {
let mut map = HashMapFnv::default();
map.insert(1, "Hello");
map.insert(2, ", world!");
println!("{:?}", map);
}
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