commit	127e16d338f7532478aac785ef70987ab5ef2c95	[log] [tgz]
author	James Farrell <[email protected]>	Wed Sep 18 20:37:08 2024 +0000
committer	James Farrell <[email protected]>	Wed Sep 18 20:37:08 2024 +0000
tree	c4cae01b9f1c3ddaced8853a00dde8339575f85b
parent	095b9219ab07e0cb7a51946a730735fdc5d35a5a [diff]

tree: c4cae01b9f1c3ddaced8853a00dde8339575f85b

README.md

weak-table: weak hash maps and sets for Rust

This crate defines several kinds of weak hash maps and sets. See the full API documentation for details.

Rust version support

This crate supports Rust version 1.46 and later.

Crate features

weak-table is built with the std feature, which enables functionality dependent on the std library, enabled by default. Optionally, the following dependency may be enabled:

ahash: use ahash’s hasher rather than the std hasher

If the std feature is disabled (for no_std) then the ahash dependency must be enabled.

Examples

Here we create a weak hash map and demonstrate that it forgets mappings whose keys expire:

use weak_table::WeakKeyHashMap;
use std::sync::{Arc, Weak};

let mut table = <WeakKeyHashMap<Weak<str>, u32>>::new();
let one = Arc::<str>::from("one");
let two = Arc::<str>::from("two");

table.insert(one.clone(), 1);

assert_eq!( table.get("one"), Some(&1) );
assert_eq!( table.get("two"), None );

table.insert(two.clone(), 2);
*table.get_mut(&one).unwrap() += 10;

assert_eq!( table.get("one"), Some(&11) );
assert_eq!( table.get("two"), Some(&2) );

drop(one);

assert_eq!( table.get("one"), None );
assert_eq!( table.get("two"), Some(&2) );

Here we use a weak hash set to implement a simple string interning facility:

use weak_table::WeakHashSet;
use std::ops::Deref;
use std::rc::{Rc, Weak};

#[derive(Clone, Debug)]
pub struct Symbol(Rc<str>);

impl PartialEq for Symbol {
    fn eq(&self, other: &Symbol) -> bool {
        Rc::ptr_eq(&self.0, &other.0)
    }
}

impl Eq for Symbol {}

impl Deref for Symbol {
    type Target = str;
    fn deref(&self) -> &str {
        &self.0
    }
}

#[derive(Debug, Default)]
pub struct SymbolTable(WeakHashSet<Weak<str>>);

impl SymbolTable {
    pub fn new() -> Self {
        Self::default()
    }

    pub fn intern(&mut self, name: &str) -> Symbol {
        if let Some(rc) = self.0.get(name) {
            Symbol(rc)
        } else {
            let rc = Rc::<str>::from(name);
            self.0.insert(Rc::clone(&rc));
            Symbol(rc)
        }
    }
}

#[test]
fn interning() {
    let mut tab = SymbolTable::new();

    let a0 = tab.intern("a");
    let a1 = tab.intern("a");
    let b  = tab.intern("b");

    assert_eq!(a0, a1);
    assert_ne!(a0, b);
}