tag	a439273099f8181307e9d1878af69746721dffe3
tagger	The Android Open Source Project <initial-contribution@android.com>	Thu May 19 16:14:54 2022 -0700
object	71b9a6464eaf9397bde1253103b0e002a1add8e0

commit	71b9a6464eaf9397bde1253103b0e002a1add8e0	[log] [tgz]
author	Android Build Coastguard Worker <android-build-coastguard-worker@google.com>	Thu Dec 16 00:04:37 2021 +0000
committer	Android Build Coastguard Worker <android-build-coastguard-worker@google.com>	Thu Dec 16 00:04:37 2021 +0000
tree	db428e02daefd369d8c754a70557de3b39686571
parent	70bc82f184783f0389a3dd5efcf36b38f1453127 [diff]
parent	93d924b9bab5b33fa2d3af0a2695e55dfa43f228 [diff]

tree: db428e02daefd369d8c754a70557de3b39686571

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.

This crate supports Rust version 1.32 and later.

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);
}